Edexcel A-Level Chemistry Paper 3, November 2021: Question 4

10 marks · Medium difficulty · Practical Techniques and Data Analysis

Investigate the rate of reaction between calcium carbonate and hydrochloric acid using mass loss and gas collection methods.

Practise this question

Question

Question text

4 A series of experiments was carried out to investigate the factors which affect the

rate of reaction between calcium carbonate and dilute hydrochloric acid.

CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g)

• 50.0 cm3 of hydrochloric acid was added to 10 g of calcium carbonate (an excess)

in a conical flask placed on an electronic balance.

• The loss in mass of the flask and its contents was recorded every 30 seconds for

10 minutes.

• The experiment was repeated using different sized pieces of calcium carbonate,

a different concentration of hydrochloric acid or a different temperature.

Concentration of

Size of calcium Temperature

Experiment hydrochloric acid

carbonate −3 /°C

/ mol dm

1 small pieces 0.50 20

2 small pieces 0.50 60

3 one large piece 0.50 20

4 small pieces 1.00 20

The results of Experiment 1 are shown on the graph.

1.0

0.8

0.6

Experiment 1

Loss in

mass /g

0.4

0.2

0 120 240 360 480 600

Time /s

(a) Draw curves on the graph to show the results you would expect for

Experiments 2, 3 and 4. Label the curves 2, 3 and 4.

(3)

8 (b) Determine the initial rate of reaction for Experiment 1.

You must show your working on the graph.*P67806A0836*

Include units in your answer.

(3)

1.0

0.8

0.6

Experiment 1

Loss in

mass /g

0.4

0.2

0 120 240 360 480 600

Time /s

Initial rate of reaction …

experiment that involved collecting the gas produced.

Outline the procedure that the student could use, including a diagram and the

measurements needed.

(4)

*P67806A0936*

(Total for Question 4 = 10 marks)

Mark scheme

Show the mark scheme

Question Answer Additional Guidance Mark

Number

4(a) Examples of curves (3)

• curve for Experiment 2 steeper than (1)

Experiment 1 and Experiment 4

and

finishing at the same mass loss as Experiment 1

• curve for Experiment 3 shallower than (1)

Experiment 1

and

finishing at the same mass loss

• curve for Experiment 4 steeper than (1)

Experiment 1 Allow curve for Experiment 4 steeper than Experiments 1 and 2

and and finishing at mass loss 1.0 g

finishing at mass loss 1.0 g

Number

4(b) Example of tangent (3)

• tangent drawn to curve when time = 0

tangent must touch curve for at least first 3

small squares on x axis and extend to at

least 120 s (1)

• calculation of gradient (1) gradient = 1.0 = 3.33 × 10-3 Allow 3.13 × 10-3 to 3.53 × 10-3

TE on tangent drawn or measurements from line on graph with no

tangent

Ignore SF including 1SF

• units (1)

g s−1 or g/s

stand alone mark

Number

4(c) An answer that makes reference to the following Example of diagram (4)

points:

• a diagram showing (calcium carbonate in a conical)

flask attached to a gas syringe / a delivery tube

passing into a container of water with an upturned

measuring cylinder (1)

Ignore missing labels

Ignore heat / water bath

Do not award inclusion of a condenser

Do not award test tube or beaker for collecting gas

• add the hydrochloric acid and (immediately) stopper (1) Allow carbonate added to acid and stopper the flask

the flask Allow acid in a tube / beaker in the flask and tip the flask

for them to mix

• record the volume of gas (1)

• collected at regular time intervals (1) Allow specified time intervals

Allow collected in a specified time

(Total for Question 4 = 10 marks)

How to answer it

Investigating Rates of Reaction

What this question tests

This question evaluates your ability to interpret graphical kinetic data, sketch predictive curves based on changes in temperature, surface area, and concentration, calculate initial rates of reaction using tangents, and design an alternative gas-collection experimental procedure.

Part (a): Sketching Predictive Reaction Curves

Understanding the effect of varying conditions on rate and yield

✅ Correct Answers (3 Marks)

Curve 2: Steeper than Experiment 1, but leveling off at the same final mass loss (0.50 g).
Curve 3: Shallower than Experiment 1, but leveling off at the same final mass loss (0.50 g).
Curve 4: Steeper than Experiment 1, but reaching a higher final mass loss of 1.0 g (due to doubled HCl concentration and excess CaCO₃).

💡 Key Knowledge

Temperature (Exp 2): Higher temperature increases particle kinetic energy and successful collision frequency, resulting in a steeper initial curve.
Surface Area (Exp 3): One large piece decreases surface area compared to small pieces, lowering the frequency of collisions and making the curve shallower.
Concentration (Exp 4): Doubling the HCl concentration doubles the moles of acid available, meaning double the total volume of CO₂ gas is evolved (yielding 1.0 g mass loss).

🧠 Exam Technique

When sketching curves representing increased limits (like Experiment 4), ensure your final plateau is precisely proportional to the limiting reagent change. Here, doubling concentration doubles the total mass loss from 0.50 g to 1.0 g.

❌ Common Errors

Students frequently fail to realize that changing temperature (Exp 2) or surface area (Exp 3) does not change the total amount of product formed if the limiting reagent stays the same. They incorrectly alter the final plateau height.

Part (b): Determining Initial Rate of Reaction

Using tangents on kinetic curves

📐 Step-by-Step Calculation

Step 1: Draw a precise tangent to the curve at time = 0 s. It must touch the curve accurately for at least the first 3 small squares.
Step 2: Extend the tangent line across the graph grid to form a large triangle (e.g., up to t = 300 s).
Step 3: Read off the change in mass (Δy) and change in time (Δt) from your straight-line construction.
Step 4: Calculate the gradient: Gradient = Δy / Δt = 1.0 / 300 = 3.33 × 10⁻³ .

✅ Final Answer & Mark Breakdown (3 Marks)

Mark 1: Correctly drawn tangent at t = 0 s.
Mark 2: Accurate calculation of the gradient (accepted range: 3.13 × 10⁻³ to 3.53 × 10⁻³ ).
Mark 3: Correct units: g s⁻¹ (or g/s ).

❌ Common Errors & Traps

A major loss of marks occurs when students calculate average rate over a time interval (e.g., dividing 0.50 by 240) instead of drawing a tangent for the initial rate at t = 0. Always construct a large triangle to minimize gradient reading errors.

Part (c): Designing an Alternative Gas-Collection Method

Practical setup for measuring gas evolution

✅ Required Procedure Points (4 Marks)

Mark 1 (Apparatus): A diagram showing a conical flask containing calcium carbonate connected via a bung and delivery tube to either a gas syringe OR an inverted measuring cylinder/burette in a trough of water.
Mark 2 (Mixing): Add the hydrochloric acid and immediately insert/secure the bung to prevent gas escape.
Mark 3 (Measurement): Measure/record the volume of gas produced.
Mark 4 (Timing): Collect measurements at regular, specified time intervals.

💡 Examiner Guidance & Setup Tips

Keep your apparatus diagrams simple and clearly labeled. Ensure gas-tight seals are implied (bung tightly fitted into the conical flask). Avoid unworkable setups like using standard test tubes instead of robust gas collection vessels or water troughs.

Topics

Physical Chemistry · Core Practicals · Core Practical 1: Measuring the molar volume of a gas · Topic 9: Kinetics I

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 3, November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.