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Transition metals Flashcards

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1
Q

What is the formal definition of a transition element

A

A transition element forms at least one stable ion with a part-full d shell of electrons.

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2
Q

List the four main chemical properties of all transition metals

A
  • Variable oxidation states: Transition metals have more than one oxidation state in their compounds eg. Cu(I) and Cu(II)
  • Colour: The majority of transition metal ions are coloured
  • Catalysis: Catalysts affect the rate of reaction without being used up or chemically changed themselves. Many transition metals and their compounds show catalytic activity.
  • Complex formation: Transition metals form complex ions. A complex ion is formed when a transition metal ion is surrounded by ions or other molecules, called ligands, which are bonded to it by coordinate bonds.
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3
Q

Why are some key example of transition metal catalysts and what they catalyse

A
  • Iron is the catalyst in the haber process
  • Vanadium (V) oxide is the catalyst for the contact process.
  • Manganese (IV) oxide is the catalyst in the decomposition of hydrogen peroxide.
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4
Q

What is a ligand

A

An ion or molecule with a lone pair of electrons that forms a co-ordinate bond with a transition metal

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5
Q

What are some key examples of ligands

A

H2O
NH3
Cl-
CN-

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6
Q

What is the coordination number

A

The number of coordinate bonds to ligands that surround the d-block metal ion

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7
Q

What shape are ions with a co-ordination number of six normally

A

Octahedral or square planar

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8
Q

What shape are ions with a co-ordination number of four normally

A

Tetrahedral

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9
Q

What are aqua ions

A
  • The species that forms when the salt of a transition metal is dissolved in water.
  • The positively charged metal ion becomes surrounded by water molecules acting as ligands.
  • Normally there are six water molecules in an octahedral arrangement
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10
Q

What are multidentate ligands

A

Molecules or ions which have more than one atom with a lone pair of electrons which can bond to a transition metal ion

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11
Q

List the three bidentate ligands you need to know

A

1) Ethane-1,2-diamine
2) Ethanedioate (oxalate) ion
3) Benzene-1,2-diol

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12
Q

How does a ethane-1,2-diamine ligand bind to a transition metal

A
  • Each nitrogen atom has a lone pair which can form a coordinate bond to the metal ion.
  • The name of this ligand is often abbreviated to en
  • It is a neutral ligand
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13
Q

What is the formula of the ethanedioate (oxalate) ion

A

C2O4 2-

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14
Q

What is the multidentate ligand you may be asked about in the exam

A

EDTA 4-

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15
Q

How does EDTA 4- bind to transition metals

A

It acts as a hexadentate ligand using lone pairs on four oxygen and both nitrogen atoms

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16
Q

What are chelates

A

Complex ions with polydentate ligands

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17
Q

What can chelates be used for

A

To remove d-block metal ions from solution

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18
Q

What is the chelate effect and explain it

A
  • Chelate effect= chelate complexes with multidentate ligands are favoured over monodentate ligands.
  • This is because when chelates replace water/other monodentate ligands, the number of particles on the product side of the equation increases.
  • This means that there is an increase in entropy which drives the reaction to the right.
  • Eg. If EDTA 4- replaces six water ligands around a transition metal, there are 6 product molecules as opposed to two reactant molecules.
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19
Q

What type of isomers do transition metal complexes form

A

Geometrical isomers (E/Z isomers) and optical isomers (non-superimposable mirror images of each other).

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20
Q

In what shape of transition metal complexes does geometrical isomerism occur

A

In octahedral and square planar complexes

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21
Q

When do transition metal complexes form optical isomers

A

When there are two or more bidentate ligands in a complex

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22
Q

Why are transition metal complexes coloured

A
  • Transition metal compounds are coloured because they have part filled d-orbitals
  • It is therefore possible for electrons to move from one d-orbital to another.
  • In an isolated transition metal atom, all the d-orbitals are of exactly the same energy, but in a compound, the presence of other atoms nearby makes the d orbitals have slightly different energies.
  • When electrons move from one d-orbital to another of a higher energy level (called an excited state), they often absorb energy in the visible region of the spectrum equal to the difference in energy between the levels.
  • This colour is therefore missing from the spectrum and you see the combination of colours that are not absorbed.
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23
Q

What is the equation which links the frequency of light absorbed by the transition metal to the d-orbital energy difference

