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2022 O Level Chem P1 Q18 — Rate of Reaction Solution

Rate of Reaction 2022 O Level Pure Chemistry · Paper 1 · Q18

Question

In experiment 1, 1.0 g of zinc granules and 50 cm³ of 0.50 mol/dm³ sulfuric acid react to produce hydrogen gas. The volume of hydrogen produced is measured. The results for experiment 1 are plotted on the graph. The experiment is repeated with one change. The results for experiment 2 are plotted on the graph. Which change is made in experiment 2? Question diagram

Question diagram

AA catalyst is used.
BMore zinc granules are used.
CThe reaction is at a higher temperature.
DThe sulfuric acid is more concentrated.

Correct answer: B

Video Solution

Explanation

Answer: B. Curve 2 starts steeper and finishes at a higher plateau than curve 1. The steepness is the rate at \(t = 0\); the plateau height is the total volume of \(\text{H}_2\) produced when the reaction is complete (so it tells you how many moles of limiting reactant there were).

If experiment 2 produces more total \(\text{H}_2\), then it had more limiting reactant. The acid was fixed at 50 cm³ of 0.50 mol/dm³ → 0.025 mol acid → 0.0125 mol Zn needed. Increasing zinc (option B) past the original 1.0 g still gives more total \(\text{H}_2\) because experiment 1's zinc was the limiting reactant.

A (catalyst) and C (higher temperature) speed the reaction up but don't change the plateau — they'd give a steeper curve to the same height. D (more concentrated acid) increases the moles of acid but doesn't help if zinc was already limiting. Only B explains both a steeper start AND a higher plateau.

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