Rates of reaction – IGCSE Chemistry exam questions with mark scheme

1Grade 4–54 marks

Magnesium ribbon reacts with dilute hydrochloric acid.

Explain, in terms of particles, why the reaction is faster at a higher temperature. [4]

particles have more (kinetic) energy / move faster(1)
particles collide more often / more frequent collisions / more collisions per second (ignore 'more collisions' alone)(1)
more particles / collisions have energy equal to or greater than the activation energy (allow a greater proportion of collisions are successful)(1)
so more successful collisions per unit time / higher frequency of successful collisions(1)
Answer: particles move faster, collide more often and more collisions have energy ≥ activation energy
Examiner tip: 'More collisions' is not enough — you need 'more FREQUENT' collisions and more with energy above the activation energy.

2Grade 6–74 marks

A student adds marble chips (calcium carbonate) to excess dilute hydrochloric acid and measures the volume of carbon dioxide given off. The graph shows the results.

(a) Calculate the mean rate of reaction during the first 20 seconds. Give the unit. [2]

(b) Explain why the gradient of the graph decreases and eventually becomes zero. [2]

(a) 46 ÷ 20 = 2.3 (allow a reading of 45–47 cm³, giving 2.25–2.35)(1)
(a) cm³/s (accept cm³ s⁻¹ / cm³ per second)(1)
(b) reactants are used up / concentration of acid decreases / marble chips get smaller, so there are fewer collisions per second (so rate decreases)(1)
(b) gradient becomes zero when all the calcium carbonate has reacted / reaction stops because the marble chips are used up (reject 'acid used up' — the acid is in excess)(1)
Answer: (a) 2.3 cm³/s (b) reactants used up so fewer collisions per second; reaction stops when all the CaCO₃ has reacted
Examiner tip: Read the graph carefully at exactly 20 s, and always include the unit — it is often a separate mark.

3Grade 4–54 marks

Hydrogen peroxide decomposes slowly to form water and oxygen. Manganese(IV) oxide is a catalyst for this reaction.

(a) Explain how a catalyst increases the rate of a reaction. [2]

(b) Describe how a student could show that manganese(IV) oxide is not used up in the reaction. [2]

(a) provides an alternative pathway / route (allow so more particles / collisions have energy equal to or greater than the activation energy)(1)
(a) with a lower activation energy (allow 'lowers the activation energy')(1)
(b) weigh the manganese(IV) oxide before the reaction and filter, dry and reweigh it after(1)
(b) mass is unchanged / the same(1)
Answer: (a) alternative pathway with lower activation energy (b) weigh before; filter, dry, reweigh – same mass
Examiner tip: Catalysts do NOT give particles more energy — they lower the activation energy.

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