Surface Area & Rate of Reaction
Cambridge IGCSE Chemistry 0620 · Rates of Reaction · Core + Supplement
Section A
The Trend
Q1–5
Section B
Collision Theory
Q6–10
Section C
Real Hazards
Q11–15
🔑 THE GOLDEN MARK SCHEME FORMULA
For any "explain why surface area increases rate" question, the mark scheme always wants: Surface area increases → More particles exposed → Higher frequency of successful collisions → Faster rate.
The Trend
Which reacts faster with dilute hydrochloric acid: 10 g of large marble chips or 10 g of powdered marble? Give one reason.
A student measures the volume of CO₂ gas produced when excess marble chips react with 50 cm³ of dilute HCl. She repeats the experiment using the same mass of powdered marble. How does the initial rate of reaction change? How does the total volume of CO₂ produced change?
Iron wool burns much more vigorously than an iron nail in oxygen. Both have the same mass. Explain why.
A student investigates the rate of reaction between zinc and sulfuric acid using zinc granules, then repeats with zinc powder. Sketch the shape of the rate-of-production-of-hydrogen graph for both experiments on the same axes. Label each curve.
A pharmacist wants to make a drug dissolve more quickly in the stomach. Suggest how the tablet's physical form could be changed to achieve this.
Collision Theory Explanations
Explain, in terms of particles and collisions, why reducing the particle size of a solid reactant increases the rate of reaction.
A student says: 'When I grind the solid, the particles have more energy, which is why the reaction is faster.' Identify the error in this statement and write a correct explanation.
Calcium carbonate (marble chips) reacts with hydrochloric acid: CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g). A student uses 5 g of large marble chips. Explain, using collision theory, what happens to the rate of reaction as the experiment proceeds.
Two experiments are set up: Experiment 1 uses 2 g of sugar in 100 cm³ of water at 20°C. Experiment 2 uses 2 g of powdered sugar in 100 cm³ of water at 20°C. Compare the rates of dissolving and explain the difference.
Explain why a log on a campfire burns slowly, but wood shavings from the same log ignite almost instantly.
Industrial Hazards — Real-World Surface Area
Flour mills have exploded catastrophically when fine flour dust suspended in the air ignited. Explain why flour dust is so much more dangerous than a bag of flour, using the concept of surface area.
Coal dust explosions in mines have killed thousands of miners throughout history. Explain why coal dust suspended in mine air is far more dangerous than coal seams in the mine walls.
Explain why safety regulations in pharmaceutical factories require that airborne drug powder concentrations be kept below specific limits.
A sugar refinery experienced a catastrophic explosion in 2008 (Imperial Sugar, Georgia, USA) caused by sugar dust. (a) Explain why the sugar dust was far more reactive than bulk sugar. (b) Suggest TWO safety measures the factory should implement to prevent future explosions.
A student claims: 'Surface area is the most important factor controlling the rate of reaction for solid reactants.' Evaluate this claim. In your answer, consider whether surface area is more or less important than temperature and concentration for different reaction scenarios.
⚡ THE ULTIMATE SURFACE AREA RULE
Surface area ONLY changes the RATE of reaction — it NEVER changes the TOTAL YIELD. The amount of product is determined by the limiting reactant, not by how the solid is divided. Write this on your hand before every exam.
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