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Chemistry Y13 > Transition Metals > Flashcards

Transition Metals Flashcards

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1
Q

What is a transition metal?

A

a transition metal has an incomplete d sub-shell either as an element or either as one of it’s common ions

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2
Q

where on the periodic table are transition metal?

A

D Block - large central block of the periodic table

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3
Q

what does this definition mean?

A

this definition means that not all the elements or ions of metals in the D-block can be considered as

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4
Q

what are the 4 characteristic properties of the transition metals?

A
  1. the formation of complexes
  2. the formation of coloured ions
  3. variable oxidation states
  4. catalytic activity
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5
Q

what are the characteristics of all transition metals the result of?

A

the result of their electronic structure (incomplete d sub-shell in atoms or ions)

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6
Q

How many electrons can the 3d sub shell hold?

A

10 electrons

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7
Q

3d elements are the first…?

A

10 elements in the first row of the d-block

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8
Q

which of the first 10 elements have a abnormal electronic structure/ deviate from the pattern?

A
  • chromium
  • copper
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9
Q

Draw the structure of the first row of transition metal atoms

A
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10
Q

how can we abbreviate electron structure?

A

using elements zero

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11
Q

why are the electron structures of chromium and copper slightly different?

A

chromium has the structure [Ar] 4s1 3d5 rather than [Ar] 4S2 3d4. This is because having a half-filled sub-shell (d5) gives it extra stability (same for copper)

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12
Q

why is Cu2+ considered a transition metal and not Copper?

A
  • the structure of copper does not have an incomplete d sub-shell However cu2+ does have an incomplete d sub-shell, therefore cu2+ is a transition metal ion
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13
Q

why is zinc not classified as a transition metal

A
  • the structure of copper does not have an incomplete d sub-shell However Zn2+ does have an incomplete d sub-shell, therefore Zn2+ is a transition metal ion
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14
Q

what ions do transition metals produce when they lose an electron?

A

positive ions

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15
Q

why are 4s electrons lost first before 3d electrons?

A

3d sub-shells are lower in energy than 4s therefore the 4S electrons are lost before the 3d electrons

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16
Q

which two ions are non-transitional and why?

A

Sc3+ and Cu+ because they do not have an incomplete d sub-shell

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17
Q

what do transition metal compounds not have?

A

do not have transition metal ions with outer-s-electrons

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18
Q

what gives transition metal ions their properties?

A

the ions have an incomplete d sub-shell and it is the arrangement that gives the ions their characteristic properties

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19
Q

Explain, In terms of electronic structures of their ions, why a solution of iron (II) is rapidly oxidised to iron (III).

A
  • Iron (II) has the electronic structure of [Ar] 3d6 and Iron (III) has the electronic structure [Ar] 3d5
  • iron 3 has a half-filled d sub shell, so has extra stability
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20
Q

Complex Formation

A
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21
Q

what does a complex consist of?

A

a complex consists of a central transition metal ion or atoms surrounded by ligands

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22
Q

What is a ligand?

A

Ligands is a species that donates a lone pair of electrons to the metal ion and form co-ordinate bonds (dative)

Ligand = Lewis base = Nucleophile

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23
Q

what is the co-ordinate bonds formed by?
State what the Transition metal ion and the ligand act as?

A

is formed by the overlap of a ligand orbital (containing a lone pair of electrons) with a vacant orbital on the transition metal ion.

Transition metal acts as a Lewis Acid = electron pair acceptor

Ligand acts as a Lewis Base = electron pair donor

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24
Q

What is a complex?

A

a complex consists of a central transition metal ion surrounded by ligands

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25
Q

what is the co-ordination number?

A

the co-ordination number of a complex is the number of atoms bonded to the central metal ion

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26
Q

the co-ordination number differs from it’s oxidation state when..?

A

in a complex compound

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27
Q

in other words,..?

A

the number of atoms bonded to the central atom is more than the normal bond forming ability of the metal

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28
Q

what is usually the co-ordination number?

A

2, 4 or 6 (coordinate bonds formed)

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29
Q

what does the co-ordination number determine?

A

determines the shape of the complex

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30
Q

what appearance does Anhydrous copper sulphate (II) (CuSO4) take normally?

A

White solid

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31
Q

Anhydrous copper sulphate forms what colour in water?

A

blue (solution)

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32
Q

why does Anhydrous copper sulphate turn blue when reacted with water?

A

this is because the Cu2+ ions get surrounded by water molecules that bond to the Cu2+ ion

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33
Q

what complex ion is formed?

A

[Cu(H2o)6]2+ - Hexaaquacopper(II) ion

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34
Q

Draw the complex ion, Hexaaquacopper (II)

A
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35
Q

what do water molecules act as?

A

ligands

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36
Q

how many atoms (water) are bonded to the central atom?
therefore…?

A

6 - therefore the co-ordination number if 6

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37
Q

explain bonding in this complex ion?

A

each water molecule oxygen oxygen atom has donated 1 lone pair of electrons into a vacant orbital on the Cu2+ ion to form a co-ordinate bond

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38
Q

draw a word equation to show the formation of Hexaaquacopper (II)

A

CuSO4 —- H2O—–> [Cu(H2O)6]2+

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39
Q

what is the co-ordination number and the oxidation state of Copper?

A

co-ordination number = 6
oxidation state = +2

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40
Q

what is this reaction the test for?

A

test for water

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41
Q

Naming complex ions
1st part of the name =
second part of the name =
third part of name =
4th part of the name =

A
  1. number of ligands
  2. ligand
  3. metal
  4. oxidation state of the metal
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42
Q

what type of ligand is water?

A

water is a neutral ligand

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43
Q

what does this mean?

A

the overall charge on the complex ion is the same as the charge on the central metal ion

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44
Q

Give an example for a complex ion where the charge on the ligand is taken into account

A

tetrachlorocuprate (II) ion [Cu(Cl)4]2-

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45
Q

what oxidation sate does Na+, K+, and Ag+ have?

A

+1

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46
Q

what oxidation state does Mg2+, Ca2+, Ba2+ have?

A

+2

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47
Q

what oxidation sate does F-, Cl-, I- have?

A

-1

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48
Q

what oxidation state does O2- and S2- have?

A

-2

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49
Q

the oxidation state of the central atom in a complex ion is the charge…?

A

it would have if it was a simple ion (not bonded to other species)

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50
Q

what will the sum of the oxidation states in a neutral compound be?

A

zero

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51
Q

How does this differ for charged ions?

A

the sum of the oxidation state in an ion is equal to the overall charge of the ion

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52
Q

what are the following ligand charges:
water =
ammonia =
hydroxide ion =
chloride =
cyanide ion =

A

water = zero
ammonia = zero
hydroxide ion = -1
chloride = -1
cyanide ion = -1

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53
Q

calculate the following oxidation states of the metal ion:
1. [MnO4-]-
2. [VO3]-
3. [Cr2O7]2-
4. [Agcl2]-
5. [Cu(NH3)4(H2O)2]2+
6. [Ag(CN)2]-

A
  1. Mn=+7
  2. V = +5
  3. Cr = +6
  4. +1
  5. +2
  6. +1
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54
Q

How does the name of the ligand change when they’re in a complex?

