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AQA A Level Chemistry > 2.4 Transition Metals > Flashcards

2.4 Transition Metals Flashcards

1
Q

Define a transition metal

A

An element which forms at least one stable ion with an incomplete d subshell.

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2
Q

Where are transition metals located in the periodic table ?

A

In the d subshell (between g2 and g3)

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3
Q

What are some physical characteristics of transition metals ?

A

•Good conductors of heat and electricity
•High mp and bp
•Shiny
•Low reactivity

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4
Q

What are the characteristic chemical properties of transition metals ?

A

•Variable oxidation states
•Form coloured compounds in solutions
•Good catalysts
•Form complex ions.

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5
Q

Define the term ‘complex ion’

A

A transition metal is central in a molecule surrounded by ligands (other molecules/ions) that are co-ordinate bonded to it.

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6
Q

Give three examples of transition metals catalysts and the reactions

A

Iron - Haber process
Vanadium oxide (V2O5) - Contact process
MnO2 - Decomposition of H2O2.

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7
Q

From what subshell do transition metals lose electrons first from when forming ions ?

A

4s.

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8
Q

Define the term ligand

A

•An ion or molecule with at least one lone pair of electrons
•Donates them to a transition metal to form a co-ordinate bond and thus a complex ion.

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9
Q

Define the term mono/unidentate ligand

A

A ligand that forms one co-ordinate bond to the central transition metal in a complex (one lone pair to donate).

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10
Q

Define the term bidentate ligand

A

A ligand that forms two co-ordinate bonds with a central transition metal in a complex ion (two lone pairs to donate).

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11
Q

Define the term multidentate ligand

A

A ligand that forms three or more co-ordinate bonds with a central TM ion in a complex ion.

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12
Q

Give some examples of common monodentate ligands

A

•Cl-
•H2O
•NH3
•CN-

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13
Q

What is the formula for an ethanedioate ion ? How many co-ordinate bonds can it form in a complex ?

A

•C2O4 2-
•Forms two co-ordinate bonds, bidentate

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14
Q

—

A

—

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15
Q

How many co-ordinate does ETDA 4+ form ?

A

6.

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16
Q

Define the term co-ordination number

A

The number of co-ordinate bonds a transition metal has made with surrounding ligands in a complex ion.

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17
Q

What is the Chelate effect ?

A

Chelate complexes with multidentate ligands are favoured over monodentate ligands or ligands that form fewer co-ordinate bonds per molecule.

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18
Q

Explain the chelate effect in terms of entropy and the reaction that is occurring

A

•The number of molecules increases when multidentate ligands displace ligands that form fewer co-ordinate bonds per molecule
•This gives a significant increase in entropy
•The Gibbs free energy change is <0 which makes it a feasible reaction
•Therefore a more stable complex ion is formed.

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19
Q

What ion is typically formed when a transition metal dissolves in water ?

A

•An aqua ion
•6 H2O ligands form around the central ion
•Octahedral complex ion shape.

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20
Q

If a transition metal has two ligands in a complex ion what is it’s shape ?

A

Linear.

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21
Q

If a transition metal has 4 ligands in a complex ion what is its shape ?

A

Tetrahedral

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22
Q

Name an exception to the rule that a complex ion with 4 ligands has a tetrahedral shape

A

Platin (platinum 2+) forms square planar molecules.

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23
Q

What shape is a complex ion if it has 6 ligands ?

A

Octahedral

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24
Q

How can complex ions display cis-trans isomerism ? What shapes of ions does this apply to ?

A

•Ligands can differ in the way they are arranged in space
•When two of the same ligand are on opposing sides of the central ion the complex is a cis isomer
•When two of the same ligand are on the same side of the central ion the complex is a trans isomer
•This applies to square planar and octahedral complex ions.

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25
Q

What conditions are needed for a complex ion to display optical isomerism ?

A

•Typically applies to octahedral molecules with two or more bidentate ligands
•This means the mirror images will not superimpose.

