Rates of Reaction
5 marks · EasySome students investigate how quickly a metal carbonate reacts with dilute acid. They want to investigate the effect of changing the surface area of the solid on the rate of reaction.
Describe how the students could carry out an experiment to investigate the effect of surface area on the rate of reaction between calcium carbonate and dilute hydrochloric acid.
State one way the results from this experiment could be measured.
Explain, using collision theory, why smaller pieces of calcium carbonate react faster than large pieces when added to hydrochloric acid.
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(a) Method describes same quantities of acid and marble, but marble in different particle sizes/powder; measure how quickly reaction proceeds. · must show Use same mass of large chips and powder, React with same volume and concentration of acid
(b) Measure mass lost or volume of gas produced over time · must show Mass loss OR gas volume collection
(c) Smaller pieces have greater surface area; more frequent collisions between reactant particles; increases rate. · must show Increased surface area, More collisions
Common mistakes: Forgetting to mention control variables; Mixing up rate and total volume
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Marking points
M1 (a) · 1 mark · Describes controlling variables: using same mass/type of calcium carbonate, same volume and concentration of acid.
M2 (a) · 1 mark · Mentions comparing different surface areas (e.g. chips vs powder) and starting reaction the same way.
B1 (b) · 1 mark · States mass loss or gas collection (volume) as a way of measuring rate.
E1 (c) · 1 mark · Explains increased surface area leads to more collisions between particles.
E2 (c) · 1 mark · Relates increased collisions to faster reaction/rate.
Worked solution
a) Use two samples of calcium carbonate with equal mass, one as large chips and one powdered. Add to separate flasks with the same volume and concentration of hydrochloric acid. Mix and observe the rate.
b) The rate can be measured by recording the loss of mass using a balance, or by collecting the volume of carbon dioxide gas produced over time.
c) Smaller pieces have a larger surface area than large ones, so there are more places where collisions can happen. More frequent effective collisions between acid and marble particles result in a faster reaction.