A
  • Water changes to Aqua
  • Chloride changes to Chloro
  • Ammonia changes to Ammine
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55
Q

what are the prefixes for each ligand?

A

Di, Tri, Tetra, Hexa

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56
Q

How does the metal ion’s name change when it forms a negatively charged complex:
silver =
Copper =
iron =
manganese =
Chromium =
Colbalt =
Vanadium =

A

silver = Argentate
Copper = Cuprate
iron = ferrate
manganese = manganate
Chromium = Chromate
Colbalt = Colbaltate
Vanadium = Vanadate

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57
Q

Name the following compounds:
1. [Cu(H2O6]2+
2. [CoCl4]2-
3. [Cr(NH3)6]3+
4. [Ag(NH3)2]+

A
  1. Hexaaquacopper(II)
  2. tetrachlorocolbaltate (II)
  3. Hexaamminechromium (III)
  4. Diamminesilver(I)
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58
Q

Different types of Ligand

A
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59
Q

what is a complex ion?

A

a complex ion consists of a central transition metal ion surrounded by ligands

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60
Q

what is a ligand?

A

A ligand is a species that donates one or more lone pairs of electrons to a metal ion (to form a co-ordinate bond)

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61
Q

How are Ligands classified?

A

ligands are classified by the number of co-ordinate bonds that they are bale to form in complexes (i.e the number of atoms that can donate a lone pair of electrons)

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62
Q

What are Monodentate ligands?

A

Monodentate ligands form one co-ordinate bond to the central metal ion (i.e one atom from the ligand donate one lone pair of electrons)

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63
Q

what does Monodentate mean?

A

‘single-toothed’ - one biting point

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64
Q

what are examples of Monodentate ligands?

A

H2O, NH3, CL-, OH-, CN-

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65
Q

Which two examples of Monodentate ligands are similar, explain why?

A

Both H2O and NH3 are similar in size (both small) and are both uncharged

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66
Q

In Monodentate ligands, the co-ordination number is equal to…?

A

the number of ligands attached to the central transition metal ion

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67
Q

Draw the complex ion:
- Hexaaquairon (II) ion
- Tetrachlorocolbaltate (II) ion
- Diamminesilver (I) ion

A
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68
Q

what is a bidentate ligand?

A

bidentate ligands form two co-ordinate bonds to the central transition metal ion (i.e the ligand has two electron pair donor atoms)

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69
Q

what are examples of Bidentate ligands? Draw each of the structures out?

A
  1. 1,2-diaminoethane H2NCH2CH2NH2 - bonds through 2 Nitrogens
  2. ethanedioate C2O4- - bonds through 2 oxygens
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70
Q

Draw the complex ion, where Chromium is the central transition metal ion, and three 1,2-diaminoethane ligands are bonded to the central metal ion. (Charge on the ion is 3+)

A
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71
Q

What is the co-ordination number of the above ligand?

A

There is only three ligands but the Co-ordination number is 6

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72
Q

what is a multidentate ligand?

A

multidentate ligands can form three or more co-ordinate bonds to the central transition metal ion (i.e the ligand has three or more electron pair donor atoms)

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73
Q

what can form the anion EDTA^4-?

A

Bis[di(carboxymethyl)amino]ethane or H4EDTA

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74
Q

how many bonds does EDTA^4- form?

A

forms 6 bonds with the metal ion

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75
Q

Draw the structure of EDTA4-, Highlight the 6 lone pairs!

A
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76
Q

Draw hoe EDTA4- surrounds the metal ion?

A
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77
Q

what ratio does the metal ion and EDTA ligand form?

A

1:1 complex - 1 metal ion and 1 EDTA ligand

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78
Q

what are the complexes formed called when Bidentate or Multidentate ligands bond to the central transition metal ion

A

Chelates

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79
Q

what does the word Chelate mean?

A

‘Crab claw’

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80
Q

why are Chelates more stable than Monodentate ligands?

A

Chelates are often more stable than Monodentate ligands since their formation results in an increase in entropy (an increase in system disorder)

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81
Q

what is this called?

A

The chelate effect

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82
Q

what is EDTA commonly used as?

A

as a chelating effect and it’s complex are extremely stable

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83
Q

Draw out the word equation to show how EDTA forms a very stable complex?

A

[Cu(H2o)6]2+ + EDTA4- ——> [Cu(EDTA4-)]2- + 6H20

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84
Q

in the above example, explain how this leads to chelate effect?

A

the formation of the chelate complex leads to an increase in entropy (more disordered system since more chemical species are formed - chelate effect.

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85
Q

what is the feasibility for all chemical reactions?

A

is a balance between the enthalpy change and the entropy change for the reaction (ΔG)

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86
Q

what is the dominant factor in the formation of EDTA complexes?

A

In the formation of EDTA complexes, the entropy change is the dominant factor

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87
Q

Uses of EDTA

A
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88
Q

what ability does EDTA have that makes it very useful?

A

has the ability to ‘trap metal ions’

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89
Q

what are the many uses of EDTA?

A
  • used as an antidote to metal poisoning
  • used in blood transfusions ans surgical operations to remove calcium ions which cause blood clotting
  • used to remove calcium ions from hard water
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90
Q

How is EDTA able to act as an antidote for metal poisoning?

A

it can remove dangerous metal ions (e.g lead and mercury) from the blood (excreted as the stable EDTA complex)

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91
Q

Describe the effect of hardwater on pipes?

A

Hard water can lead to the blockage of water pipes and form scum on bathwater

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92
Q

what products contain EDTA and why?

A

EDTA removes calcium ions from hard water, therefore, many cleaning products contain EDTA e.g shampoo

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93
Q

Blood contains which metal Complex?

A

Iron(II) complex called Haem

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94
Q

What species of Ligand does Haem/iron complex (II) contains?

A

contains Multidentate ligand

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95
Q

How are atoms arranged around the Iron (II)?

A

octahedrally

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96
Q

How many atoms is bonded to the Iron in a plane?

A

The iron (II) is bonded to 4 nitrogen atoms in a plane

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97
Q

where do the four atoms come from?

A

they come from a porphyrin molecule

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98
Q

what is the Porphyrin molecule?

A

a large organic tetradentate ligand

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99
Q

what else is the iron bonded to?

A

the iron is also bonded to another nitrogen atom above the plane from a protein called globin

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100
Q

what is the final 6th position of the iron octahedral complex occupied by?

A

by an oxygen molecule (oxyhaemoglobin) or a water molecule

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101
Q

What bonds does Oxygen form to the iron (II) and what does this allow?

