Edexcel A-Level Chemistry Paper 3, June 2019: Question 6

14 marks · Hard difficulty · Open Response

Deduce the coordination number and overall charge of a chromium complex ion, explain factors affecting transition metal ion colours using iron or copper examples, plot a graph of partial pressure against time for the catalytic decomposition of ammonia, determine the rate equation and rate constant, and describe the stages in heterogeneous catalysis.

Practise this question

Question

Exam question in multiple parts about transition metals and catalysis. Part (a) shows the structure of a Cr3+ complex ion with Cl and bidentate ligands, asking for coordination number and overall charge. Part (b) asks to give examples illustrating factors affecting transition metal complex colours using iron or copper. Part (c) gives experimental data for the decomposition of ammonia catalysed by tungsten, including a table of partial pressure against time, a grid for plotting a graph, and subsequent questions asking to deduce the rate equation, calculate the rate constant with units, and describe the stages of heterogeneous catalysis.
Question text

6 This question is about transition metals and their ions.

(a) The shape of a complex ion formed from Cr3+ ions is shown.

Cl

H2N NH2

H2C CH2

Cr

H2C CH2

H2N NH2

Cl

(i) State the coordination number of Cr3+ in this complex ion.

(1)

(ii) State the overall charge on this complex ion.

(1)

(b) The complex ions of transition metals have different colours in aqueous solution.

Two factors that affect the colour of the solution are the oxidation number of the

central metal ion, and the ligands present.

Give examples to illustrate these factors by referring to complex ions

of iron and/or copper. Include the formula and colour of each complex.

An explanation of why transition metal ions are coloured is not required.

(3)

(c) Tungsten wire catalyses the decomposition of ammonia.

2NH3(g) o N2(g) + 3H2(g)

In an experiment, the following results were obtained. 13

Time /s*P58308A01332*Partial pressure of ammonia / kPa

0 0.350

100 0.335

200 0.319

300 0.303

400 0.287

500 0.271

(i) Plot a graph of partial pressure of ammonia against time.

(2)

(ii) Deduce the rate equation for this reaction by using your graph in (c)(i).

Justify your answer.

(2)

… 14

… *P58308A01432*

(iii) Use the graph to calculate the rate constant. Include units in your answer.

(2)

(iv) Describe the stages in the catalytic decomposition of ammonia by tungsten.

(3)

(Total for Question 6 = 14 marks)

Mark scheme

Show the mark scheme Mark scheme showing acceptable answers and additional guidance for all parts of Question 6, including expected values for coordination number (6) and charge (1+), examples of iron and copper complex colours for different oxidation states and ligands, a completed graph template with labeled axes and straight line, the deduced zero-order rate equation, gradient calculation for the rate constant with units (e.g. kPa s-1), and the three stages of heterogeneous catalysis (adsorption, bond breaking, desorption).

How to answer it

Transition Metals and Reaction Kinetics Study Guide

📋 What this question tests

This question assesses core transition metal chemistry (coordination numbers, charges, and factors affecting complex ion colours) alongside advanced reaction kinetics (interpreting zero-order graphs, calculating rate constants with units, and describing heterogeneous catalysis mechanisms).

Question 6(a): Complex Ion Structure, Coordination Number, and Charge

✅ Correct Answers

  • (i) Coordination number: 6 (or six)
  • (ii) Overall charge: 1+ (or +1)

💡 Key Knowledge

The coordination number is the number of coordinate (dative covalent) bonds formed to the central metal ion. Remember that bidentate ligands (like ethanediamine, H₂NCH₂CH₂NH₂ ) form two coordinate bonds each, while monodentate ligands ( Cl⁻ ) form one.

🧠 Exam Technique

Count carefully: there are two bidentate ligands (contributing 4 coordinate bonds) and two monodentate chloride ligands (contributing 2 coordinate bonds), giving a total of 6. For charge, sum the oxidation states/charges: Cr³⁺ + 2(neutral bidentate ligand) + 2( Cl⁻ ) = (+3) + 0 + 2(-1) = +1.

