Edexcel IAL Chemistry A2 Topic 17 Transition Metals

[Maximum number: 3]

This question is about mercury, Hg, and its compounds.
Mercury is a liquid element in the same group of the Periodic Table as zinc.
The electronic configuration of mercury is $[\mathrm{Xe}] 4 \mathrm{f}^{14} 5 \mathrm{~d}^{10} 6 \mathrm{~s}^{2}$ .

Mercury forms compounds in either the +1 or +2 oxidation states.

Explain why mercury is classified as a d-block element but is not a transition element.
(3)

An explanation that makes reference to the following points:

- $\left(\mathrm{Hg}^{+}\right.$ is $\left.[\mathrm{Xe}] 4 \mathrm{f}^{14}\right) 5 \mathrm{~d}^{10} 6 \mathrm{~s}^{1}$

and $\left(\mathrm{Hg}^{2+}\right.$ is $\left.[\mathrm{Xe}] 4 \mathrm{f}^{14}\right) 5 \mathrm{~d}^{10}\left(6 \mathrm{~s}^{0}\right)$

(1)

- (d-block element as last) electron goes into a (5)d-orbital(s) (when the electronic configuration is written according to the Aufbau principle)

(1)

- (not transition element as) $\mathrm{Hg}^{+}$ and $\mathrm{Hg}^{2+}$ /(stable) ions do not have incompletely filled (5)d-orbital(s)

(1)

Accept use of d-subshell for d-orbital(s) Allow use of d-shell for d-subshell Penalise use of just d-block for d-shell once only Penalise use of 3d/4d for 5d once only

Allow Hg loses (only) its 6s electrons (when forming ions/compounds)

Do not award answer in terms of the electronic configuration of an ion of mercury

Allow $\mathrm{Hg}^{+}$ and $\mathrm{Hg}^{2+}$ /(stable) ions have completely full (5)d-orbital(s) Ignore any reference to d-d transitions / other transition element properties

Do not award answer in terms of the electronic configuration of the element / an Hg atom

Expert

[Maximum number: 1]

Which d-block elements of Period 3 do not follow the periodic pattern in the electronic configuration of their atoms?

A

chromium and copper

B

chromium and manganese

C

scandium and copper

D

scandium and zinc

[Maximum number: 6]

This question is about the chemistry of vanadium.
The standard electrode potentials of some vanadium species are shown.

Table for Question 3 — Edexcel A-Level Chemistry A2

(a)

Explain the highest stable oxidation state formed by vanadium, by referring to its electronic configuration.

[ 3 ]

(b)

Most vanadium produced is used to make a steel alloy called ferrovanadium. The vanadium content of ferrovanadium may be determined by a titration method.

Procedure
- The sample of ferrovanadium is dissolved in chloric(V) acid.

The vanadium species formed is VO_3^-.
- The resulting solution is transferred to a 250.0 cm^3 volumetric flask, washings added and the solution made up to the mark with distilled water and mixed.
- Using a pipette, 25.0 cm^3 of the solution is transferred to a conical flask and 25.0 cm^3 of a 0.250 mol dm^-3 solution of iron(II) sulfate, FeSO_4, is added.
The iron(II) ions react with the VO_3^- ions:

\mathrm{VO}_{3}^{-}+4 \mathrm{H}^{+}+\mathrm{Fe}^{2+} \rightleftharpoons \mathrm{VO}^{2+}+2 \mathrm{H}_{2} \mathrm{O}+\mathrm{Fe}^{3+}

- The resulting solution is titrated against potassium manganate(VII) to determine the amount of iron(II) ions remaining.

\mathrm{MnO}_{4}^{-}+8 \mathrm{H}^{+}+5 \mathrm{Fe}^{2+} \rightarrow \mathrm{Mn}^{2+}+5 \mathrm{Fe}^{3+}+4 \mathrm{H}_{2} \mathrm{O}

In an experiment, the mass of ferrovanadium used was 4.87 g , the concentration of potassium manganate(VII) was 0.0195 mol dm^-3 and a mean titre of 22.50 cm^3 was obtained.

[ 1 ]

(i)

Give the colour of the solution at the end-point of the titration.

