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International A LevelChemistryEdexcelExam Questions5: Transition Metals and Organic Nitrogen Chemistry5.2 Transition Metals & Complexes

Transition Metals & Complexes (Edexcel International A Level Chemistry)

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5.2 Transition Metals & Complexes

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5.2 Transition Metals & Complexes

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1a
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A series of reactions with iron and iron complexes was carried out.

Reaction 1 A sample of iron was heated with hydrochloric acid and a pale green hydrated salt A with molar mass 198.8 g mol–1 was crystallised from the solution.
Reaction 2 Salt A was dissolved in water forming a pale green solution containing complex ion B. On addition of excess aqueous potassium cyanide, KCN, the solution turned yellow due to the formation of complex ion C.
Reaction 3 Chlorine gas was bubbled through the solution containing complex ion C forming a red solution of complex ion D.
Salt E, the potassium salt of complex ion D, was then crystallised from the solution.

Deduce the formula of the hydrated salt A.
You must show your working.

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1b1 mark

Give the formula of complex ion B.

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1c1 mark

Complex ion C has six cyanide ligands.
Draw the structure of C, clearly showing its three‐dimensional shape.

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1d
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The percentage composition by mass of salt E is

K = 35.6%   Fe = 17.0%   C = 21.9%   N = 25.5%

Calculate the empirical formula of salt E.

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1e2 marks

Write the ionic equation for the reaction of complex ion C with chlorine to form complex ion D.

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1f2 marks

Complete the table, using ✓ as appropriate, to identify the type of each reaction.

  Neutralisation Ligand exchange Redox
Reaction 2      
Reaction 3      

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1a5 marks

This question is about silver and silver compounds.

Glass decorations are made reflective by coating their inner surface with silver. This is achieved by using the reaction between silver nitrate solution, ammonia and glucose, under alkaline conditions.

Initially the colourless complex ion diamminesilver(I), [Ag(NH3)2]+ , forms.

i)
Explain the shape of [Ag(NH3)2]+ .
(3)
ii)
Explain why [Ag(NH3)2] + is colourless.
(2)

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1b3 marks

The diamminesilver(I) complex then reacts with glucose forming silver and an organic compound, Y. Two other products also form.

i)
Complete the equation for the reaction.

(2)

q10b-paper-5-jan-2022-edexcel-ial-chemistry

ii)
Draw the structure of Y.

(1)

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1c1 mark

The overall reaction in a silver cell used in watch batteries is

Ag2O (s) + H2O (l) + Zn (s) → 2Ag (s) + Zn(OH)2 (s)

The half‐equation for the reaction at the positive electrode is

Ag2O (s) + H2O (l) + 2e− → 2Ag (s) + 2OH− (aq)

Deduce the half‐equation for the reaction at the negative electrode.
State symbols are not required.

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2a2 marks

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 (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.

Explain, in terms of electronic structure, why iron(III) compounds are more stable than iron(II) compounds.



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2b4 marks

The third ionisation energy of iron is 2958 kJ mol−1.

i)
Write the equation for the third ionisation energy of iron.
Include state symbols.
(1)
ii)
Explain how stable iron(III) ions can be formed from iron(II) ions in aqueous solution. Refer to the relevant energy changes of these ions only.
(3)

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2c3 marks

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.

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2d
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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, begin mathsize 16px style Fe open parentheses H S C H subscript 2 C O O H close parentheses subscript 3 superscript 3 plus end superscript end style . The solution was made up to 500 cm3 in a volumetric flask and thoroughly mixed.

q21d-paper-5-june-2021-edexcel-ial-chemistryThe transmittance of the resulting solution was determined using a colorimeter and found to be 39.8%.

i)
Using the calibration graph, determine whether or not the iron concentration in this sample of sodium carbonate meets the stated specification.
(4)
ii)
Suggest what type of ligand thioglycolic acid is in the iron(III) thioglycolic acid complex. Justify your answer.
(2)

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2e4 marks

Iodide ions are oxidised to iodine by peroxodisulfate ions.

2I− (aq) + S2begin mathsize 16px style O subscript 8 to the power of 2 minus sign end style (aq) → I2 (aq) + 2Sstraight O subscript 4 superscript 2 minus end superscript (aq)

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

i)
State why the catalyst is described as ‘homogeneous’.
(1)
ii)
Write two equations to show how iron(II) ions catalyse this oxidation.
State symbols are not required.
(2)

iii)
Suggest how iron(II) ions lower the activation energy of this reaction.
(1)

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2f1 mark

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

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