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26
Q

What happens to Co2+, Cu2+ and Fe3+ ‘s co-ordinate numbers when Cl- ligands replace NH3 or H2O ligands ?

A

It decreases from 6 to 4 as Cl- is a much larger ligand than H2O or NH3.

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27
Q

What is haem - it’s metal ion, co-ordination number and ligands ?

A

•A molecule which makes up proteins
•It has an Fe2+ central ion
•It has a coordination number of 6
•4 of these bonds form to porforin, one forms to nitrogen and one forms to O2.

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28
Q

How does haemoglobin transport oxygen ?

A

•O2 forms a weak co-ordinate bond with the Fe2+ ion
•It is then transported around the body
•The bond breaks when the haemoglobin reaches the cell and O2 is released.

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29
Q

Why is CO monoxide toxic ?

A

•CO coordinately bonds to the Fe2+ ion of Haemoglobin and is a favoured ligand
•This means it bonds more strongly than just O2 and therefore stops O2 from being transported around the body.

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30
Q

Why are transition metal compounds coloured ?

A

•TMs have partially filled d subshells
•In compounds (when ligands have bonded to the ion) the d subshell splits into two energy levels
•Electrons can gain a precise amount of energy from a photon, become excited and move to a high energy level
•The energy of the photon = difference between levels / gap in energy
•The energy of the photon is released to the frequency of the light
•The photon which is absorbed is missing from the light which is then reflected, we see the combination of all remaining frequencies.

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31
Q

How do you calculate ΔE from f and / or gamma?

A

ΔE=hf=hc/gamma

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32
Q

What effects the colour of a transition metal compound ?

A

ΔE affects the frequency of the photons which are absorbed and thus the colour
ΔE is changed by oxidation state of the TM, number / type of ligands, shape and co-ordination number.

33
Q

State the colour for each of the following vanadium species: VO2 , VO 2+, V 3+, V2+

A

VO2 + : Yellow
VO 2+ : Blue
V 3+ : Green
V 2+ : Violet

34
Q

What can you use to reduce vanadium ?

A

Zinc

35
Q

What colour is aqueous Fe 2+ ?

A

Green

36
Q

What colour is aqueous Fe 3+ ?

A

Pale brown

37
Q

What colour is aqueous Cr 2+ ?

A

Blue

38
Q

What colour is aqueous Cr 3+ ?

A

Red/violet

39
Q

What colour is aqueous Co 2+ ?

A

Brown

40
Q

What colour is aqueous Co 3+ ?

A

Yellow

41
Q

What does a colorimeter do ?

A

Measures the percentage absorbance of a particular wavelength of light of a solution.

42
Q

How would you use a colorimeter experimentally ?

A

Use solutions of known concentrations to form a calibration graph; find concentration of unknown solution.

43
Q

What information can a colorimeter give you ?

A

The concentration of a certain ion in a solution.

44
Q

Why can transition metals have variable oxidation states ?

A

They have partially filled d subshells and so can use 4s and 3d electrons.

45
Q

What is the use of the [Ag(NH3)2]+ complex ion ?

A

It is the Tollens Reagent and is used to test for aldehydes and ketones. Positive result is a silver mirror forming with aldehyde, no change for ketone.

46
Q

What colour is MnO4 - ?

A

Deep purple

47
Q

What colour is Mn 2+ ?

A

Pink

48
Q

What is the equation for the reduction of MnO4 - into Mn 2+ ?

A

MnO4 - + 8H+ + 5e- -> Mn 2+ + 4H2O

49
Q

Why are redox titrations involving transition metals said to be self indicating ?

A

The reaction will usually involve a colour change as the metal is changing oxidation state, sometimes an indicator is still needed.

50
Q

What colour is Cr2O7 ?

A

Orange

51
Q

What colour is Cr3+ ?