A
  • /\E = hV where:
  • E is the energy
  • V is the frequency
  • h is Plancks constant
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24
Q

What colour are Fe 2+ complexes

A

Green

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25
Q

What colour are Fe 3+ complexes

A

Pale brown

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26
Q

What colour are Cr 2+ complexes

A

Blue

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27
Q

What colour are Cr 3+ complexes

A

Red-violet

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28
Q

What colour are Co 2+ complexes

A

Brown

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29
Q

What colour are Co 3+ complexes

A

Yellow

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30
Q

How do you measure the concentration of solutions of coloured transition metal compounds

A
  • A colorimeter is used to detect the amount of light that passes through the solution.
  • The more concentrated a solution, the less light passes through the solution.
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31
Q

What is the equation of a managante ion

A

MnO4-

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32
Q

When does potassium manganate (VII) act as an oxidising agent

A

In acidic solution

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33
Q

Describe what happens when titrating potassium manganate and a compound containing Fe 2+ ions

A
  • Using a burette, gradually add potassium manganate (VII) solution to a solution containing Fe 2+ ions, acidified with dilute sulfuric acid.
  • The purple colour disappears as the MnO 4- ions are converted to pale pink Mn 2+ ions to leave a virtually colourless solution.
  • Once just enough MnO 4- ions have been added to react with all the Fe 2+ ions, one more drop of MnO 4- ions will turn the solution purple.
  • This is the end point of the titration.
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34
Q

In what ratio do Fe 2+ ions react with manganate ions in a redox titration

A

5:1

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35
Q

What colour is the Fe 2+ ion in solution

A

Pale green

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36
Q

What colour are MnO 4- ions in solution

A

Intense purple

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37
Q

What colour are Fe 3+ ions in solution

A

Pale violet

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38
Q

What colour are Mn 2+ ions in solution

A

Pale pink

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39
Q

Why can’t you use hydrochloric acid (as an alternative to sulfuric acid) to supply the H+ ions in the reaction between potassium manganate and Fe 2+ (aq)

A
  • Using electrode potential and EMF values, you can see that the reaction where MnO 4- ions oxidise the Cl- ions in hydrochloric acid is feasible and so occurs.
  • This would affect the titration because the manganate ions must only be oxidising the Fe 2+ ions.
  • This is not an issue with sulfuric acid because the manganate ions do not oxidise sulfate ions.
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40
Q

What is the typical species which represents how a transition metal exists in an acidic solution

A

M(H2O)6 2+

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41
Q

What is the typical species which represents how a transition metal exists in a neutral solution

A

M(H2O)4(OH)2

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42
Q

What is the typical species which represents how a transition metal exists in an alkaline solution

A

M(H2O)2(OH)4 2-

43
Q

What are the two types of catalyst

A

Homogeneous catalyst and heterogeneous catalyst

44
Q

Describe what a heterogeneous catalyst is and how it works

A
  • A heterogeneous catalyst is a catalyst which is in a different phase (solid/liquid/gas) than the reactants.
  • They are usually present as solids, whilst the reactants may be gases or liquids.
  • Their catalytic action occurs on the solid surface.
  • The reactants pass over the catalyst surface, which remains in place so that the catalyst is not lost and does not need to be separated from the products.
45
Q

What are two ways in which you can make heterogeneous catalysts more effecient

A
  • Increase their surface area
  • Spread the catalyst onto sun inert support medium , or even impreganate it into one.
  • This increases the surface-to-mass ratio so that a little goes a long way; the more expensive catalysts are often used in this way.
46
Q

What is catalytic poisoning (heterogeneous catalysts) and what is another reason why a hetergeneous catalyst may not last forever

A
  • When the surface of the catalyst becomes covers with unwanted impurities that block its action.
  • Reason 2: the finely divided catalyst may gradually be lost from the support medium.
47
Q

Give the equation for the Haber process

A

N2 (g) + 3H2 (g) <=> 2NH3 (g)

48
Q

Describe the catalyst used in the Haber process

A
  • The catalyst used is iron which is present as pea-sized lumps to increase its surface area.
  • The iron catalyst lasts about five years before it becomes poisoned by impurities in the gas stream such as sulfur compounds and has to be replaced.
49
Q