[ 1 ]

(c)

In the manufacture of sulfuric acid, vanadium(V) oxide, V_2 O_5, is the catalyst used in the conversion of sulfur dioxide to sulfur trioxide:

2 \mathrm{SO}_{2}+\mathrm{O}_{2} \rightleftharpoons 2 \mathrm{SO}_{3}

Write two equations to show a possible mechanism for this reaction.
State symbols are not required.

[ 2 ]

[Maximum number: 3]

This question is about the chemistry of vanadium.
The standard electrode potentials of some vanadium species are shown.

Table for Question 4 — Edexcel A-Level Chemistry A2

Explain the highest stable oxidation state formed by vanadium, by referring to its electronic configuration.

[Maximum number: 1]

A ligand must be an

A

electron-pair donor

B

electron-pair donor and negatively charged

C

electron-pair acceptor

D

electron-pair acceptor and negatively charged

[Maximum number: 1]

Transition metal compounds can show a number of different types of isomerism. Hydration isomerism is where different numbers of water molecules act as ligands. The name chromium(III) chloride is given to several chemical compounds with the formula $\mathrm{CrCl}_{3} \cdot \times \mathrm{H}_{2} \mathrm{O}$ , including a number of hydration isomers.

Complexes can exhibit both cis-trans and optical isomerism. The cis- and trans-isomers of diamminedichloroplatinum(II) are commonly known as cis-platin and trans-platin.

cis-platin

trans-platin

In chemotherapy medication, cis-platin is used to treat a number of cancers including testicular cancer and breast cancer, while trans-platin has no beneficial effect against cancer. The cis-isomer is effective because it binds with the deoxyribonucleic acid (DNA) molecules in a cancerous cell through adenine and guanine groups. This interferes with the replication of the cell and results in its destruction.

Figure for Question 6 — Edexcel A-Level Chemistry A2

This works in three steps.
Step 1 Slow substitution of one chloride ligand by a water.
Step 2 This water ligand is easily displaced by guanine or adenine in the DNA strand.

Step 3 Finally the second chloride ligand is displaced by a different guanine or adenine from a different part of the strand.

Describe how guanine and adenine can bind to the platinum ion.

[Maximum number: 1]

The diagram summarises some reactions of copper compounds.

Figure for Question 7 — Edexcel A-Level Chemistry A2

T and V are the same type of chemical species.

Name this type of chemical species.

[Maximum number: 3]

Solution A and solution B are aqueous solutions.
The compound in each solution contains one cation and one anion.
The cations are different but the anions are the same.
Both solutions are green.

(a)

Give the formulae of three cations which could be responsible for the green colour.

[ 2 ]

(b)

A student added dilute aqueous sodium hydroxide, drop by drop, to samples of each solution.
Initially a green precipitate formed in both solutions.
More aqueous sodium hydroxide was added until the sodium hydroxide was present in excess.
The precipitate produced from solution A was insoluble and turned brown on standing.
The precipitate produced from solution B dissolved to give a dark green solution C.

[ 1 ]

(i)

Give the formula of the species responsible for the dark green colour in solution C.

[ 1 ]

$\left[\mathrm{Cr}(\mathrm{OH})_{6}\right]^{3-}$

Allow $\mathrm{Cr}(\mathrm{OH})_{6}{ }^{3-}$

[Maximum number: 3]

21
Iron Chemistry

Iron is a typical transition metal. Due to the similar energies of the 3d and 4s electrons, iron forms compounds in a number of oxidation states.

Iron(II) and iron(III) are the most common oxidation states, and iron(III) is the most stable.

Iron ions form many complexes, including that in haemoglobin which is responsible for oxygen transport in the blood of most vertebrates. The haemoglobin-iron complex with oxygen is responsible for the red colour of blood.

Iron(III) ions may be detected in solution by the addition of thioglycolic acid ( $\ce{HSCH2COOH}$ ). All the water ligands of the iron(III) ion are replaced giving a complex with an intense red colour which can be detected in very low concentrations.

The complexes of iron(II) and iron(III) usually have a coordination number of six and are octahedral but the chloro complexes have a coordination number of four and are tetrahedral.

Iron and its compounds can act as catalysts. The element catalyses the Haber process, acting as a typical heterogeneous catalyst. However, the compounds and complexes of iron are usually homogeneous catalysts.

Invertebrates use a copper complex, haemocyanin, to transport oxygen. Blue oxyhaemocyanin gives invertebrate blood its characteristic colour.

