OCR A-Level Chemistry AS Depth in chemistry (02), June 2022: Question 3

12 marks · Medium difficulty · Structured Questions

Investigate the rate of reaction of zinc carbonate with hydrochloric acid using a mass-time graph, collision theory, rate calculation, and enthalpy profile diagrams with catalysts.

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Question

Question text

3 A student investigates some reactions of zinc compounds and zinc metal.

(a) The student investigates the rate of reaction between zinc carbonate, ZnCO3(s), and dilute

hydrochloric acid, HCl(aq).

ZnCO3(s) + 2HCl(aq) ZnCl2(aq) + CO2(g) + H2O(l)

The student follows the method outlined below:

• Add 50 cm3 of dilute HCl (aq) into a conical flask at 20 °C.

• Place the flask on a top-pan balance.

• Add an excess of ZnCO3(s) to the flask.

• Record the mass of the flask and contents on the top-pan balance every 30 seconds.

The student plots a graph of mass against time, shown in Fig. 3.1 below.

92.6

92.5

92.4

92.3

92.2

92.1

92.0

91.9

ass/g 91.8

91.7

91.6

91.5

91.4

91.3

91.2

91.1

91.0

0 50 100 150 200 250 300 350 400

time/s

Fig. 3.17

(i) The graph shows that the reaction gets slower over time, and eventually stops.

Explain why, in terms of collision theory.

… [3]

(ii) Using the graph in Fig. 3.1, find the rate of reaction, in g s–1, at 50 seconds.

Show your working on the graph and in the space below.

rate of reaction = … g s–1 [2]

(iii) The student repeats the experiment but heats 50 cm3 of dilute hydrochloric acid up to

40 °C before adding the ZnCO3(s).

On Fig. 3.1, sketch the curve the student would obtain. [2]

(b) The student investigates the reaction between zinc and dilute sulfuric acid.

Zn(s) + H SO (aq) ZnSO (aq) + H (g) ΔH = –140 kJ mol–1

24 4 2

Copper(II) sulfate is a catalyst for this reaction.

• The student adds a piece of zinc to each of two test tubes.

• The student adds a few drops of aqueous copper(II) sulfate to one of the test tubes,

forming a pale blue solution.

• The student adds an excess of dilute sulfuric acid to each test tube.

(i) Describe two differences the student would observe between the test tubes.

1 …

2 …

[2]

(ii) Using the axes below, sketch an enthalpy profile diagram for the reaction with and

without the catalyst.

On your diagram, include the following labels:

• ΔH, the enthalpy change

• Ea, the activation energy without a catalyst

• Ec, the activation energy with a catalyst.

Enthalpy

Zn(s)+H2SO4(aq)

Progress of reaction

[3]

Mark scheme

Show the mark scheme

Question Answer Marks Guidance

element

3 (a) (i) Rate 3

(Acid) concentration decreases AO1.1 IGNORE amount of acid decreases

Response MUST imply a volume and NOT area,

e.g. fewer particles/molecules/ions

in same space /volume

Collisions

Fewer collisions per second IGNORE responses not linked to rate, e.g.

OR less frequent collisions AO1.1 • ‘fewer collisions’

• fewer successful collisions

• fewer collisions, less chance of collisions

No link to rate.

Reaction stops

(Acid/reactant/limiting reagent) has reacted/been AO2.3

used up AW

14 AO

element

(ii) Tangent on graph 2 DO NOT ALLOW interpolation (taking a direct

drawn at approximately t = 50 s (± 10 s) AO3.1 reading from graph), answer must be derived from

taking a gradient

Calculation of rate

= Gradient (y/x) of tangent drawn

92.4 – 91.0 1.4 –3 –1 ALLOW ECF from incorrectly drawn tangent

e.g. = = 6.36 × 10 (g s ) AO3.2

220 220

ALLOW range of 5.7 x 10-3 to 6.9 x 10-3 in

calculation of tangent (rounded to 1 d.p.)

IGNORE units

IGNORE sign

Tolerance of readings:

y axis should be ± 0.02 g

(i.e. within 1 square)

x axis should be ± 5 min

(i.e. within 1 of a square)

(iii) Slope is steeper 2 AO3.2

AND ×2

levels off earlier

Same loss in mass, i.e. levels off at ~91.55 g Tolerance ± 1 small square

element

(b) (i) More vigorous bubbling 2 AO2.7 AW, e.g. bubbles/fizzes more quickly

×2

Zinc dissolves/disappears more quickly For 1 alternative marking point ALLOW responses

related to displacement of Cu from CuSO4 by Zn:

EITHER red/brown/black precipitate/solid

formed OR (blue solution) turns colourless

(ii) 3 ANNOTATE ANSWER WITH TICKS AND

CROSSES ETC

IGNORE state symbols

∆H DO NOT ALLOW –∆H

DO NOT ALLOW double headed arrow on ∆H

ALLOW ∆H arrow even with small gap at the

top and bottom,

∆H ∆H labelled with product (ZnSO4 + H2) below i.e. line does not quite reach reactant or

reactant product line.