What is the equation for the contact process

A

2SO2 + O2 <=> 2SO3

50
Q

Describe the catalyst used in the contact process

A
  • The contact process in catalysed by vanadium (V) oxide V2O5 which catalyses in in two steps:
  • First the vanadium (V) oxide oxidises sulphur dioxide to sulfur trioxide and is itself reduced to vanadium (IV) oxide V2O4.
  • The vanadium (IV) oxide is then oxidised back to vanadium (V) oxide by oxygen.
  • The Vanadium (V) oxide is regenerated unchanged.
  • Each of the two steps has a lower activation energy than the uncatalysed single step and therefore the reaction goes faster.
51
Q

Give the two equations that show how vanadium (V) oxide V2O5 catalyses the contact process

A

SO2 + V2O5 —> SO3 +V2O4
2V2O4 + O2 —> 2V2O5

52
Q

What is a homogeneous catalyst

A

A catalyst that is in the same phase as the reactant

53
Q

What is an example of a homogeneous transition metal catalyst

A

Peroxodisulfate ions oxidise iodide ions to iodine and this reaction is catalysed by Fe 2+ ions

54
Q

Describe how Fe 2+ ions ctalayse the oxidation of iodide ions to iodine by peroxodisulfate ions and why it is useful

A
  • The catalysed reaction takes place in two steps
  • First the peroxodisulfate ions oxidise iron (II) to iron (III).
  • The Fe 3+ then oxidises the I- to I2, regenerating the Fe 2+ ions so that none are used up in the reaction.
  • So iron first gives an electron to the peroxodisulfate and later takes on back from the iodide ions.
  • This is useful because the uncatalysed reaction takes place between two ions of the same charge which repel, therefore giving as high activation energy.
  • Both steps of the catalysed reaction involve reactions between oppositely charged ions which explains the increase in rate of reaction.
55
Q

Give the two equations which show how Fe 2+ ions catalyse the oxidation of iodide ions to iodine by peroxodisulfate ions

A
  • S2O8 2- (aq) + 2 Fe 2+ (aq) —> 2SO4 2- (aq) + 2Fe 3+ (aq)
  • 2Fe 3+ (aq) + 2I- (aq) —> 2Fe 2+ (aq)+ I2 (aq)
56
Q

What is autocatalysis

A
  • Autocatalysis is when one of the products of the reaction is a catalyst for the reaction.
  • These reaction start slowly at an uncatalysed rate- as the concentration of the product that is also the catalyst builds up, the reaction speeds up to the catalysed rate.
  • From then on it behaves like a normal reaction.
57
Q

What is an example of an auto catalysed reaction

A

The reaction between potassium manganate and ethanedioic acid

58
Q

What happens when the salt of a transition metal is dissolved in water

A
  • The water molecules cluster around the metal ions so it exists as a complex ion.
  • Six water molecules act as ligands bonding to the metal ion in an octahedral arrangement.
  • These ions are called aqua ions.
59
Q

Why are solutions of Fe 2+ not noticeably acidic whereas a solution of Fe 3+ is a stronger acid than ethanoic acid

A
  • The Fe 3+ ion is both smaller and more highly charged than Fe 2+ (it has a higher charge density) making it more strongly polarising.
  • So in the [Fe(H2O)6]3+ (aq) ion, the iron strongly attracts electrons from the oxygen atoms of the water ligands, so weakening the O—H bonds in the water molecules.
  • This complex ion will then readily release an H+ ion making the solution acidic.
  • Fe 2+ is less polarising so fewer O—H bonds break in solution.
60
Q

What is the general rule relating to the acidity of the aqua ions of transition metals

A

The aqua ions of M3+ are significantly more acidic than those of M2+

61
Q

What was Lavosiers (1777) theory of acidity

A

He proposed that all acids contain oxygen

62
Q

What was Davy’s theory of acidity (1816)

A

All acids contain hydrogen

63
Q

What was Liebig’s theory of acidity

A

Leibig (1838) defined acids as substances containing hydrogen which could be replaced by a metal.

64
Q

Why was Liebigs theory an improvement on Davy’s

A
  • It was an improvement on Davy’s theory as it explains why not all hydrogen-containing compounds are acidic eg.ammonia.
  • There must be something special about that hydrogen that makes it replaceable by a metal.
65
Q

What was Arrhenius’ (1887) theory of acidity

A

Arrhenius thought of acids as producing hydrogen ions, H+

66
Q

What is the theory of acidity we use today

A

Brønsted-Lowry

67
Q

What is the Lewis theory of acidity that is used today (as well as the Brønsted-Lowry)

A

Lewis theory regards acids as electron pair acceptors and bases as electron pair donors in the formation of coordinate covalent bonds.