Explain why oxyhaemocyanin and oxyhaemoglobin have different colours.

[Maximum number: 3]

The diagram summarises some reactions of copper compounds.

Figure for Question 10 — Edexcel A-Level Chemistry A2

T and V are the same type of chemical species.

Explain why T is coloured while V is colourless. A detailed explanation of the fact that T is coloured is not required.
(3)

[Maximum number: 1]

The structure of the complex $\ce{Cr2(CH3CO2)4(H2O)2}$ is shown.

What is the coordination number of chromium in this complex?

Figure for Question 11 — Edexcel A-Level Chemistry A2

A

two

B

four

C

six

D

twelve

[Maximum number: 2]

The diagram summarises some reactions of copper compounds.

Figure for Question 12 — Edexcel A-Level Chemistry A2

T and V are the same type of chemical species.

Suggest an explanation for the change of V into T on shaking.

[Maximum number: 2]

This question is about compounds and complex ions of iron.

In aqueous solution, iron(II) exists as the complex ion $\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}$ .

Explain how water acts as a monodentate ligand.

An explanation that makes reference to the following points:

- because it forms one dative (covalent) / co-ordinate bond (to $\mathrm{Fe}^{2+}$ )

(1)

- using a lone pair (of electrons) on oxygen

(1)

Allow 'a dative/co-ordinate bond'

Allow oxygen donates a pair of electrons Ignore water uses a lone pair of electrons COMMENT

Allow M2 for a diagram showing the oxygen lone pair forming the co-ordinate bond but annotation needed to score M1

Number

Admer

T

[Maximum number: 2]

This question is about compounds and complex ions of iron.

In aqueous solution, iron(II) exists as the complex ion $\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}$ .

Explain the shape of the $\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}$ complex ion.

An explanation that makes reference to the following points:

- octahedral because there are six pairs of electrons

(1)

- which are as far apart as possible to minimise repulsion (1)

Allow this shown on a diagram
Allow octahedral because there are 6 coordinate bonds/coordination number is 6
Ignore just octahedral because there are 6 ligands
Do not award if bond angle other than $90^{\circ} /$ $90^{\circ}$ and $180^{\circ}$ stated
Allow repel/arrange/shape to maximum separation
Do not allow repulsion between atoms or water
molecules or ligands

[Maximum number: 1]

21
Iron Chemistry

Iron is a typical transition metal. Due to the similar energies of the 3d and 4s electrons, iron forms compounds in a number of oxidation states.

Iron(II) and iron(III) are the most common oxidation states, and iron(III) is the most stable.

Iron ions form many complexes, including that in haemoglobin which is responsible for oxygen transport in the blood of most vertebrates. The haemoglobin-iron complex with oxygen is responsible for the red colour of blood.

Iron(III) ions may be detected in solution by the addition of thioglycolic acid ( $\ce{HSCH2COOH}$ ). All the water ligands of the iron(III) ion are replaced giving a complex with an intense red colour which can be detected in very low concentrations.

The complexes of iron(II) and iron(III) usually have a coordination number of six and are octahedral but the chloro complexes have a coordination number of four and are tetrahedral.

Iron and its compounds can act as catalysts. The element catalyses the Haber process, acting as a typical heterogeneous catalyst. However, the compounds and complexes of iron are usually homogeneous catalysts.

Give a possible reason why the chloro complexes of iron ions have a coordination number of four rather than six.

[Maximum number: 2]

Transition metal compounds can show a number of different types of isomerism. Hydration isomerism is where different numbers of water molecules act as ligands. The name chromium(III) chloride is given to several chemical compounds with the formula $\mathrm{CrCl}_{3} \cdot \times \mathrm{H}_{2} \mathrm{O}$ , including a number of hydration isomers.

Complexes can exhibit both cis-trans and optical isomerism. The cis- and trans-isomers of diamminedichloroplatinum(II) are commonly known as cis-platin and trans-platin.

cis-platin

trans-platin

In chemotherapy medication, cis-platin is used to treat a number of cancers including testicular cancer and breast cancer, while trans-platin has no beneficial effect against cancer. The cis-isomer is effective because it binds with the deoxyribonucleic acid (DNA) molecules in a cancerous cell through adenine and guanine groups. This interferes with the replication of the cell and results in its destruction.

