Rates of Reaction
6 marks · NormalA student investigates the reaction between zinc granules and dilute hydrochloric acid. She wants to find out how both the surface area of zinc and the concentration of acid affect the rate of reaction.
State what will happen to the rate of reaction if she uses powdered zinc instead of granules, keeping everything else the same.
Explain, in terms of particles and collisions, why your answer in (a) is correct.
Next, she repeats the experiment using the same mass of zinc powder but halves the concentration of hydrochloric acid. Predict how the rate of reaction changes, compared to the experiment with original concentration.
Explain your answer in (c).
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) The rate increases
(b) Powdered zinc has a greater surface area so more particles are exposed; there are more successful collisions per second. · must show reference to more particles exposed or surface area, reference to more (frequent) collisions per second
(c) The rate decreases
(d) The concentration of acid is lower, so there are fewer acid particles in the same volume. This means successful collisions happen less frequently per second. · must show reference to fewer particles or lower concentration, reference to fewer collisions per second or less frequent collisions
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Correctly states that the rate increases when using powdered zinc.
B1 (b) · 1 mark · Mentions increased surface area or more particles exposed with powder.
B2 (b) · 1 mark · Mentions more frequent or more collisions per second between zinc and acid particles.
B1 (c) · 1 mark · Correctly predicts the rate decreases when concentration is halved.
B1 (d) · 1 mark · Mentions fewer acid particles or lower concentration in the same volume.
B2 (d) · 1 mark · Mentions fewer collisions per second (frequency is less) between acid and zinc.
Worked solution
a) The rate increases when powdered zinc is used.
b) Powdered zinc has a greater surface area than granules. This means more zinc particles are exposed for reaction with acid, so there are more successful collisions every second.
c) The rate decreases when the concentration of acid is halved.
d) Lower concentration means fewer acid particles in the same volume, so collisions with zinc become less frequent every second, resulting in a slower rate.