❌ Common Errors

Students often miscount the coordination number by treating bidentate ligands as single bonds or forgetting to include the monodentate ligands surrounding the complex.

Marks: 6(a)(i) = 1 mark, 6(a)(ii) = 1 mark.

Question 6(b): Factors Affecting Transition Metal Complex Colours

💡 Key Knowledge

The colour of a transition metal complex is determined by d-d transitions. Two main factors alter this colour by changing the energy gap (ΔE) of the split d-orbitals: (1) Oxidation number of the central metal ion, and (2) Identity of the ligands present.

✅ Correct Answers & Examples

  • Factor 1 - Oxidation State: Same metal with different oxidation states.
    Example: [Fe(H₂O)₆]²⁺ is green AND [Fe(H₂O)₆]³⁺ is yellow/orange/red-brown.
  • Factor 2 - Ligand Identity: Same metal and oxidation state with different ligands.
    Example: [Cu(H₂O)₆]²⁺ is blue AND [CuCl₄]²⁻ is yellow.

❌ Common Errors & Examiner Guidance

Do not mix up different metals when illustrating a factor—you must compare the same metal across different conditions. Ensure chemical formulae and corresponding colours match strictly. Square brackets are leniently marked if omitted, but formulae must be chemically sound.

Marks: Total 3 marks (1 for stating the oxidation state factor with example, 2 for the ligand factor with formula and colour pairs).

Question 6(c): Kinetics of Ammonia Decomposition over Tungsten

Part (i): Plotting the Graph

🧠 Plotting Requirements

  • Time must be on the x-axis, clearly labelled with units ( Time / s ).
  • Partial pressure of ammonia must be on the y-axis with units ( Partial pressure of ammonia / kPa ).
  • The scale must use at least half the available grid space. Note: The y-axis does not need to start at 0 based on data ranges (0.271 to 0.350).
  • All points accurately plotted with a clean straight line of best fit.

Part (ii): Deducing the Rate Equation

✅ Correct Answer & Justification

Rate equation: rate = k (or rate = k × p(NH₃)⁰ )

Justification: The graph of partial pressure against time is a straight line (constant gradient), meaning the rate of reaction is independent of the concentration/partial pressure of ammonia.

❌ Common Errors

Failing to explicitly link the linear/straight-line nature of the graph to a zero-order dependence. Simply stating "it's zero order" without referencing the graph's constant gradient loses the justification mark.

Part (iii): Calculating the Rate Constant

📐 Step-by-Step Calculation

  1. Find the gradient: Calculate the gradient of your line of best fit (change in y / change in x). Using points from the mark scheme: (0.271 - 0.350) / (500 - 0) = -0.079 / 500 = -1.58 × 10⁻⁴ . (Magnitude taken for k : 1.58 × 10⁻⁴ ).
  2. Acceptable range: Any value between 1.50 × 10⁻⁴ and 1.65 × 10⁻⁴ .
  3. Units: kPa s⁻¹ (or equivalent units like Pa s⁻¹ ).

Part (iv): Heterogeneous Catalysis Stages

💡 Three Key Stages of Heterogeneous Catalysis

  1. Adsorption: Reactant gases ( NH₃ ) adsorb onto the surface of the tungsten catalyst.
  2. Reaction / Bond Breaking: Bonds within the reactant molecules weaken and break (specifically the N-H bonds).
  3. Desorption: Product gases ( N₂ and H₂ ) desorb and leave the surface of the catalyst.
Marks: 6(c)(i) = 2, 6(c)(ii) = 2, 6(c)(iii) = 2, 6(c)(iv) = 3. Total for Question 6 = 14 marks.

Topics

Inorganic Chemistry · Physical Chemistry · Topic 15: Transition Metals · Topic 16: Kinetics II

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 3, June 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.