Figure for Question 16 — Edexcel A-Level Chemistry A2

This works in three steps.
Step 1 Slow substitution of one chloride ligand by a water.
Step 2 This water ligand is easily displaced by guanine or adenine in the DNA strand.

Step 3 Finally the second chloride ligand is displaced by a different guanine or adenine from a different part of the strand.

trans-Platin binds to DNA.

Suggest why this does not damage the DNA.

[Maximum number: 2]

21
Iron Chemistry

Iron is a typical transition metal. Due to the similar energies of the 3d and 4s electrons, iron forms compounds in a number of oxidation states.

Iron(II) and iron(III) are the most common oxidation states, and iron(III) is the most stable.

Iron ions form many complexes, including that in haemoglobin which is responsible for oxygen transport in the blood of most vertebrates. The haemoglobin-iron complex with oxygen is responsible for the red colour of blood.

Iron(III) ions may be detected in solution by the addition of thioglycolic acid ( $\ce{HSCH2COOH}$ ). All the water ligands of the iron(III) ion are replaced giving a complex with an intense red colour which can be detected in very low concentrations.

The complexes of iron(II) and iron(III) usually have a coordination number of six and are octahedral but the chloro complexes have a coordination number of four and are tetrahedral.

Iron and its compounds can act as catalysts. The element catalyses the Haber process, acting as a typical heterogeneous catalyst. However, the compounds and complexes of iron are usually homogeneous catalysts.

The presence of iron in sodium carbonate can affect its properties; the higher the quality of the sodium carbonate, the lower the proportion of iron.

The proportion of iron in a laboratory grade anhydrous sodium carbonate was listed as less than 20 ppm by mass.

In an experiment to check this specification, 20 g of the sodium carbonate was dissolved in sulfuric acid, and thioglycolic acid added in excess to form the iron(III) thioglycolic acid complex, $\mathrm{Fe}\left(\mathrm{HSCH}_{2} \mathrm{COOH}\right)_{3}^{3+}$ . The solution was made up to $500 \mathrm{~cm}^{3}$ in a volumetric flask and thoroughly mixed.

Colorimeter calibration graph

The transmittance of the resulting solution was determined using a colorimeter and found to be 39.8%.

Suggest what type of ligand thioglycolic acid is in the iron(III) thioglycolic acid complex. Justify your answer.

(thioglycolic acid is a) bidentate (ligand)

(1)

- because there are three ligands per complex ion and the coordination number of $\mathrm{Fe}^{3+}$ is (usually) six

OR

can form dative bonds using the lone pairs on the SH and the COOH groups

(1)

Allow three ligands replace six (monodentate) water ligands

Accept S and O atoms Ignore just 'forms two dative covalent bonds’

Do not award two dative covalent bonds from COOH

[Maximum number: 2]

This question is about compounds and complex ions of iron.

Haemoglobin is an iron(II) complex which carries oxygen around the body. Part of the structure of oxyhaemoglobin is shown.

Figure for Question 18 — Edexcel A-Level Chemistry A2

The four nitrogen atoms are part of a multidentate ligand in the haem group.
Explain, in terms of the iron(II) complex, why carbon monoxide is toxic.

An explanation that makes reference to the following points:

- carbon monoxide replaces / takes the place of the oxygen molecule / ligand

(1)

- (and it may be toxic) because it binds strongly to the

(1) $\mathrm{Fe}^{2+}$ ion

Accept ligand substitution / exchange reaction between oxygen and carbon monoxide COMMENT

The question refers to oxygen being carried around and so there needs to be explicit reference and not just implied that to it being replaced/substituted or its place being taken

Figure for Answer to Question 18 — Edexcel A-Level Chemistry A2

Number

Ader

[Maximum number: 2]

This question is about transition metals and their compounds.

Vanadium can exist in the oxidation states +2,+3,+4 and +5.

Complete the table to show the colours of the ions that each of these oxidation states has in aqueous solution.

Table for Question 19 — Edexcel A-Level Chemistry A2

Oxidation state of vanadium	Colour of aqueous solution
+3	green
+4	blue
+5	yellow or colourless

[Maximum number: 5]

This question is about transition metals and their compounds.

Vanadium can exist in the oxidation states +2,+3,+4 and +5.

Predict the oxidising agent that could convert vanadium(II) ions to vanadium(III) ions and then vanadium(IV) ions, but not to vanadium(V) ions, under standard conditions.