A

forms weak co-ordinate bonds. This allows Haemoglobin to transport oxygen around our bodies through the blood

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102
Q

what else can the final 6th position of Haemoglobin be occupied by?

A

Carbon monoxide

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103
Q

Why is carbon monoxide able to occupy the final 6th position?

A

Carbon monoxide form strong co-ordinate bond with the iron (II) than an oxygen molecule (carboxyhaemoglobin)

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104
Q

What is reduced when carbon monoxide is present in the blood?

A

When carbon monoxide is present in the blood, the oxygen-carrying capacity is greatly reduced.

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105
Q

Why is carbon monoxide toxic?

A

Carbon monoxide is toxic because it can replace oxygen co-ordinately bonded to Fe(II) in haemoglobin

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106
Q

Draw the structure of Oxyhaemoglobin

A
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107
Q

How to answer a question on Chelate effect? - try to find mark scheme answer!!

A
  • formation of a VERY STABLE chelate complex
  • leads to an increase in entropy
  • give example e.g [Cu(H2o)6]2+ + EDTA4- ——> [Cu(EDTA4-)]2- + 6H20
  • there 2 species at the start, 7 at the end = more disordered system = increase in entropy
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108
Q

Shapes of Complex ions

A
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109
Q

what is the most common Complex ion shape?

A

Octahedral - bond angle of 90°

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110
Q

what type of ligand forms Octahedral complex ions?

A

Monodentate ligands

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111
Q

what is the co-ordination number in an octahedral complex?
For monodentate ligands, how many ligands are there and how many co-ordinate bonds are formed ?

A
  • in octahedral complex ions , co-ordination number is 6
  • For monodentate ligands, this means that 6 ligands form 6 co-ordinate bonds to the central transition metal ion
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112
Q

what complex does the first row of transition metal ions form with water?

A

form Octahedral Hexaaqua ions

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113
Q

Draw the two main structure of the octahedral ion [Cu(H2O)6]2+

A
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114
Q

which two ligands usually form an octahedral shape and have a co-ordination number of 6?

A

Water and ammonia

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115
Q

why do water an ammonia usually form Complex containing a co-ordination number of 6 therefore an octahedral shape?

A
  • water and ammonia are small ligands/ small in size
  • therefore, 6 water and or ammonia can fit around the central transition metal ion
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116
Q

Draw octahedral complex [Fe(NH3)4(H20)2]2+

A
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117
Q

what is the second most common shape a complex ion forms?

A

Tetrahedral shape

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118
Q

what is the bond angle and co-ordination number of a tetrahedral complex?

A

Bond angle = 109.5 °
Co-ordination number = 4

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119
Q

what type of ligand form Tetrahedral complexes?

A

Monodentate ligands - large negatively charged ligands

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120
Q

How many Monodentate ligands will form bonds in a tetrahedral complex?

A

This means that 4 ligands can form 4 co-ordinate bonds to the central transition metal ion

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121
Q

Give examples of the type of Ions that form complexes tetrahedral in shape?

A
  • Chloride ions
  • Bromide ions
  • Iodide ions
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122
Q

why can chloride only form 4 bonds whereas ammonia and water can form 6?

A
  • Chloride ion ligands are larger than water and ammonia
  • Therefore, there is only space for 4 chloride ion ligands around the central transition metal ion
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123
Q

why are chloride ions further apart in the tetrahedral arrangement than they are in an octahedral arrangement?

A

because there is less electron cloud repulsive forces between ligands

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124
Q

Draw the tetrahedral complex [CuCl4]2-

A
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125
Q

why don’t fluoride ions form tetrahedral complex but form octahedral complex instead?

A

Fluoride ions ligands are smaller than Chloride ion ligands

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126
Q

what are less common complex shapes?

A
  • square planar = Bond angle of 90°
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127
Q

which Important molecule has a square planar complex?

A

CISPLATIN

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128
Q

what is cisplatin?

A

Cisplatin is a Anti-Cancer drug used in Chemotherapy

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129
Q

what type of cancer has it been used to treat?

A

Lung and bladder

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130
Q

what type of complex does Cisplatin form, How many bonds?

A

Cisplatin is a 4 co-ordinate, square planar complex of Platinum (II)

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131
Q

what are the two ligands present in the molecule?

A

2 chloride ions (CL-) ligands and 2 NH3 ligands

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132
Q

Draw the structure of Cisplatin.

A
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133
Q

What is cancer caused by?

A

cancer is caused when cells mutate and start dividing uncontrollable to form tumours

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134
Q

in order for a cell to divide, what needs to happen?

A

DNA needs to replicate - so the two strands of DNA have to unwind so they can be replicated/copied

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135
Q

How does cisplatin effect this (DNA replication)?

A

Cisplatin stops this happening properly(stops DNA replication), so that tumour cells stop reproducing and cancer cells can not longer divide and grow/replicate

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136
Q

what type of reaction occurs between the ligands and Cisplatin?

A

Cisplatin does a ligand replacement reaction with DNA in which a co-ordinate bond is formed between platinum and a nitrogen atom on guanine

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137
Q

Draw How cisplatin undergoes a ligand replacement reaction with DNA, until

A
  • the nitrogen atom on guanine can bond to the platinum and replace the chloride
  • a second nitrogen atom from a nearby guanine molecule (on the same strand or on the other strand) can bond to the platinum and replace the second chloride too
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138
Q

These cisplatin complexes cause the strands to..?

A

Kink - they can’t unwind and be coiled properly - so they can’t replicate properly

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139
Q

what is a downside of Cisplatin?

A
  • cisplatin can bind to DNA in normal cells as well as cancer cells - so healthy cells can be prevented from dividing too
  • so causes problems for healthy cells that replicate frequently e.g hair + blood cells
  • therefore, cisplatin has serious risks associated with it’s use and a number of TOXIC side effects
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140
Q

what are common side effects of using cisplatin?

A
  • kidney damage, nerve damage, hearing loss, nausea, vomiting, alopecia and electrolyte disturbances
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141
Q

How can we reduce the effect of side effects?

A
  • lowering the dosage and also using methods that can help deliver the drug directly to the tumour
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142
Q

so what does society/chemists need to asses the balance..?

A

society needs to asses the balance between the benefits and the adverse effects of drugs, such as the anti-cancer drug cisplatin

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143
Q

even though cisplatin has many side effects it’s use is widespread, why?

A

Cisplatin’s use is widespread in medicine since the balance of long-term positive effects (curing cancer) outweigh the negative short-term side effects

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144
Q

Draw the structure of Transplatin

A

(chlorides are on the opposite sides of the complex to each other)

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145
Q

How is Cisplatin different to Transplatin

A

has different biological effects to cisplatin

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146
Q

what complexes have a bond angle of 180°?