68
Q

What is the general rule for the carbonates of transition metal ions

A

In general, carbonates of transition metal ions in oxidation state +2 exist, whilst those of ions in the 3+ state do not.

69
Q

What colour is the solution containing [Fe(H2O)6]3+ (aq)

A

Pale brown

70
Q

What colour is the solution containing [Fe(H2O)6]2+

A

Pale green

71
Q

What test allows you do distinguish between dilute solutions containing [Fe(H2O)6]3+ and [Fe(H2O)6]2+ where it is hard to tell the difference

A
  • Add dilute alkali, which precipitates the hydroxides whose colours are obviously different:
  • Iron II hydroxide is green
  • Iron III hydroxide is brown
72
Q

What does it mean if something is amphoteric

A

It shows both acidic and basic properties

73
Q

What is an example of an amphoteric compound

A

Ammonium hydroxide

74
Q

What are the possibilities when replacing water as a ligand in ligand substitution reactions

A
  • The water molecules may be replaced by neutral ligands such as ammonia.
  • The water molecules may be replaced by negatively charged ligands, such as chloride ions.
  • The water molecules may be replaced by bi- or multidentate ligands- this is called chelation.
  • Replacement of water ligands may be complete or partial.
75
Q

Why is there no change in co-ordination number or charge when water ligands are replaced by ammonia ligands

A

Both ligands are uncharged and of similiar size

76
Q

What is the complication that’s occurs when M2+ aqua ions undergo ligand substitution reactions with ammonia

A

Because ammonia is a base as well as a ligand, and therefore contains OH- ions, a precipitate may form and redissolve.

77
Q

Describe what happens when an aqua ion of cobalt 2+ undergoes ligand substitution with ammonia

A
  • The first step is the formation of a blue hydrated cobalt (II) hydroxide when ammonia is added.
  • This is produced by the loss of a proton from each of two of the six water molecules co-ordinated to the Co 2+ ion.
  • Here ammonia is acting as a base.
  • If you add more of the concentrated ammonia, then both OH- ions and all the four water ligands are replaced by ammonia.
  • This causes the blue precipitate to dissolve to form a pale yellow solution.
78
Q

Why are all four ammonia ligands and both hydroxide ligands replaced by ammonia when more concentrated ammonia is added to Co 2+ aqua ion after two molecules of ammonia have already been added

A
  • Ammonia is a better ligand than water
  • The high concentration of ammonia displaces equilibria to the right.
79
Q

What is the overall equation for the replacement of a cobalt 2+ aqua ion with ammonia

A

Co(H2O)4(OH)2 + 6NH3 (aq) <=> [Co(NH3)6]2+ (aq) + 4H2O (l) + 2OH- (aq)

80
Q

Describe what happens when a copper 2+ aqua ion undergoes ligand substitution with ammonia

A
  • The ligand replacement is partial- only four of the water ligands are replaced.
  • The ammonia first acts as a base, removing protons from two of the water molecules in [Cu(H2O)6]2+ to form [Cu(OH)2(H2O)4] (s).
  • The first thing seen is the pale blue precipitate of copper hydroxide.
  • When more of the concentrated ammonia is added, the precipitate dissolves to form a deep blue solution containing [Cu(NH3)4(H2O)2]2+
  • The ammonia has replaces both of the OH- ligands and two of the H2O ligands.
81
Q

What is the overall reaction that occurs when a copper 2+ aqua ion undergoes ligand substitution with ammonia

A

[Cu(H2O)6]2+ + 4NH3 <=> [Cu(NH3)4(H2O)2]2+ + 4H2O

82
Q

Why is the shape of the octahedron of the [Cu(NH3)4(H2O)2]2+ ion slightly distorted

A

Because the Cu—O bonds are longer and therefore weaker than the Cu—N bonds.

83
Q

Describe what happens when cooper 2+ ions undergo ligand substitution with chloride ions

A
  • When aqueous copper ions react with concentrated hydrochloric acid there is a change in both charge and co-ordination number.
  • Concentrated HCl provides a high concentration of Cl- ligands
  • The pale blue colour of the [Cu(H2O)6]2+ ion is replaced by the yellow [CuCl4]2- ion. (Although the solution may look green as some [Cu(H2O)6]2+ will remain).
  • This replacement takes place in steps.
84
Q

Why is [Cu(H2O)6]2+ six co-ordinate but [CuCl4-]2- four co-ordinate

A

Cl- is larger than H2O and fewer ligands can physically fit around the central copper ion.