Only use data from the tables shown.
Include equations and $E_{\text {cell }}^{\ominus}$ values for the two reactions that occur.

Vanadium systems

Table for Question 20 — Edexcel A-Level Chemistry A2

Oxidising agents

An answer that makes reference to the following points:

- equation for oxidation of $\mathrm{V}^{2+}$ to $\mathrm{V}^{3+}$

(1)

- $E^{\circ}$ cell for oxidation of $\mathrm{V}^{2+}$ to $\mathrm{V}^{3+}$

(1)

- equation for oxidation of $\mathrm{V}^{3+}$ to $\mathrm{VO}^{2+}$

(1)

- $E^{\circ}{ }_{\text {cell }}$ for oxidation of $\mathrm{V}^{3+}$ to $\mathrm{VO}^{2+}$

(1)

- $\mathrm{VO}^{2+}$ is not oxidised to $\mathrm{VO}_{2}{ }^{+} /$ any further as $E^{\circ}$ cell is $-0.2(\mathrm{~V}) /$ negative

(1)

Examples of equations:

Allow multiples Ignore state symbols even if incorrect Ignore uncancelled $\mathrm{H}^{+} / \mathrm{H}_{2} \mathrm{O}$

Penalise uncancelled electrons once only

$\mathrm{NO}_{3}{ }^{-}+2 \mathrm{H}^{+}+\mathrm{V}^{2+} \rightarrow \mathrm{NO}_{2}+\mathrm{H}_{2} \mathrm{O}+\mathrm{V}^{3+}$

Allow $\mathrm{Cu}^{2+}+\mathrm{V}^{2+} \rightarrow \mathrm{Cu}^{+}+\mathrm{V}^{3+}$

Allow $1 / 2 \mathrm{Br}_{2}+\mathrm{V}^{2+} \rightarrow \mathrm{Br}^{-}+\mathrm{V}^{3+}$ $E^{\circ}{ }_{\text {cell }}=(+) 1.06(\mathrm{~V})$

TE on $\mathrm{Cu}^{2+} / \mathrm{Br}_{2}$ chosen as oxidising agent

With $\mathrm{Cu}^{2+} E^{\ominus}{ }_{\text {cell }}=(+) 0.41(0)(\mathrm{V})$

With $\mathrm{Br}_{2} E^{\circ}{ }_{\text {cell }}=(+) 1.35(\mathrm{~V})$ $\mathrm{NO}_{3}{ }^{-}+\mathrm{V}^{3+} \rightarrow \mathrm{NO}_{2}+\mathrm{VO}^{2+}$

Allow $1 / 2 \mathrm{Br}_{2}+\mathrm{V}^{3+}+\mathrm{H}_{2} \mathrm{O} \rightarrow \mathrm{Br}^{-}+\mathrm{VO}^{2+}+2 \mathrm{H}^{+}$ $E^{\circ}{ }_{\text {cell }}=(+) 0.46(\mathrm{~V})$

With $\mathrm{Br}_{2} E^{\circ}{ }_{\text {cell }}=(+) 0.75(\mathrm{~V})$

Allow this shown in an equation

[Maximum number: 1]

This question is about cell reactions involving chromium.
Use your Data Booklet when answering this question.

State the essential condition required for these reactions to occur.

[Maximum number: 1]

Solution A and solution B are aqueous solutions.
The compound in each solution contains one cation and one anion.
The cations are different but the anions are the same.
Both solutions are green.

The student added hydrogen peroxide solution to C and warmed the mixture, which turned yellow. The yellow solution was boiled to remove excess hydrogen peroxide, cooled and then pure ethanoic acid was added, drop by drop. The yellow solution gradually turned orange.

Identify, by name or formula, the ion responsible for the final orange colour of the solution.

[Maximum number: 1]

This question is about copper and some of its compounds.

Two tests were carried out on separate samples of an aqueous solution of copper(II) sulfate.

Test 1

A few drops of aqueous sodium hydroxide were added to a sample of the copper(II) sulfate solution.

State what you would see.

[Maximum number: 2]

This question is about transition metals and their compounds.

Three reactions were carried out on a solution containing $\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}$ ions.

Figure for Question 24 — Edexcel A-Level Chemistry A2

Complete the equation for Reaction 1. State symbols are not required.