A

Linear complexes

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147
Q

what type of ions form Linear complexes?

A

Silver ions (Ag+) and Copper ions prefer to form linear complexes

148
Q

How many ligands bond to the silver ion?

A

2 ligands bond to the silver ion and form a bond angle of 180°

149
Q

Ag+ and Cu+ form linear complexes. Explain why these ions are colourless?

A
  • Ag+ and Cu+ have D10 ions - they have no vacant d orbtal and are therefore colourless
  • (they are not transition metal)
150
Q

Many silver compounds are insoluble in water except for 3, what are they?

A
  • [Ag(NH2)2]+
  • [Ag(S2O3)2]3-
  • Ag(CN)2]-
151
Q

give the formula and name of the silver complex in Tollens reagent?

A

[Ag(NH3)2]+ - diamminesilver (I)

152
Q

How is Tollen’s reagent formed?

A

this complex is formed by mixing silver nitrate and aqueous ammonia to form the colourless solution Tollen’s reagent

153
Q

when aldehydes react with Tollens, they produce a..?

A

silver mirror - Ag+ ions are reduced to Ag metal

154
Q

which complex is used in Film photography (the ‘fixing’ process)?

A

[Ag(S2o3)2]3-

155
Q

which Complex is used in Silver-plating?

A

[[Ag(CN)2]-

156
Q

Shapes of complex ions with Bidentate and Multidentate ligands

A
157
Q

when complexes have bidentate and multidentate ligands, what does the shape of the ion depend on?

A

the shape of the ion depends on the number of co-ordinate bonds formed

158
Q

Draw the structure of 1,2-diaminoethane, what complex does it form?

A

forms an octahedral complex - forms 6 co-ordinate bonds

159
Q

what Complex does EDTA^4- form?

A

forms an octahedral complex (6 co-ordinate bonds)

160
Q

Stereoisomerism in complexes

A
161
Q

what are stereoisomers?

A

stereoisomers have the same molecular and same structural formula but their bonds are arranged differently in space.

162
Q

complex can show either..?

A

optical or cis-trans isomerism (E-Z stereoisomerism)

163
Q

what type of complexes can show optical isomerism?

A

octahedral complexes can display optical isomerism when 3 bidentate ligands bonds with the central transition metal ion

164
Q

Draw the two optical isomers of 1,2-diaminoethane?

A
165
Q

what are the two key properties of these two optical isomers?

A
  • non-super imposable and mirror images
166
Q

when does Cis-trans isomerism occur in Square-planar complexes?

A

Cis-trans isomerism can occur in square-planar complexes where 2 pairs of ligands are present e.g cisplatin and transplatin

167
Q

when does Cis-trans isomerism occur in octahedral complexes?

A

cis-trans isomerism can occur in octahedral complexes when there are 4 ligands of one type and 2 ligands of another type present.

168
Q

Variable Oxidation States

A
169
Q

why do transition metals have variable oxidation states?

A

transition metals have 4s and 3d electrons are available for bonding

170
Q

what does the oxidation sate of a transition metal in a particular species help determine?

A

the colour of the species and it’s chemical properties

171
Q

What is meant by transition metals have variable oxidation states?

A

transition metals have the ability to change oxidation state in chemical reactions

172
Q

What is the oxidation states of the first row of transition metals?

A
173
Q

What are the common oxidation states?

A

+2 and +3

174
Q

What does the oxidation state of +2 correspond to?

A

the loss of ‘4s’ electrons - (the 4s electrons are used in bonding)

175
Q

For electrons with oxidation states above +, what electrons are involved in bonding?

A

3d electrons are also involved in bonding

176
Q

why is the +2 Oxidation state so common?

A
  • the +2 oxidation state becomes more stable and more common across the period
  • As increasing nuclear charge causes 3d electrons to be held more tightly
177
Q

when does oxidation states increase?

A

from scandium to manganese

178
Q

Explain why Cr does not have an oxidation state of +7?

A

would have to remove the an electron from the next full outer shell

179
Q

Explain why Fe does not have an oxidation state of +7?

A
  • the increase in nuclear charge across the period, means that 3d electrons are held more tightly.
  • This is a tipping point around the Mn
  • beyond the Mn nuclear charge dominates
180
Q

Explain why Mn2+ is a particularly stable ion?

A
  • extra stability associated with a half-filled d sub shell
181
Q

Lower Oxidation states are found as..?

A

simple ions e.g M2+ or M3+

182
Q

Lower oxidation sates are also..?

A

Reducing e.f Cr2+ and Fe2+

183
Q

Which two ions will we never find as individual ions in ionic compunds?

A

Mn7+ and Cr6+

184
Q

Why is this?

A

the higher oxidation states are usually found in complex ions or in compounds with very electronegative elements such as oxygen
e.g VO2+, VO3-, MnO4-, CrO4-, Cr2O72-

185
Q

The higher oxidation states are said to be..?

A

Oxidising

186
Q

Which two ions are powerful oxidising agents.

A

MnO4- and Cr2O72-

187
Q

What are Redox reactions?

A

redox reactions are those that involve both reduction and oxidation

188
Q

Why do transition metals take place in redox reactions?

A

because they have variable oxidations state

189
Q

The transition metal is either..?

A

Oxidised or reduced and the transition metal ion changes it’s oxidation states

190
Q

Redox Titrations

A
191
Q

Redox reactions can be used in what?

A

Quantitative volumetric analysis

192
Q

What is Volumetric quantitative analysis?

A

Volumetric analysis is a quantitative analytical method of determining the amount of substance contained in a sample solution by gradually adding a standard solution of known concentration and measuring the volume at the time of reaction.

193
Q

What can we use to determine the endpoint of a titration?

A

the end-points can be determined by the use of indicators and electrical methods

194
Q

what is an important property of some redox reactions involving transition metal complexes and why?

A

Some important redox reactions involving transition metals complexes are self-indicating due to the products having different colours

195
Q

Which are two common quantitative oxidising agents?

A

Potassium Manganate (VII) and Potassium dichromate (Vi) in ACID solution

196
Q

What is good about using potassium manganate (VII) in redox titrations?

A

no indicator is needed - so it is self-indicating

197
Q

What colour is potassium manganate (VII)?

A

purple

198
Q

When potassium manganate is reduced what colour does it form?

A

colourless solution, when potassium manganate is added gradually, at the end-point a colour change is observed

199
Q

Which will one drop of excess potassium manganate produce at the end-point?

A

At the end-point 1 drop of excess potassium manganate will produce a permanent pale pink colouration in the solution

200
Q

What is the initial species present and the final species present?

A

Initial: MnO4-
Final: Mn2+

201
Q

Construct the half equation for this reaction?

A

MnO4- + 8H+ + 5e- —> Mn2+ + 4H2O

202
Q

Potassium manganate (VII) only works in the presence of..?