AND AO2.1

Arrow downwards

Ea and Ec

AO1.1 ALLOW no arrowhead or arrowheads at both

Ea Ea correctly labelled end of Ea or Ec lines

AO1.1 Ea or Ec lines must reach maximum (or near to

Ec Ec correctly labelled with Ec < Ea maximum) on curve

For Ea, ALLOW AE OR AE

ALLOW marks for Ea and Ec for correctly labelled

endothermic diagram (i.e. ECF from ∆H)

Total 12

How to answer it

Investigation of Rates and Enthalpy in Zinc Reactions

What this question tests

This question assesses core physical chemistry concepts including collision theory, interpreting rate-time graphs to calculate rates via tangents, predicting the effect of temperature changes on reaction kinetics, metallic displacement reactions, and drawing/interpreting enthalpy profile diagrams with and without catalysts.

Question 3 (a) (i)

Explaining Reaction Rates via Collision Theory

✅ Correct Answer

(Acid) concentration decreases as HCl is consumed.
Fewer collisions per second / less frequent successful collisions occur between particles.
The reaction stops when the limiting reagent (acid) has been completely used up.

💡 Key Knowledge

Collision theory explanations require precise vocabulary. You must link concentration to frequency of collisions in a fixed volume. Saying "particles slow down" is incorrect because temperature remains constant.

❌ Common Errors

Students frequently lose marks by stating "fewer collisions" without specifying a time unit (must be per second or frequency). Another major error is mentioning "less surface area of solid", ignoring the fact that solid zinc carbonate is explicitly added in excess.

Mark allocation: 3 marks total (1 mark per key marking point).

Question 3 (a) (ii)

Calculating Rate from a Graph at a Given Time

📐 Step-by-Step Calculation

Step 1: Draw a precise tangent to the curve at exactly t = 50 s touching the curve at that point.
Step 2: Choose a large, clear triangle on your tangent line to minimise reading errors.
Step 3: Read the change in mass ( Δy ) and change in time ( Δx ). For example: Δy = 92.4 - 91.0 = 1.4 g and Δx = 220 s .
Step 4: Calculate the gradient: Rate = 1.4 / 220 = 6.36 × 10⁻³ g s⁻¹ .

🧠 Exam Technique

Examiners heavily penalize "interpolation" (taking a direct slope calculation from points on the curve instead of a constructed tangent line). Always draw your tangent spanning a wide interval to ensure accurate gradient calculation.

Mark allocation: 2 marks (1 for correct tangent drawn, 1 for correct gradient calculation within the accepted range of 5.7 × 10⁻³ to 6.9 × 10⁻³ g s⁻¹ ).

Question 3 (a) (iii)

Effect of Increased Temperature on Rate Curves

✅ Correct Sketch Guidance

The new curve must start at the exact same initial mass point ( 92.5 g ).
The initial slope must be steeper (representing a faster initial rate due to higher temperature and increased collision energy).
The curve must level off earlier at the exact same final mass ( ~91.55 g ) because the limiting reagent amount remains unchanged.

❌ Common Errors

Students often draw the final plateau higher or lower than the original curve. Remember that the total mass of carbon dioxide lost must be identical because the number of moles of limiting acid is identical.

Mark allocation: 2 marks.

Question 3 (b) (i)

Catalytic Displacement Reactions

✅ Correct Observations

Observation 1: More vigorous bubbling / fizzing occurs in the test tube with the copper(II) sulfate catalyst.
Observation 2: Zinc dissolves / disappears more quickly, OR a red/brown solid coats the zinc / blue solution turns colourless (due to copper displacement).

💡 Key Knowledge

A catalyst provides an alternative reaction pathway with a lower activation energy. In this heterogeneous/homogeneous transition, aqueous copper(II) ions interact with zinc metal to form a electrochemical couple, speeding up hydrogen gas evolution while depositing copper metal.

Mark allocation: 2 marks.

Question 3 (b) (ii)

Enthalpy Profile Diagram with Catalysts

✅ Correct Diagram Features

Products level: Drawn lower than reactants Zn(s) + H₂SO₄(aq) , labelled ZnSO₄ + H₂ , reflecting exothermic enthalpy change ( ΔH = -140 kJ mol⁻¹ ).
Enthalpy change (ΔH): Vertical arrow pointing downwards from the reactant energy level to the product energy level.
Activation energy without catalyst (Eₐ): Tall curve peak originating from reactants.
Activation energy with catalyst (Eᶜ): Lower curve peak beneath the uncatalyzed peak, clearly labelled.

🧠 Exam Technique

Do not use double-headed arrows for ΔH or activation energies; direction matters. The arrow for ΔH must point strictly downwards since the reaction is exothermic. Ensure activation energy peaks reach the maximum of their respective curves.

Mark allocation: 3 marks.

Topics

Module 3: Periodic table and energy · Module 5: Physical chemistry and transition elements · Practical Activity Groups · 3.2 Physical chemistry · 5.1 Rates, equilibrium and pH · PAG 9: Rates of reaction – continuous monitoring method

Question and mark scheme from the OCR A-Level Chemistry examination, AS Depth in chemistry (02), June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.