\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}+\ldots \ldots \ldots \ldots \ldots \mathrm{NH}_{3} \rightarrow

correct formula of iron(III) hydroxide

(1)

- rest of equation correct, conditional on correct precipitate

(1)

Examples of equation:
$\begin{aligned} & {\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}+3 \mathrm{NH}_{3} \rightarrow \mathrm{Fe}(\mathrm{OH})_{3}+3 \mathrm{H}_{2} \mathrm{O}+3 \mathrm{NH}_{4}^{+}} & \begin{array}{l} \mathrm{Ir} {\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}+3 \mathrm{NH}_{3} \rightarrow \mathrm{Fe}(\mathrm{OH})_{3}\left(\mathrm{H}_{2} \mathrm{O}\right)_{3}+3 \mathrm{NH}_{4}^{+}} \end{array} \end{aligned}$

Allow $\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{3}(\mathrm{OH})_{3}$

Ignore state symbols, even if incorrect Ignore square brackets around iron(III) hydroxide formulae

[Maximum number: 2]

This question is about copper and some of its compounds.

Two tests were carried out on separate samples of an aqueous solution of copper(II) sulfate.

Test 2

A few drops of concentrated hydrochloric acid were added to another sample of the copper(II) sulfate solution.
More of the concentrated hydrochloric acid was added until it was present in excess.

Describe the changes that would be observed during this test.

[Maximum number: 2]

This question is about compounds and complex ions of iron.

Ethanedioate ligands react with iron(II) ions in aqueous solution.

\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}+2 \mathrm{C}_{2} \mathrm{O}_{4}^{2-} \rightarrow\left[\mathrm{Fe}\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)_{2}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\right]^{2-}+4 \mathrm{H}_{2} \mathrm{O}

Explain, in terms of entropy, why this reaction is feasible.

[Maximum number: 6]

This question is about transition metals and their compounds.

Transition metals, their compounds and their ions can act as heterogeneous and homogeneous catalysts.

Compare and contrast these two types of catalytic behaviour.
Include one specific example from transition metal chemistry of each of these types of catalyst and a reaction in which it is used.

This question assesses the student's ability to show a coherent and logically structured answer with linkages and fully sustained reasoning.

Marks are awarded for indicative content and for how the answer is structured and shows lines of reasoning.

The following table shows how the marks should be awarded for indicative content.

Figure for Answer to Question 27 — Edexcel A-Level Chemistry A2

Guidance on how the mark scheme should be applied.

The mark for indicative content should be added to the mark for lines of reasoning. For example, a response with five indicative marking points that is partially structured with some linkages and lines of reasoning scores 4 marks ( 3 marks for indicative content and 1 mark for partial structure and some linkages and lines of reasoning).

If there were no linkages between the points, then the same indicative marking points would yield an overall score of 3 marks (3 marks for indicative content and no marks for linkages).

In general it would be expected that 5 or 6 indicative points would get 2 reasoning marks, and

The following table shows how the marks should be awarded for structure and lines of reasoning

Comment: Look for the indicative marking points first, then consider the mark for the structure of the answer and sustained line of reasoning.

Indicative content

3 or 4 indicative points would get 1 mark for reasoning, and 0, 1 or 2 indicative points would score zero marks for reasoning.

If there is any incorrect chemistry, deduct mark(s) from the reasoning. If no reasoning mark(s) awarded do not deduct mark(s). e.g. iron catalysing formation of ammonia from nitrogen and hydrogen but naming it the Contact Process / incorrect formula e.g. for persulfate ions

Allow correct formulae for names

Do not award examples that are not transition metals, ions or compounds

- IP1 Comparison - Activation energy

both catalysts increase the rate of reaction by providing an alternative route / mechanism with a lower activation energy

- IP2 Phase

a heterogeneous catalyst is in a different phase from the reactants and a homogeneous catalyst is in the same phase as the reactants / all solutions / gases

- IP3 Example of heterogeneous

example of a heterogeneous catalyst and reaction it catalyses e.g. iron and Haber Process, nickel and hydrogenation of alkenes, platinum in a catalytic converter / with CO and NO

- IP4 Example of homogeneous example of a homogeneous catalyst and reaction it catalyses e.g. iron(II) / iron(III) ions and reaction between iodide ions and persulfate ions

- IP5 Mechanism of heterogeneous reactant molecules are adsorbed onto the catalyst surface, the bonds are weakened, reaction takes place then the product molecules are desorbed

Allow this shown on a Maxwell-Boltzmann distribution / reaction profile diagram

Allow (physical) state for phase Allow heterogeneous catalysts are easy to separate from the reaction mixture / reactants / products and homogeneous catalysts are difficult to separate from the reaction mixture / reactants / products

[Maximum number: 2]

This question is about the chemistry of vanadium.
The standard electrode potentials of some vanadium species are shown.

