AQA A-Level Chemistry Paper 2, 2023: Question 1

7 marks · Easy difficulty · State/Explain/Numerical

Rates of Reaction This question explores reaction rates, focusing on the use of a conical flask with NaHCO₃ and acids, calculating the rate of CO₂ loss from experimental data, deducing units for the rate, sketching a reaction curve for chloroethanoic acid compared to ethanoic acid, and explaining why chloroethanoic acid is stronger.

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AQA A-Level Chemistry Paper 2, 2023: Question 1
Question text

01 This question is about rates of reaction.

Figure 1 shows apparatus used to measure the rate of reaction when an acid reacts

with an excess of solid sodium hydrogencarbonate, NaHCO3

When different monoprotic organic acids are used, the rates at which gas escapes

can be used to compare the strengths of the acids.

A timer is started when the NaHCO3 is added to the acid and

the mass of CO2 gas lost is recorded at regular intervals.

(It is assumed that any change in mass is due to the loss of CO2)

Figure 1

01.1 Suggest a reason why using a conical flask instead of a beaker would give more

accurate results in this experiment.

[1 mark]

Figure 2 shows the results of this experiment when 25.0 cm3 of a

2.23 mol dm–3 solution of ethanoic acid reacts with an excess of NaHCO

Figure 2

01.2 Use Figure 2 to calculate the rate of reaction at 2 minutes.

Deduce the units of your calculated rate.

[3 marks]

Rate Units

01.3 Chloroethanoic acid is a stronger acid than ethanoic acid.

Sketch, on Figure 2, the curve you would expect when 25.0 cm3 of a 2.23 mol dm–3

solution of chloroethanoic acid reacts with an excess of NaHCO3

Suggest why chloroethanoic acid is a stronger acid than ethanoic acid.

[3 marks]

Mark scheme

Show the mark scheme Mark scheme for AQA A-Level Chemistry Paper 2, 2023: Question 1

Question Answers Additional Comments/Guidelines Mark

Reduces loss of liquid droplets Allow description of reduction of loss of liquid 1

01.1

(AO3)

M1 Tangent drawn at 2 mins

Conseq to their M1

M2 Gradient of tangent = (0.50 +/– 0.05) If convert mins to sec M2 = 7.80 x 10-3 (7.0 x 10-3

to 8.6 x 10-3) and award M3 conseq

M3 g min–1

If M1 not awarded then allow average rate

01.2 calculated M2 = 1.05

(3 x AO2)

If M1 not awarded then allow average rate and if

120 sec used for time allow M2 =0.0175 and can

score M3 for g s-1

Penalise g/min

M1 Curve steeper at first & flattens at same point on y axis

M2 Cl is an electron withdrawing group or negative inductive effect

M3 Weakens the O-H bond / increase polarity of O-H bond

Allow opposite argument

M2 CH3 electron donating or positive inductive

effect

M3 Makes O-H bond stronger / decrease polarity

of O-H bond

Also allow answers that discuss the carboxylate 3

01.3 ion

M2 Cl Electron withdrawing group (3 x AO3)

M3 makes RCOO– less negative / delocalises the

negative charge more / more stable ion (so

RCOO– less likely to accept H+ )

How to answer it

Rates of Reaction

Question

(a) Suggest why using a conical flask instead of a beaker gives more accurate results in this experiment.

(b) Use Figure 2 to calculate the rate of reaction at 2 minutes and deduce the units of the rate.

(c) Sketch the curve for the reaction of chloroethanoic acid with NaHCO3 and explain why it is stronger than ethanoic acid.

Answer: Part (a)

A conical flask is used because:

  • It reduces the loss of liquid droplets during the reaction.
Misconception: Some students assume the conical flask affects the reaction rate itself. Its purpose is to minimise experimental error, not to change the chemistry.

Answer: Part (b)

To calculate the rate at 2 minutes:

  1. Draw a tangent to the curve at 2 minutes.
  2. Calculate the gradient of the tangent:
    Gradient = Δy / Δx = (0.50 ± 0.05) g / (0.6 ± 0.1) min = 0.83 g min-1
  3. Convert units if necessary:
    0.83 g min-1 = 0.0138 g s-1

Units: g min-1 (or g s-1 if converted).

Tip: Always use a ruler to draw tangents and ensure the gradient calculation uses accurate Δx and Δy values.

Answer: Part (c)

The curve for chloroethanoic acid would be steeper at the start but flatten at the same point on the y-axis. This is because chloroethanoic acid reacts faster but produces the same amount of CO2 (same molar ratio).

Explanation:

  • Chloroethanoic acid is stronger than ethanoic acid because the Cl atom withdraws electrons, increasing the polarity of the O-H bond.
  • This makes the O-H bond easier to break, increasing the release of H+ ions.
Misconception: Students may think the stronger acid produces more CO2. However, the volume of CO2 depends only on the moles of acid, not its strength.

Final Notes

Key points to remember:

  • Always consider experimental design to minimise errors (e.g., choice of apparatus).
  • Use tangents to determine reaction rates from graphs.
  • Acid strength depends on the molecule's structure, including electron-withdrawing or donating groups.

Topics

Physical Chemistry · Required Practicals · 3.1.3 Bonding · 3.1.5 Kinetics · Required Practical 7: Measuring the rate of reaction by an initial rate method

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 2, 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.