A

Only works in the presence of excess acid (since H+ ions are required for the reaction)

203
Q

What happens when there is no excess Acid?

A

if there is no excess H+ ions the solution goes brown as MnO2 forms instead of Mn2+

204
Q

We must use an appropriate acid, What properties must the acid have and why?

A
  • Be strong (a high concentration of H+ ions needed)
  • Not be an oxidising agent (an oxidising agent may react with the reducing agent we are trying to analyse)
  • not be a reducing agent (reducing agents will be oxidised by the manganate (VII) ions
205
Q

Why can Hydrochloric acid not be used as a reducing agent?

A

Hydrochloric acid can not be used because this can be oxidised to chlorine

206
Q

Why can’t nitric and concentrated sulphuric acid be used?

A

Nitric acid and concentrated sulphuric acid can not be used because these are oxidising agents

207
Q

Why can we not used Ethanoic acid?

A

Ethanoic acid can not be used because this is a weak acid which will not produce a high enough concentration of H+ ions

208
Q

What Suitable acid can we used?

A

DILUTE Sulphuric acid

209
Q

How can we use Potassium manganate (VII) in titration?

A
  1. Potassium manganate solution goes into the burette
  2. The conical flask contains the solution of the reducing agent we are trying to analyse (e.g Fe2+ solution)
  3. the solution in the conical flask is made up using dilute H2SO4 (to provide an excess of H+ ions)
  4. The end-point of a titration is when there is a slight excess of Potassium Manganate and the solution in the flask goes from colourless to pale pink
  5. When there is an excess of Potassium manganate then we know the reaction is complete (all Fe2+ ions have been oxidised)
210
Q

How do we know when the reaction is complete?

A

When there is an excess of Potassium manganate then we know the reaction is complete (all Fe2+ ions have been oxidised)

211
Q

Potassium manganate in titrations can be used to estimate the concentration of what in a solution?

A

Potassium manganate can be used to estimate the concentration of iron (II) ions in a solution

212
Q

What does Potassium manganate oxidise Iron (II) ions to?

Show this in an equation

A

Potassium Manganate (VII) can oxidise Iron (II) to Iron (III)

Fe2+ —> Fe3+ + e-

213
Q

What is manganate reduced to?

Show this in an equation

A

Manganate is reduced to manganese

MnO4- + 8H+ + 5e- —> Mn2+ + 4H2O

214
Q

How do we get the overall equation?

A

To get the overall equation we need to combine the two half equation

215
Q

Before we combine the equations, what must we do to the equation?

A

we must balance them, make sure that there is the same amount of electrons in each half equation

216
Q

What is the overall equation for redox reaction of potassium manganate with Fe2+ ions?

A

MnO4- + 8H+ + 5Fe2 —> Mn2+ + 4H2O + 5Fe3+

217
Q

Potassium Dichromate (VI) in Titrations

A
218
Q

When potassium dichromate (VI) is used in titrations, what is needed?

A

An indicator is needed

219
Q

What indicator could we use?

A

e.g sodium diphenylaminesulphonate

220
Q

What colour does sodium diphenylaminesulphonate turn at the end-point of titration?

A

Turns from colourless to purple at the end-point

221
Q

describe how chromium (Cr2O7^2-) is reduced from +6 to +3 oxidation state

A

Cr2O7^2- + 14H+ + 6e- —> 2Cr3+ + 7H2O

222
Q

what is required for this reaction?

A

Excess Acid since H+ is a reactant

223
Q

In this reaction, how can H+ ions be provided?

A

By using dilute sulphuric acid or dilute hydrochloric acid

224
Q

Why can dilute hydrochloric acid be used this time?

A

dilute hydrochloric acid can be used this time because the dichromate (Vi) ions are not powerful enough to oxidise the chloride ions into chlorine

225
Q

Describe how we can use Potassium Dichromate in titration?

A
  1. the KCr2O7 solution goes into the burette
  2. the conical flask containing solution of the reducing agent we are analysing (e.g Fe2+ solution)
  3. the solution in the conical flask is made up using dilute H2SO4 or dilute HCL (to provide an excess of H+ ions
  4. sodium diphenylaminesulphonate indicator turns from colourlesss to purple at the endpoint
226
Q

Write an overall equation to show the oxidation of iron (II) by Cr2O2^2-?

A

Cr2O72- + 14H+ + 6Fe2+ —> 2Cr3+ + 7H2O + 6FE3+

227
Q

so 1 mole of Cr2O7^2- requires, how many moles of Fe2+?

A

6 omoles of Fe2+

228
Q

Complete example question 1 and 2

A
229
Q

Why does our body need Iron?

A

The body needs iron for good health (it’s an essential component of haem

230
Q

What does a lack of iron do?

A

Leads to anaemia

231
Q

How is anaemia treated?

A

Iron tablets

232
Q

Why do blood donors have to take iron tablets?

A

to restore iron levels after giving blood

233
Q

Formation Of Coloured Ions

A
234
Q

If you find a coloured solution in chemistry what does it usually indicate?

A

That there is a transition metal present

235
Q

Different transition metal complexes..?

A

are shown by a large variety of colours

236
Q

What is transition metal chemistry also referred to?

A

transition metals are at the very heart of a branch of chemistry called Colour Chemistry

237
Q

Why are transition metals coloured/

A

Because they absorb visible light

238
Q

Some light is reflected, what does this light contain?

A

the reflected light contains the frequencies not absorbed

239
Q

Why can Transition metal complexes be identified b their colour?

A

transition metals complexes have characteristic colours and therefore transition metal ions can be identified by their colour

240
Q

What affects the colour of a transition metal?

A

1) the type of ligand
2) the co-ordination number
3) the oxidation state of the transition metal

241
Q

What happens when any of these factors change?

A

The colour of the transition metal changes

242
Q

When does a colour change of the transition metal usually occur?

A

colour changes often occur in transition metal ion chemical reactions due to change in one or more of the above factors

243
Q

Describe the colour change when we CHANGE THE LIGAND of Chromium from Hexaaqua to Hexaammine?
[Cr(H2O)6]3+ —> [Cr(NH3)6]3+

A

turns from a red-violet colour to a purple

244
Q

Describe the colour change of [Cu(H2O)6]2+ —–> [Cu(NH3)4(H2O)2

A

Turns from blue to blue-violet

245
Q

Describe the colour change when we change the co-ordination number of Copper in the following reaction: [Cu(H2O)6]2+ —-> [CUCl4)2-

A

Blue to Yellow-green colour

246
Q

Describe the colour change when we change the co-ordination number of Colbalt in the following reaction:
[Co(H2O)6]2+ —–> [CoCl4]2-

A

pink to blue colour change

247
Q

Describe the colour change when we change the oxidation state of the following of the following:
1. [Fe(H2O)6]2+ —–> [Fe(h2O)6]3+
2. [Cr(H2O)6]3+ —–> [Cr(H2O)6]2+

A
  1. green to very pale violet
  2. red-violet to blue
248
Q

Describe the colour change when all of these factors have been changed for the following compounds:
1. [CrO4]2- —–> [Cr(H2O)4(Cl)2]+
2. [Mn(H2O)6]2+ —–> [MnO4-]-

A
  1. Yellow to green colour change
  2. Very pale pink to purple colour change
249
Q

Which transition metals are coloured?

