Explaining what's happening
5 marks · HardA student compares the rates of reaction between two samples of marble chips (calcium carbonate) and dilute phosphoric acid at room temperature. In experiment 1, large marble chips are used; in experiment 2, the marble chips are crushed into a fine powder. The same mass of marble and the same volume of acid are used in both experiments.
State which experiment will have the faster initial rate of reaction, and give a reason using collision theory.
Draw a suitable diagram to show the difference in surface area between the marble chips in experiment 1 and the powder in experiment 2.
Explain, using collision theory, why the reaction stops even though there is excess marble in both experiments.
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.
Show answer
(a) — · must show Experiment with powdered marble is faster, Greater surface area leads to more frequent collisions
(b) — · must show Block diagram, e.g., large chip vs. small particles, Surface area clearly larger for powder
(c) — · must show Reaction stops when acid is all used up, No more acid particles left to collide with marble particles
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Correctly states: experiment 2 (powder) has faster initial rate.
M1 (a) · 1 mark · Explains: more surface area means more collisions per second, leading to faster reaction (collision theory).
B2 (b) · 1 mark · Suitable diagram: shows large chip vs. small powder pieces, or annotated sketch clearly indicating increased surface area.
M2 (c) · 1 mark · Explains: reaction stops when all acid is used up.
A2 (c) · 1 mark · Links stop to: no acid particles left to collide with marble, so no more successful collisions occur.
Worked solution
a) Experiment 2, with powdered marble, will have a faster initial rate. This is because the powder has a much greater surface area, so more acid particles can collide with marble particles per second. According to collision theory, this increases the chance of successful collisions.
c) The reaction stops when all of the acid has been used up. Once the acid particles are gone, there are no more collisions between acid and marble particles, so no further reaction can take place, even though marble is still present.