9701 MCQ Solution 12

Question 12: [Physical > Reaction Kinetics]:

A piece of zinc foil dissolved completely in 20cm³ of a dilute sulphuric acid solution and the volume of hydrogen evolved was noted at equal, short time intervals. Another piece of zinc foil of the same surface area and mass was added to 40cm³of the same solution of dilute sulphuric acid.

How will the initial rate of reaction and the total volume of hydrogen evolved in this second experiment compare to the first experiment?

	initial rate of reaction	total volume of hydrogen evolved
A	no change	decrease
B	no change	no change
C	increase	no change
D	increase	increase

Reference: Past Exam Paper – November 2002 Paper 1 Q12

Solution 12:

Answer: B.

In the first experiment the piece of zinc foil completely dissolved in the 20cm³ solution. Thus we know that the solution is in excess already. Increasing the amount of solution will not produce more hydrogen gas since the amount of zinc, the limiting reactant, is still the same. Thus options A and D are eliminated. 

The zinc metal used in the second experiment had the same surface area as the piece of metal in the first experiment. This means that the contact point on which reactions can occur for both experiments are the same. Hence the rate of the reaction remains the same. This eliminates option C, leaving option B as the correct answer.