85
Q

What forms when carbonate ions are added to M2+ ions

A

Precipitates of the metal carbonates

86
Q

What forms when carbonate ions are added to M3+ ions and why

A

Bubbles of carbon dioxide- this is a reflection of the greater acidity of [M(H2O)6]3+ compared with [M(H2O)6]2+

87
Q

What transition metal complexes have a linear shape

A

Ag+ complexes

88
Q

What shape are all platinum complexes

A

Square planar

89
Q

What is the bond angle in a square planar molecule

A

90 degrees

90
Q

What is the bond angle in an octahedral molecule

A

90 degrees

91
Q

What are the factors that affect the colour of a transition metal complex

A

1) Identity of metal
2) Oxidation state of metal
3) Identity of ligands
4) Co-ordination number

92
Q

Why does a change in one of the factors that affects the colour of a transition metal complex cause said change

A

If any of the factors are changed, then the size of the energy gap between the higher and lower d orbitals changes, and so the frequency of light absorbed changes and so the colour seen changes.

93
Q

What is the relationship between pH and redox reactions of transition metals

A

In general, it is easier to:
- Oxidise a transition metal in alkaline conditions
- Reduce a transition metal in acidic conditions.

94
Q

What type of catalyst is Vanadium V oxide that catalyses the contact process

A

Heterogeneous

95
Q

Explain why dilute sulfuric acid is a suitable acid for acidifying manganate ions for a REDOX titration

A
  • Hydrochloric acid cannot be used as the MnO4- would also oxidise Cl- to Cl2 so affect the volume of KMnO4 required in the titration.
  • Concentrated sulfuric acid or concentrated nitric acid cannot be used as they are oxidising agents themselves so affect the volume of KMnO4 required in the titration.
  • Ethanoic acid cannot be used as it is a weak acid so would not provide enough H+ ions.
96
Q

Titrations containing manganate ions are autocatalysed- what species acts as the catalyst

A

Mn 2+

97
Q

In a redox titration with potassium manganate, what is the setup of the apparatus

A
  • The purple potassium manganate is in the burette.
  • The sample being analysed is in a flask with dilute sulfuric acid.
98
Q

If the iron is not in the +2 oxidation state before a redox titration with manganate, what is done

A
  • if it is the element Fe (0) then it is reacted with sulfuric acid to oxidised it to Fe 2+ for analysis.
  • If it is Fe 3+, then it is reacted with zinc to reduce it to Fe 2+ ready for analysis (the remaining Zn must be removed first to stop it reducing Fe 3+ formed in the titration back to Fe 2+ ).
99
Q

What is the reacting ratio between ethandioate ions and manganate ions in a redox titration

A

2.5 : 1

100
Q

What is the bond angle for tetrahedral complexes

A

109.5

101
Q

Describe what happens, in terms of transition metals, when Tollens reagent is used to test for an aldehyde

A
  • Tollens reagent contains [Ag(NH3)2]+
  • A silver mirror is formed in the presence of aldehydes
  • Ag + in [Ag(NH3)2]+ is reduced to Ag (0) in silver mirror.
  • The aldehyde is oxidised to a carboxylic acid.
102
Q

Describe what happens, in terms of transition metals, when Fehlings reagent is used to test for an aldehyde

A
  • Fehlings solution contains Cu 2+
  • A brick-red precipitate of (Cu2O) is formed in the presence of Aldehydes.
  • Cu 2+ is reduced to Cu + in Cu2O
  • Aldehyde is oxidised to a carboxylic acid.
103
Q

Describe what happens, in terms of transition metals, when acidified potassium dichromate is used to test for primary and secondary alcohols and aldehydes

A
  • The formula for acidified potassium dichromate is H+/K2Cr2O7
  • Colour change form orange (Cr2O7)2- to green Cr3+
  • Cr 6+ in (Cr2O7)2- is reduced to Cr 3+
  • The alcohol or aldehyde is oxidised
104
Q

What is the colour change in a redox titration using potassium manganate in the burette

A

Colourless to purple

Inorganic Chemistry (5 decks)

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