A

Transition metals that have an incomplete 3d sub-shell are coloured

250
Q

What makes Transition metal compounds Coloured?

A
  • when a ligand bonds with a transition metal ion the 3d sub-shell splits to form 2 slightly different energy levels
  • Electrons in the lower 3d sub-shell absorb energy from visible and Ultraviolet radiation which promotes (excites) electrons to a higher 3d sub-shell
251
Q

What are these electron promotions called?

A

d-d transitions

252
Q

In order for electrons to be promoted to a higher energy state (d-d transitions), which condition need to be fulfilled?

A
  • the 3d sub-shell must be partially filled or there will not be room for any excited electrons
253
Q

Because of this D10 or D0 ions are..?

A

colourless (need incomplete d-sub-shell for d-d transitions

254
Q

What does the difference in energy between the two set’s of sub-shells dictate?

A

The difference in energy (ΔE) between 2 sets of 3d sub-shells dictates the frequency of visible light absorbed and hence the colour of the complex.

255
Q

the colour associated with d-d transitions are not particularly intense. When are more particular colours observed?

A

more intense colours are observed when the transition involves an electron moving from a Ligand orbital to a metal (or vice-versa). These are called charge transfer transitions

256
Q

What is the equation for Difference in energy ΔE (between the two levels)?

A

ΔE = hv
where:
v = frequency S-1 or Hz
h= planks constant (6.63 X 10-34 Js)

257
Q

When is energy (J) absorbed?

A

Energy (J) is absorbed when an electron is promoted from a higher (excited) energy state.

258
Q

If we are not given the wavelength to calculate ΔE, which equation must we use to find wavelength?

A

v = cλ
c = speed of light (3 x 10^8)
λ = wavelength (m)

259
Q

Why might Ultraviolet radiation carry out the excitation instead of visible light?

A

Energy levels may be too far apart for visible light to be able to carry out excitation. Higher energy Ultraviolet light may be absorbed instead (cannot be detected by the human eye) - other means of detection is required

260
Q

What is the colour that we observe due to?

A

The colour we observe is due to the frequencies that are not absorbed (i.e those that are reflected or transmitted)

261
Q

when light of a particular colour is absorbed, what is reflected?

A

it’s complimentary colour is reflected

262
Q

How to we identify this?

A

on a colour wheel, the colour opposite the colour absorbed is observed

263
Q

So using this logic, why is a solution of Hexaaquacopper(II) ions blue-green?

A

this is because red-yellow wavelengths are absorbed (and blue-green is not absorbed (and reflected))

264
Q

what can the intensity of the colours shown by a solution determine?

A

the intensity of colours shown by a solution of a transition metal ion can be used to determine the concentration of that ion.

265
Q

How can we identify the concentration of them ion?

A

using a spectrophotometer A.K.A colorimeter

266
Q

Describe how a colorimeter works?

A
  • light accross a range of frequencies (a spectrum) is passed though a sample
  • the light that emerges on the other side of the sample is detected and analysed
  • the amount of light absorbed is proportional to the concentration of the absorbing species in the solution under test (Beer-Lambert Law)
267
Q

How can we work out the concentration of the absorbing species?

A

by measuring how much light is absorbed at a particular frequency

268
Q

How can we determine the concentration of a transition metal ion in solution using Ultraviolet-visible spectrophotometry (colorimetry).

A

1) Add a suitable ligand that will intensify the colour (e.g EDTA). This leads to a significant difference in absorption between different solution concentrations
2) Make several different known concentrations of the solution (e.g FeSO4 solution). these are called standards
3) Measure the absorption of all the known concentrations (the more concentrated the solution the more light it absorbs)
4) Plot a graph of concentration vs. absorption (a calibration graph)
5) measure the absorption of an unknown sample and use the calibration curve to determine the unknown concentration

269
Q

What type of line would we expect to see?

A

a straight line graph - absorption is directly proportional to concentration

270
Q

what would be on each axis?

A

Absorption on the Y axis and Concentration the X axis

271
Q

CATALYSIS

A
272
Q

What is a catalyst?

A

a catalyst provides an alternate reaction pathway that has a lower activation energy

273
Q

Draw a graph to show how a catalyst affects the rate of reaction?

A
274
Q

Describe the effect that a catalyst has on the rate of reaction

A
  • in the presence of a catalyst more of the reactant particles will have energy in excess of the lowered activation energy
  • therefore there is an increase in the percentage of successful collisions, resulting in an increase in the rate of reaction (Maxwell-Boltzman)
275
Q

explain the effect of a catalyst on the rate of an equilibrium reaction?

A

in a reversible reaction a catalyst increases the rate of the forward and the backward reactions equally. The equilibrium position is reached more quickly but there is no change in the position of equilibrium.

276
Q

what property of transition metals allows them to act as catalysts in a wide range of inductrial processes

A

Transition metals and transition metal compounds have variable oxidation states which allows many of them to act as catalysts in a wide range of industrial processes

277
Q

What is specific about a catalyst?

A

it is specific for a particular reaction

278
Q

What do successful industrial processes make?

A

make the maximum amount of product in the shortest possible time

279
Q

Therefore, Catalysts are at the..?

A

heart of the chemical industry

280
Q

What specific catalysts can transition metals act as?

A

Heterogenous and Homogenous catalysts

281
Q

What is a heterogenous catalysts?

A

A catalyst that acts in a different phase (state) from the reactants is called a heterogenous catalyst.

282
Q

Where do these reactions occur?

A

these reactions occur on surfaces

283
Q

What is a homogenous catalyst?

A

a catalyst that acts in the same phase (state) as the reactants is called a homogeneous catalyst

284
Q

How do these reactions occur?

A

These reactions proceed through an intermediate species

285
Q

Give Four examples of a heterogenous

A

The Haber Process
N2 (g) + 3H2 (g) ⇌ 2NH3 (g) (Fe iron being used)

The formation of epoxyethane
C2H4 (g) + 1/2 O2 (g) — Ag(s)—-> + epoxyethane (g)

Hydrogenation reactions
C2H4(g) + H2 —–Ni (s) —> CH3CH3

The contact Process
2SO2 (g) + O2 (g) ⇌ 2SO3 (V2O3)

286
Q

How do Heterogenous catalysts work?

A
  • Heterogenous catalysis occurs on the surface of the solid catalyst
  • when a reactant approached the solid, there is a tendency for them to react with the surface atoms, forming weak temporary bonds
  • the process is called adsorption
287
Q

What is adsorption/ what is the surface adsorption theory?

