Changing the surface area of the reactants
5 marks · HardA student conducts an experiment to discover how changing the surface area of a solid affects the rate of its reaction with a liquid. She reacts 4.0 g of zinc with 25 cm³ of excess dilute sulfuric acid. She performs the same reaction twice:
Describe a safe method she could use to compare the reaction rate using zinc granules and zinc powder, and state a measurement that would provide evidence for which reaction is faster.
Explain, in terms of particle collisions, why the reaction rate changes when using zinc powder instead of granules.
Suggest one other variable, apart from surface area, that must be kept the same to ensure a fair comparison.
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(a) Use equal masses of zinc granules and powder in separate conical flasks with identical volumes and concentrations of acid. Measure and compare the time taken for the reaction to finish or the rate at which hydrogen gas is produced.
(b) Zinc powder has a greater surface area so more particles are exposed to the acid, leading to more frequent successful collisions per second, so the reaction is faster.
(c) Temperature or concentration of sulfuric acid (or volume of sulfuric acid).
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Describes using both forms (granules and powder) with same mass/acid volume in a conical flask.
A1 (a) · 1 mark · Names a correct measurement to compare rate (e.g. time for reaction, volume of gas produced over time).
M2 (b) · 1 mark · States zinc powder has greater surface area/more particles exposed.
A2 (b) · 1 mark · Links greater surface area to more frequent (successful) collisions, increasing rate.
B1 (c) · 1 mark · Names any other variable that should be kept constant (e.g., temperature, acid concentration).
Worked solution
a) The student should add 4.0 g of zinc granules to a conical flask with 25 cm³ of dilute sulfuric acid, repeat with 4.0 g zinc powder in a fresh flask. She can measure the volume of hydrogen gas given off every 30 seconds, or time until bubbling stops.
b) Zinc powder has a larger surface area than granules. More particles are in contact with acid, so collisions between zinc and acid particles happen more often. This increases the rate of reaction.
c) The temperature of the acid (or concentration or volume) must be kept constant to ensure a fair comparison.