AQA A-Level Chemistry AS Paper 2, June 2025: Question 20

1 mark · Medium difficulty · Multiple Choice

Deduce which reaction conditions produce a curve with the same initial rate but double the volume of hydrogen gas collected compared to an initial reaction between magnesium and hydrochloric acid.

Practise this question

Question

Question 20 shows a line graph plotting the volume of hydrogen gas in cm³ against time in minutes. Curve R begins at the origin and levels off at approximately 4 major grid units vertically. Curve S starts at the origin with the exact same initial gradient as curve R, but levels off at exactly double the height (8 major units). Four multiple-choice options (A to D) vary the mass of magnesium ribbon (0.48 g vs 0.96 g) and the volume/concentration of hydrochloric acid (100 cm³ or 200 cm³ of 1.0 or 2.0 mol dm⁻³).
Question text

20 A student adds 0.48 g of magnesium ribbon to 100 cm3 of

1.0 mol dm–3 hydrochloric acid.

The student records the volume of hydrogen collected at regular time intervals.

Line R on the graph shows the results.

Which of these reactions gives results shown by line S?

Assume that the reactions occur at the same temperature.

[1 mark]

0.48 g of magnesium reacting with 200 cm3 of

A –3

1.0 mol dm hydrochloric acid

0.48 g of magnesium reacting with 100 cm3 of

B –3

2.0 mol dm hydrochloric acid

0.96 g of magnesium reacting with 200 cm3 of

C –3

1.0 mol dm hydrochloric acid

0.96 g of magnesium reacting with 200 cm3 of

D –3

2.0 mol dm hydrochloric acid

Mark scheme

Show the mark scheme Mark scheme table for question 20 showing the correct answer is C (0.96 g of magnesium reacting with 200 cm³ of 1.0 mol dm⁻³ hydrochloric acid), worth 1 mark with assessment objective AO3.

0.96 g of magnesium reacting with 200 cm3 of 1.0 mol dm–3

20 C 1 (AO3)

hydrochloric acid

How to answer it

AQA A-Level Chemistry • Physical Chemistry

Rates of Reaction & Stoichiometry (Graph Interpretation)

What this question tests

This multiple-choice question assesses your ability to combine limiting reagent stoichiometry with reaction kinetics. Specifically, you must determine:

  • How the total yield (plateau height) depends on the limiting reactant.
  • How the initial rate of reaction (initial gradient at t = 0) depends on reactant concentration.

Question 20 Analysis

Reaction: Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)

📐 Step 1: Analyse Initial Line R

  1. Moles of Mg:
    n(Mg) = mass / Aᵣ = 0.48 / 24.3 ≈ 0.020 mol
  2. Moles of HCl:
    n(HCl) = conc × vol = 1.0 × (100/1000) = 0.10 mol
  3. Identify the Limiting Reagent:
    Ratio is 1 Mg : 2 HCl, so 0.020 mol of Mg requires only 0.040 mol of HCl.
    HCl is in excess ( 0.10 > 0.040 mol ).
    Mg is the limiting reagent, producing 0.020 mol H₂ .

📐 Step 2: Analyse Curve S from the Graph

  • Final Volume (Plateau):
    Line R plateaus at 5 major grid lines.
    Line S plateaus at 10 major grid lines.
    Line S produces exactly double the volume of H₂ (0.040 mol H₂).
    Therefore, we need 2 × 0.48 g = 0.96 g of Mg that reacts completely.
  • Initial Rate (Gradient at t = 0):
    Notice the tangent at origin ( t = 0 ) for curve S is identical to curve R. They start with the same initial slope.
    Therefore, the initial concentration of HCl must remain unchanged at 1.0 mol dm⁻³.

✅ Correct Answer: Option C

0.96 g of magnesium reacting with 200 cm³ of 1.0 mol dm⁻³ hydrochloric acid

  • n(Mg) = 0.96 / 24.3 = 0.040 mol (doubled yield of H₂).
  • n(HCl) = 1.0 × 0.200 = 0.20 mol (HCl is still in large excess since only 0.080 mol is needed).
  • [HCl] = 1.0 mol dm⁻³ , so the initial collision frequency and initial rate are the same as Line R.

🧠 Exam Technique: Two-Step Elimination

  1. Check the Height (Total Amount):
    Plateau S is 2× Line R. Since Mg is limiting, doubling the gas requires doubling the mass of Mg (0.96 g).
    Immediately eliminate A and B.
  2. Check the Initial Gradient (Initial Rate):
    Both curves leave the origin along the exact same path. Same initial rate means same initial concentration of acid.
    Eliminate D (concentration was doubled to 2.0 mol dm⁻³).

❌ Common Errors & Misconceptions

  • Selecting D: Students see that the acid volume and mass doubled, and mistakenly think [HCl] must also double. Doubling [HCl] to 2.0 mol dm⁻³ would make the initial tangent twice as steep at the origin.
  • Selecting A or B: Forgetting to check which reagent is limiting. Since HCl was already in excess in the first experiment, increasing the acid volume or concentration without adding more Mg will produce no extra gas.
  • Confusing Rate with Yield: Remembering that concentration affects the gradient (rate), while the amount of limiting reactant determines the final height (yield).
Mark Scheme Breakdown: [1 mark] for selecting C (AO3 – Analysis and evaluation of experimental data).

Topics

Physical Chemistry · 3.1.2 Amount of Substance · 3.1.5 Kinetics

Question and mark scheme from the AQA A-Level Chemistry examination, AS Paper 2, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.