OCR A-Level Chemistry Unified chemistry (03), June 2022: Question 4

8 marks · Hard difficulty · Structured Questions

Explain the effect of temperature and silver nitrate addition on the cobalt complex equilibrium, including rates and enthalpy changes.

Practise this question

Question

Two-part exam question about a cobalt complex equilibrium. Part (a) provides a colorimetry experiment graph showing absorbance versus time for 50 C and 90 C, requiring students to explain initial rates and predict the sign of delta H. Part (b) shows concentration-time graphs after adding silver nitrate at time t1, with subparts asking why chloride ion concentration drops sharply and explaining the shifts in concentrations of CoCl4 2-, Cl-, and Co(H2O)6 2+.
Question text

4 Two students plan to investigate Equilibrium 4.1, shown below.

CoCl 2−(aq) + 6H O(l) [Co(H O) ]2+(aq) + 4Cl −(aq) Equilibrium 4.1

42 2 6

blue pink

(a) The students are supplied with the equilibrium mixture in Equilibrium 4.1 at room

temperature.

• One student heats 20 cm3 of the mixture to 50 °C.

• The other student heats 20 cm3 of the mixture to 90 °C.

The students use colorimetry to observe how the colour of the equilibrium mixture changes

over time.

• The colorimeter is set up so that the greater the absorbance, the greater the

concentration of [Co(H O) ]2+.

• The initial absorbance is set to zero.

• The absorbance is recorded every 30 seconds.

The students plot the graph below from the results of the experiment.

50°C

90°C

Absorbance

0,0 Time/s

Use the graph and relevant chemical theory to answer the following. Include all reasoning:

• Explain the different initial rates at 50 °C and 90 °C.

• Predict the sign of ∆H for the forward reaction in Equilibrium 4.1.

… [4]

(b) The students investigate how addition of aqueous silver nitrate, AgNO3(aq), affects the

equilibrium position in Equilibrium 4.1.

The graph shows the changes in the equilibrium concentrations of CoCl 2–, Cl – and

[Co(H O) ]2+ after addition of the AgNO (aq).

26 3

The AgNO3(aq) is added at time = t1.

CoCl 2–

Cl –

Concentration

[Co(H O) ]2+

0 t Time

(i) Explain why the Cl – concentration drops sharply at time = t .

… [1]

(ii) Explain the changes in concentration of CoCl 2–, Cl – and [Co(H O) ]2+ after time = t .

42 6 1

Refer to Equilibrium 4.1 in your answer.

… [3]

Mark scheme

Show the mark scheme Mark scheme table detailing correct answers for parts (a), (b)(i), and (b)(ii). Part (a) awards marks for faster rate at 90 C, more particles exceeding activation energy, lower [Co(H2O)6]2+, and an exothermic/negative delta H. Part (b)(i) awards 1 mark for mentioning Cl- reacting with Ag+ to form an AgCl precipitate. Part (b)(ii) awards 3 marks for noting [CoCl42-] decreases and [Co(H2O)6]2+ increases, the 4x stoichiometric increase for Cl-, and the equilibrium shifting to the right.

Question Answer Marks AO Guidance

element

4 (a) 4

ORA for 50 ºC

At 90 ºC/higher temperature

• Faster rate AND more frequent collisions AO2.7 IGNORE more successful collisions

×1

• More particles have the activation energy/Ea or

AO1.2 ALLOW more molecules have enough

greater energy to react

×1

ALLOW atoms/molecules/ions

• [Co(H O) ]2+ is lower

26 AO2.3 ALLOW decreases

×1

• (forward reaction) ∆H –ve OR exothermic

AO1.2

×1

(b) (i) Cl– /It/They react with AgNO / Ag+ /silver ions 1 AO3.2 IGNORE chlorine/Cl for chloride ion

OR

AgCl formed IGNORE AgCl2

OR

Ag+ + Cl– → AgCl

22 element

(ii) 3 AO3.1

[CoCl 2–] decreases AND [Co(H O) ]2+ increases ×2

42 6

IGNORE missing charges and small slips in

formulae, e.g. CoCl4 missing bracket, etc

Cl– increase is 4 × change in [CoCl 2–] / [Co(H O) ]2+ IGNORE Cl– for changes in concentration

42 6

Equilibrium shifts to right AO3.2 ALLOW suitable alternatives for ‘shifts to

×1 right’, e.g. towards products

OR in forward direction OR ‘favours the right’

How to answer it

Transition Metal Equilibria & Kinetics

📌 What this question tests

This question assesses your understanding of transition metal complex equilibria, the kinetics of temperature changes on initial rates (Maxwell-Boltzmann distribution), enthalpy changes in reversible reactions, and Le Chatelier's principle involving precipitation reactions (ionic equations and stoichiometry).

