The graph in Fig. 12.1 shows the volume of gas collected against time for the reaction of the same mass of marble chips with dilute hydrochloric acid in two...
Question 1 Report
The graph in Fig. 12.1 shows the volume of gas collected against time for the reaction of the same mass of marble chips with dilute hydrochloric acid in two experiments, A and B. Experiment B used a higher concentration of acid.
Fig. 12.1
(a) Use the graph to find the volume of gas collected in experiment A after 20 s. [1]
(b) State which experiment, A or B, was faster. [1]
(c) Explain how the graph shows which experiment was faster. [1]
(d) Both curves level off at the same final volume of gas. Explain why. [2]
(e) Use the graph to describe how the rate of reaction in experiment A changes with time. [2]
(f) Suggest one reason why experiment B is faster than experiment A. [1]
Answer Details
This question tests reading a volume-of-gas against time graph and explaining rate from the shape of the curves. The steeper the curve, the faster the reaction; the height where it levels off shows the total gas made.
(a) Reading curve A at 20 s gives about 66 cm3 (accept 64 to 68) [1]. Go up from 20 s on the time axis to curve A, then across to the volume axis.
(b) Experiment B was faster [1].
(c) The graph shows this because curve B is steeper (rises more quickly) and reaches its final volume in a shorter time than curve A [1].
(d) Both curves level off at the same final volume because the same mass of marble chips (calcium carbonate) and the same limiting amount of reactant is used in each [1], so the same number of moles of carbon dioxide gas is produced in total [1]. A higher concentration changes the speed, not the amount.
(e) In experiment A the rate is fastest at the start, where the curve is steepest, because the acid is most concentrated then [1]; the rate then slows and finally stops when the curve becomes horizontal and no more gas is made, as a reactant is used up [1].
(f) Experiment B is faster because a higher concentration of acid means more acid particles in the same volume, so there are more frequent collisions with the marble chips [1].
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