Table for Question 28 — Edexcel A-Level Chemistry A2

In the manufacture of sulfuric acid, vanadium(V) oxide, V_2 O_5, is the catalyst used in the conversion of sulfur dioxide to sulfur trioxide:

2 \mathrm{SO}_{2}+\mathrm{O}_{2} \rightleftharpoons 2 \mathrm{SO}_{3}

Write two equations to show a possible mechanism for this reaction.
State symbols are not required.

[Maximum number: 1]

Which of the following is not true of the reactions occurring in the catalytic converter fitted to a car exhaust?

A

they involve heterogeneous catalysis

B

carbon monoxide is adsorbed onto the surface of the catalyst

C

nitrogen is desorbed from the surface of the catalyst

D

the products cause no harm to the environment

[Maximum number: 1]

21
Iron Chemistry

Iron is a typical transition metal. Due to the similar energies of the 3d and 4s electrons, iron forms compounds in a number of oxidation states.

Iron(II) and iron(III) are the most common oxidation states, and iron(III) is the most stable.

Iron ions form many complexes, including that in haemoglobin which is responsible for oxygen transport in the blood of most vertebrates. The haemoglobin-iron complex with oxygen is responsible for the red colour of blood.

Iron(III) ions may be detected in solution by the addition of thioglycolic acid ( $\ce{HSCH2COOH}$ ). All the water ligands of the iron(III) ion are replaced giving a complex with an intense red colour which can be detected in very low concentrations.

The complexes of iron(II) and iron(III) usually have a coordination number of six and are octahedral but the chloro complexes have a coordination number of four and are tetrahedral.

Iron and its compounds can act as catalysts. The element catalyses the Haber process, acting as a typical heterogeneous catalyst. However, the compounds and complexes of iron are usually homogeneous catalysts.

Iodide ions are oxidised to iodine by peroxodisulfate ions.

2 \mathrm{I}^{-}(\mathrm{aq})+\mathrm{S}_{2} \mathrm{O}_{8}^{2-}(\mathrm{aq}) \rightarrow \mathrm{I}_{2}(\mathrm{aq})+2 \mathrm{SO}_{4}^{2-}(\mathrm{aq})

Iron(II) ions act as a homogeneous catalyst for this reaction.

State why the catalyst is described as 'homogeneous'.

[Maximum number: 3]

21
Iron Chemistry

Iron is a typical transition metal. Due to the similar energies of the 3d and 4s electrons, iron forms compounds in a number of oxidation states.

Iron(II) and iron(III) are the most common oxidation states, and iron(III) is the most stable.

Iron ions form many complexes, including that in haemoglobin which is responsible for oxygen transport in the blood of most vertebrates. The haemoglobin-iron complex with oxygen is responsible for the red colour of blood.

Iron(III) ions may be detected in solution by the addition of thioglycolic acid ( $\ce{HSCH2COOH}$ ). All the water ligands of the iron(III) ion are replaced giving a complex with an intense red colour which can be detected in very low concentrations.

The complexes of iron(II) and iron(III) usually have a coordination number of six and are octahedral but the chloro complexes have a coordination number of four and are tetrahedral.

Iron and its compounds can act as catalysts. The element catalyses the Haber process, acting as a typical heterogeneous catalyst. However, the compounds and complexes of iron are usually homogeneous catalysts.

(a)

Iodide ions are oxidised to iodine by peroxodisulfate ions.

2 \mathrm{I}^{-}(\mathrm{aq})+\mathrm{S}_{2} \mathrm{O}_{8}^{2-}(\mathrm{aq}) \rightarrow \mathrm{I}_{2}(\mathrm{aq})+2 \mathrm{SO}_{4}^{2-}(\mathrm{aq})

Iron(II) ions act as a homogeneous catalyst for this reaction.