Part (a): Initial Rates & Enthalpy Change

Investigating the effect of temperature on Equilibrium 4.1

✅ Correct Answer Breakdown (4 Marks)

  • Initial rate: At 90 °C, the initial rate is faster due to more frequent collisions.
  • Activation Energy: More particles have energy greater than or equal to the activation energy (Ea).
  • Extent of reaction: At 90 °C, the final equilibrium absorbance is lower, meaning the equilibrium concentration of [Co(H₂O)₆]²⁺ is lower.
  • Enthalpy sign: The forward reaction is exothermic, so ΔH is negative (-ve). Raising temperature shifts equilibrium in the endothermic direction (backwards).

💡 Key Knowledge

  • Rate vs Position: Higher temperature always increases rate (faster initial gradient), but shifts the position of equilibrium depending on the sign of ΔH.
  • Colorimetry link: The question states higher absorbance equals higher [[Co(H₂O)₆]²⁺]. Looking at the graph, 50 °C gives a higher final plateau than 90 °C.

🧠 Exam Technique

  • Structure your answer clearly: split it into a kinetic argument (rate & temperature) and a thermodynamic argument (equilibrium yield & enthalpy).
  • Use comparative language ("more frequent", "higher proportion"). Avoid vague terms like "particles move faster" without linking to collision frequency.

❌ Common Errors

  • Stating that "more successful collisions" happen because particles collide harder (Examiners specifically penalise or ignore "successful" unless tied to Ea).
  • Confusing the effect of temperature on rate with its effect on equilibrium yield.
🎯 Mark Scheme Allocation (4 marks): 1 mark for faster rate/more frequent collisions at 90 °C; 1 mark for proportion of particles ≥ Ea; 1 mark for recognizing [Co(H₂O)₆]²⁺ decreases at higher temp; 1 mark for deducing ΔH is negative (exothermic).

Part (b)(i): Precipitation & Chloride Concentration

Effect of adding aqueous silver nitrate

✅ Correct Answer (1 Mark)

Chloride ions (Cl⁻) react with added silver ions (Ag⁺) to form a silver chloride precipitate (AgCl).

Ag⁺(aq) + Cl⁻(aq) → AgCl(s)

🧠 Exam Technique & Guidance

  • A simple ionic equation or clear statement that Cl⁻ is removed by forming AgCl(s) secures the mark instantly.
  • Note: Examiners accept "react with Ag⁺ / silver ions" or "AgCl formed". Ignore references to spelling variations like "chlorine" instead of chloride ion.
🎯 Mark Scheme Allocation (1 mark): 1 mark for stating Cl⁻ reacts with Ag⁺ / AgCl is formed / ionic equation.

Part (b)(ii): Le Chatelier's Principle & Stoichiometry

Explaining concentration changes after time t₁

✅ Correct Answer Breakdown (3 Marks)

  • Concentration shifts: [CoCl₄]²⁻ decreases AND [Co(H₂O)₆]²⁺ increases.
  • Stoichiometric ratio: The increase in [Co(H₂O)₆]²⁺ (and decrease in [CoCl₄]²⁻) is accompanied by a 4× larger drop in [Cl⁻] concentration, reflecting the stoichiometric coefficient of 4Cl⁻ in the equation.
  • Equilibrium shift: Equilibrium shifts to the right (towards products) to replace the removed Cl⁻ ions.

📐 Stoichiometry & Ratio Check

Look closely at the equation coefficients:

CoCl₄²⁻(aq) + 6H₂O(l) ⇌ [Co(H₂O)₆]²⁺(aq) + 4Cl⁻(aq)

Because 4 moles of Cl⁻ are produced for every 1 mole of [CoCl₄]²⁻ reacted, the vertical drop on the graph for Cl⁻ must be precisely 4 times steeper/larger than the changes in the complex ions.

❌ Common Errors

  • Failing to link the magnitude of the concentration changes to the stoichiometric balancing numbers in Equilibrium 4.1.
  • Stating the equilibrium shifts left instead of right when Cl⁻ is removed. (Remember Le Chatelier: system opposes the change by trying to produce more Cl⁻, shifting forward).
🎯 Mark Scheme Allocation (3 marks): 1 mark for stating [CoCl₄]²⁻ decreases and [Co(H₂O)₆]²⁺ increases; 1 mark for noting the Cl⁻ increase/change is 4× the change of the complex ions; 1 mark for stating equilibrium shifts to the right / towards products.

Topics

Module 5: Physical chemistry and transition elements · Module 3: Periodic table and energy · 5.1 Rates, equilibrium and pH · 5.3 Transition elements · 3.2 Physical chemistry

Question and mark scheme from the OCR A-Level Chemistry examination, Unified chemistry (03), June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.