A

when a reactant approaches the solid, there is a tendency for them to react with the surface atoms, forming weak temporary bonds

288
Q

What are the positions on the catalyst where adsorption takes place are called?

A

Called active sites

289
Q

Surface catalysis occurs in 3 different steps, what are they?

A

1) adsorption of a reactant (or reactants)
2) reaction
3) desorption of a product (or products)

290
Q

why are transition metals good catalyts?

A

transition metals are good catalysts as they have vacant d orbitals that can accept a pair of electrons to form these weak temporary bonds

291
Q

Describe and explain what adsorption can result in?

A

1) if a reactant is adsorbed then it is more likely to collide with another reactant. The collision frequency is then increased

2) a reactant may be held on the surface in a more favourable configuration. - So when an un-adsorbed reactant species collides with a adsorbed species it is more likely to result in a reaction. The activation energy is lowered in this case

3) an adsorbed reactant may undergo an internal bond breaking or rearrangement. - The reactive fragments produced react more easily. The activation energy is lowered in this case

292
Q

Describe how Adsorption could lead to the collision frequency being increased?

A

If a reactant is adsorbed then it is more likely to collide with another reactant. - the collision frequency is then increased

293
Q

Describe how Adsorption can lead to a lowered activation energy?

A

1) a reactant may be held on the surface in a more favourable configuration. - So that when an unadsorbed species reacts with an adsorbed species it is more likely to lead to a reaction. - The activation energy is lowered

2) An adsorbed reactant may undergo internal bond breaking or rearrangement. The reactive fragments produced react more easily. Thus the activation energy is lowered in this case

294
Q

Example - The Haber Process

A

N2 (g) + 3H2(g) —–Fe(s) —> 2NH3 (g)

295
Q

What are the reaction conditions for the Haber process?

A

Fe catalyst, 400°C, 200atm

296
Q

Describe adsorption in the Haber process

A

1) N2 and H2 diffuse to the Fe surface
2) they are adsorbed onto the Fe surface and internal bond breaking occurs
N2 (g) —> 2N (ads) and H2(g) —> 2H (ads)
3) N and H react on the Fe surface forming Ammonia
N (ads) + H (ads) –> NH (ads) –> NH2 (ads)–> NH3 (ads)
4) Ammonia desorbs from the surface
Nh3 (ads) –> NH3 (g)
5) Ammonia diffuses away from the surface releasing the active sites so they can adsorb more N2 and H2 to continue the process

297
Q

What do good catalysts need a balance of?

A

strong adsorption and weak adsorption

298
Q

Why does weak adsorption affect the efficiency of the catalyst?

A

weak adsorption does not encourage reactants to come together (not held on the surface long enough to react) - reaction rates are low

299
Q

Why is silver generally a poor catalyst?

A

silver is generally a poor catalyst because it fails to adsorb reactant molecules strongly enough - the reactants do not experience an enhanced opportunity to react with each other - sometimes such weak adsorption is useful

300
Q

In what reaction is silver used as a catalyst for?

A

for the formation of epoxyethane

301
Q

What does the poor catalytic activity of silver result in?

A

the poor catalytic activity of silver results in a slow release of heat energy and this reduced the chance of any unwanted processes taking place (epoxyethane is explosive)

302
Q

Explain How a strong adsorption affects the rate of reaction?

A
  • strong adsorption keeps molecules immobile and fails to regenerate active sites
  • reaction rates are low if product molecules are not desorbed from the catalyst surface
  • the catalyst surface is effectively poisoned by the product
303
Q

Which metal is a poor catalyst because it adsorbs too strongly?

A

Tungsten is a poor catalyst and is not used commercially because it adsorbs too quicky

304
Q

What is balanced adsorption?

A

A good catalyst achieves a balance between efficient adsorption of reactants and effective desorption of products

305
Q

What are good catalysts?

A

Nickel and Platinum

306
Q

Give the trend on the periodic table of adsorption of transition metals?

A
  • generally metals on the left of the transition series adsorb too strongly (e.g W)
  • And Metals on the right hand side adsorb too weakly
  • Metals in the middle of the transition series balance these features and are excellent catalysts
307
Q

why is it important to increase the surface area of a heterogenous catalyst?

A
  • heterogenous catalysis occurs on solid surfaces
  • it is therefore important to increase the surface area available for the reaction (increase the number of active sites)
308
Q

Why are expensive catalyst spread really thinly?

A

Expensive catalyst are usually spread vey thinly onto an inert support medium in order to increase the surface-to-mass ratio

309
Q

What will a good support medium have?

A

a good support medium will have a large surface area in it’s own right (e.g silica and asbestos)

310
Q

Give a good example this?

A

Catalytic converters

311
Q

Why is a catalytic converter expensive and very efficient?

A

The catalyst is a mixture of platinum, palladium and Rhodium - these metals are very expensive.
These metals are sprayed very thinly onto a ceramic honeycomb support to provide maximum surface area for minimum cost

312
Q

How can the Catalyst be poisoned?

A

The active sites play a key role in heterogenous catalysis and impurities present in the reaction can poison the catalyst by blocking the active sites

313
Q

What does this do to the catalyst?

A

This reduces the efficiency of the catalyst and increases the cost of the process

314
Q

Why do cars with catalytic converters need to avoid?

A

Leaded petrol - catalytic converters are readily poisoned by Anti-Knock additives

315
Q

What is the iron catalyst in the Haber process poisoned by, what is formed??

A

readily poisoned by sulphur compounds and Inactive surface sulphides are formed

316
Q

When do catalyst poisons need to be removed in the Haber process?

A

Catalyst poisons need to be removed from the feedstock gases in the Haber process before they can be passed over the catalyst

317
Q

The iron catalyst is poisoned by Sulphur compounds (found in the natural gas used to make the hydrogen) and what other compounds?

A

carbon monoxide, carbon dioxide and water vapour

318
Q

Example of Heterogenous Catalysis - The Contact Process

A
319
Q

What does the contact process make?

A

Sulphuric acid

320
Q

What is the catalyst?

A

Vanadium(V) oxide - V2O5 (s)

321
Q

what does Vanadium (V) oxide catalyse the reaction between?

A

Conversion of sulphur dioxide to sulphur trioxide

322
Q

How is sulphuric acid made from Sulphur trioxide?

A

Sulphur trioxide is dissolved in water to make sulphuric acid

323
Q

What is the overall equation for the contact process?

A

2SO2 (g) + O2 (g) ⇌ 2SO3 (g)
V2O5

324
Q

What property of vanadium allows the contact process to occur?

A

Vanadium has variable oxidation states

325
Q

Describe how the contact process occur in two steps?