[ 3 ]

(i)

Write two equations to show how iron(II) ions catalyse this oxidation. State symbols are not required.

[ 2 ]

equation for oxidation of iron(II) by peroxodisulfate

- equation for oxidation of iodide ions by iron(III)

Examples of equations
$\mathrm{S}_{2} \mathrm{O}_{8}^{2-}+2 \mathrm{Fe}^{2+} \rightarrow 2 \mathrm{SO}_{4}^{2-}+2 \mathrm{Fe}^{3+}$ $2 \mathrm{I}^{-}+2 \mathrm{Fe}^{3+} \rightarrow \mathrm{I}_{2}+2 \mathrm{Fe}^{2+}$

Allow multiples / equations in any order Ignore state symbols even if incorrect Two unbalanced equations with all species correct scores (1)

(ii)

Suggest how iron(II) ions lower the activation energy of this reaction.

[ 1 ]

[Maximum number: 1]

Acidified manganate(VII) ions react with ethanedioate ions.

2 \mathrm{MnO}_{4}^{-}+16 \mathrm{H}^{+}+5 \mathrm{C}_{2} \mathrm{O}_{4}^{2-} \rightarrow 2 \mathrm{Mn}^{2+}+8 \mathrm{H}_{2} \mathrm{O}+10 \mathrm{CO}_{2}

The rate of this reaction increases then decreases.
What are the reasons for these changes in rate of reaction?
A
B
C
D

Table for Question 32 — Edexcel A-Level Chemistry A2

A

kinetic energies of particles increase then reactant concentration decreases

B

Mn2+ acts as a catalyst; concentration of reactants decreases

C

MnO4− acts as a catalyst; kinetic energies decrease

D

MnO4− acts as a catalyst; concentration of reactants decreases

[Maximum number: 1]

Transition metal compounds can show a number of different types of isomerism. Hydration isomerism is where different numbers of water molecules act as ligands. The name chromium(III) chloride is given to several chemical compounds with the formula $\mathrm{CrCl}_{3} \cdot \times \mathrm{H}_{2} \mathrm{O}$ , including a number of hydration isomers.

Complexes can exhibit both cis-trans and optical isomerism. The cis- and trans-isomers of diamminedichloroplatinum(II) are commonly known as cis-platin and trans-platin.

cis-platin

trans-platin

In chemotherapy medication, cis-platin is used to treat a number of cancers including testicular cancer and breast cancer, while trans-platin has no beneficial effect against cancer. The cis-isomer is effective because it binds with the deoxyribonucleic acid (DNA) molecules in a cancerous cell through adenine and guanine groups. This interferes with the replication of the cell and results in its destruction.

Figure for Question 33 — Edexcel A-Level Chemistry A2

This works in three steps.
Step 1 Slow substitution of one chloride ligand by a water.
Step 2 This water ligand is easily displaced by guanine or adenine in the DNA strand.

Step 3 Finally the second chloride ligand is displaced by a different guanine or adenine from a different part of the strand.

Write the balanced equation for Step 1.

State symbols are not required.

An answer that makes reference to the following point:

- correct equation

$\left[\mathrm{Pt}\left(\mathrm{NH}_{3}\right)_{2} \mathrm{Cl}_{2}\right]+\mathrm{H}_{2} \mathrm{O} \rightarrow\left[\mathrm{Pt}\left(\mathrm{NH}_{3}\right)_{2}\left(\mathrm{H}_{2} \mathrm{O}\right) \mathrm{Cl}\right]^{+}+\mathrm{Cl}^{-}$

Allow ligands in any order

Allow displayed formula but ignore incorrect shapes

Ignore state symbols even if incorrect Ignore omission of square brackets

Do not award products without charges

Edexcel A-Level Chemistry A2 17 Transition Metals and Their Chemistry Questions

Exam points

Covered syllabus points

Covered syllabus points

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Question 2

Question 3

Question (a)

Question (b)

Question (i)

Question (c)

Question 4

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Question 8

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Question (b)

Question (i)

Question 9

Question 10

Question 11

Question 12

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Question 14

Question 15

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Question 17

Question 18

Question 19

Question 20

Question 21

Question 22

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Question 24

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Question 31

Question (a)

Question (i)

Question (ii)

Question 32

Question 33