A

Green

52
Q

Write a half equation for the reduction of Cr2O7 - into Cr 3+

A

Cr2O7- + 14H+ + 6e- -> 2Cr 3 + 7H2O

53
Q

What happens to aqua metal ions in acidic conditions ?

A

Presence of H+ means they get reduced

54
Q

What happens to aqua metal ions in alkaline conditions ?

A

They get oxidised

55
Q

What does the readiness of a species being oxidised/reduced depend on ?

A

E^θ values

56
Q

What can change the E^θ of a complex ?

A

pH, ligands involved

57
Q

What is a catalyst ?

A

A substance that decreases the activation energy of a compound without being directly used itself

58
Q

How do catalysts usually work ?

A

They provide an alternative pathway for the reaction to occur

59
Q

Why are transition metals good catalysts ?

A

They can exist in variable oxidation states so can provide alternate pathways easily

60
Q

Why are group 1, 2 and 3 metals not great catalysts ?

A

They can only exist in one oxidation state

61
Q

What are the advantages of using a catalyst for a reaction ?

A

It allows the reaction to be completed at lower temperatures, saving energy / recourses

62
Q

What metals are used in a catalytic converter ?

A

Platinum, Rhodium, Palladium

63
Q

What reaction does Platinum catalyse in a catalytic converter ?

A

2CO + 2NO -> 2CO2 + N2
CO + 1/2O2 -> CO2

64
Q

Define a heterogenous catalyst

A

•A catalyst that is present in the reaction in a different phase to the reactants, typically a solid
•Catalytic activity takes place on the catalysts surface as reactants pass over it

65
Q

What is the advantage of using a heterogenous catalyst ?

A

You don’t need to separate the products from the catalyst

66
Q

How do heterogeneous catalysts work ?

A

•Reactants adsorb to the catalysts surface at active sites
•This weakens the bonds of the reactant and also holds it in the correct orientation to react
•Once the reaction has occurred the product desorbs from the active site.

67
Q

What properties does a catalyst need to have to make it an efficient catalyst ?

A

•Reactants can’t adsorb too strongly as they won’t desorb
•Reactants can’t adsorb too weakly as reactant would not be held long enough and bonds wouldn’t be sufficiently weakened.

68
Q

How can you increase the efficiency of a heterogenous catalyst ?

A

•Increase surface area (more active sites)
•Spread onto an inert medium in a honeycomb structure.

69
Q

What is catalyst poisoning ?

A

When unwanted impurities adsorb to the catalysts active sites and do not desorb, this blocks the active sites of the catalysts surface.

70
Q

What effect does catalyst poisoning have on the efficiency of the catalyst ?

A

It decreases efficiency as poisoning increases.

71
Q

How else can a catalyst be degraded apart from poisoning ?

A

Finely spread out catalysts can gradually be lost from the support surface

72
Q

What is the Haber process and what catalyst is used ?

A

N2 (g) + 3H2 (g) -> 2NH3 (g)
Fe catalyst

73
Q

What is the contact process and what is the catalyst ?

A

• 2SO2 (g) + O2 (g) -> 2SO3 (g)
• Catalysed by V2O5

74
Q

What are the two reactions involved in the contact process ?

A

SO2 + V2O5 -> SO3 + V2O4
V2O4 + 1/2 O2 -> V2O5

75
Q

Why is V2O5 a good catalyst in the case of the contact process ?

A

Vanadium can change from +5 to +4 oxidation state

76
Q

Define the term homogenous catalyst

A

A catalyst that is in the same phase as the reactants

77
Q

How do homogenous catalysts work ?

A

They form intermediates to give a different reaction pathway with a different Ea.

78
Q

What reaction occurs between S2O8 2- + 2I- ?

A

S2O8 2- + 2I- -> 2SO4 2- + I2 (all aq)

79
Q

Why does the reaction between S2O8 2- + 2I- have such a high Ea ?

A

As the negative ions repel

AQA A Level Chemistry (26 decks)

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