A

1) Sulphur dioxide diffuses onto the surface of the catalyst and a redox reaction occurs. The vanadium oxide oxidises the sulphur dioxide forming sulphur trioxide:
V2O5 (s) + SO2 (g) —> V2O4 (s)+ SO3 (g)

2) The vanadium (IV) oxide formed then reacts with oxygen (and is oxidised) to regenerate Vanadium (V) oxide
2V2O4 (s) + O2 (g)—-> 2V2O5 (s)

326
Q

What is remade at the end of the catalyst?

A

V2O5 catalyst is regenerated and unchanged at the end of the reaction

327
Q

How do we know that the V2O5 acted as a catalyst?/

A

by the use of the two variable oxidation states of vanadium (+5 and +4)

328
Q

What does this catalyst allow the reaction to take?

A

the presence of vanadium oxide enables the reaction to proceed via a different route with a lower activation energy?

329
Q

E.g Making methanol

A
330
Q

What catalyst acts as a heterogenous catalyst in the synthesis of methanol?

A

Chromium (II) oxide

331
Q

Describe what reactants methanol is synthesised from?

A

Carbon monoxide + Hydrogen
CO (s) + 2H2 (g) —-Cr2O3 (s) —-> CH3OH

332
Q

What is the chromium oxide in a mixture with?

A

the chromium oxide is in a mixture with Zinc and or copper oxides

333
Q

What are the reaction conditions for making methanol?

A
  • Cr2O3 catalyst
  • High temperatures
  • High pressure
334
Q

HOMOGENOUS CATALYSIS

A
335
Q

What is Homogenous catalysis?

A

A catalyst that acts in the same phase (state) as the reactants

336
Q

What do these reactions proceed through?

A

These reactions proceed through an intermediate species

337
Q

Many transition metal compounds can act as homogenous catalysis in what type of solution?

A

Aqueous solution

338
Q

What happens during Homogenous Catalysis to the transition metal?

A

Reactions that are speeded up by Homogenous catalysis usually involve the change in oxidation state of the transition metal

339
Q

Because the transition metal has variable oxidation states, the reaction can..?

A

Proceed through an intermediate species

340
Q

Oxidation of Iodide ions by peroxodisulphate ions

A
341
Q

How are iodide ions oxidised into iodine?
Show an equation to show this?

A

iodide ions are oxidised to iodine by Peroxodisulphate ions
S2O82- + 2I- —> 2SO42- + I2

342
Q

why is this reaction very slow without a catalyst?

A

the reaction is very slow without a catalyst because because both reactant particles are negatively charged and effective collisions are unlikely as they repel each other

343
Q

what speeds up this reaction?

A

the reaction is quick is an iron (II) catalyst is added

344
Q

Why does adding Fe2+ ions speed up the reaction?
Write an equation to show this?

A

positively charged Fe2+ ions make effective collisions with negative S2O82- ions:
2Fe2+ + S2O82- ——> 2Fe3+ + 2SO42-

345
Q

What is formed in the reaction above that helps to make I2 (iodine)?
Draw an equation to show this?

A

the positively charged Fe3+ ions formed will now readily oxidise the negatively charged I- ions to I2:
2Fe3+ + 2I- —-> 2Fe2+ + I2

346
Q

What is the overall equation obtained?

A

S2O82- + 2I- —> 2SO42- + I2

347
Q

This reaction can occur both ways?

A
  • the 2 steps can occur in either order
  • this means that iron (III) ions can be added instead of Iron (II) ions to catalyse the reaction
  • Iron (II) and Iron (III) are constantly recycled by the 2 reactions
348
Q

What property of transition metals allows this reaction to proceed by an alternate route with a lower activation energy?

A

The variable oxidation state of Fe/ iron enables the reaction to proceed through an alternative route with a lower activation energy

349
Q

Autocatalysis

A
350
Q

What is autocatalysis?

A

Is where a product of a particular reaction catalyses the reaction

351
Q

What is an example of Autocatalysis?

A

Th reaction between ethanedioate ions (C2O42-) and manganate (VII) ions

352
Q

How could this reaction be carried out?

A

this reaction could be carried out as a titration where potassium manganate (VII) from a burette is run into hot, acidified solution containing ethanedioate ions.

353
Q

What is the overall equation for this reaction?

A

2MnO4- + 16H+ + 5C2O4- —> 2Mn2+ + 10CO2 + 8H2O

354
Q

How is the overall equation achieved?

A

combining the two equations:
MnO4- + 8H+ + 5e- —> Mn2+ + 4H2O and
C2O42- —-> 2CO2 + 2e-

355
Q

why is the reaction between ethanedioate ions and manganate (VII) ions slow, and why are effective collisions unlikely?

A

The reaction between MnO4- ions and C2O4- ions is slow because both ions are negatively charged and effective collisions are unlikely as they repel each other

356
Q

What is the Autocatalyst that catalyses this reaction?

A

Mn2+ ions (one of the products)

357
Q

Describe how Mn2+ ions act as an Autocatalyst in this reaction?
Write out an equation to show this reaction happening?

A

Once some Mn2+ have been formed in the early stages of the reaction, they can readily react with some MnO4- to make M3+ ions:
4Mn2+ + MnO4- + 8H+ —-> 5Mn3+ + 4H2O

358
Q

What is regenerated in this reaction?
Draw an equation to show the regeneration of the autocatalyst

A
  • the Mn3+ ions can then react with C2O4- to regenerate Mn2+ (and make CO2):
    2Mn3+ + C2O4- —> 2CO2 + 2Mn2+
359
Q

Why is the initial rate of reaction slow, but as the reaction proceeds starts to rapidly increase?

A

the initial rate of reaction is slow because both of the reactants are negatively charged, but as the concentration of Mn2+ increases, the rate of reaction rapidly increases

360
Q

Describe why in titration, the purple colour of potassium manganate (added from a burette) to a solution of hot acidified ethanedioate is not decolourised straight away at the start of the titration?

A
  • the reaction is very slow
  • Once some Mn2+ ions have been formed, further addition of potassium manganate (VII) leads to fast decolourisation
  • The rapid decolourisation continues until end-point is reached - the first permanent pink tinge
361
Q

What is the overall equation obtained for this reaction?

A

combining the two equations:
MnO4- + 8H+ + 5e- —> Mn2+ + 4H2O and
C2O42- —-> 2CO2 + 2e-

362
Q

Determination of the concentration of a solution of ethandioate ions

A
363
Q

How can the concentration of ethanedioate ions be found?

A
  • we can find out the concentration of ethanedioate ions by titration with potassium manganate (VII)
364
Q

A 25.0cm3 acidified solution of ethanedioate ions was titrated with 0.0160 moldm-3 solution of KMnO4, exactly 32.5cm3 of KMnO4 solution was required to reach the end-point. Calculate the concentration of ethanedioate ions present in the solution.

A

0.052 moldm-3

365
Q

DONE!!!

A

Chemistry Y13 (13 decks)

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