5

5a. [1 mark] 

Students were asked to investigate how a change in concentration of hydrochloric acid, HCl, affects the initial rate of its reaction with marble chips, CaCO3.

They decided to measure how long the reaction took to complete when similar chips were added to 50.0 cm3 of 1.00 mol dm−3 acid and 50.0 cm3 of 2.00 mol dm−3 acid.

Two methods were proposed:

(1) using small chips, keeping the acid in excess, and recording the time taken for the solid to disappear

(2) using large chips, keeping the marble in excess, and recording the time taken for bubbles to stop forming.

Annotate the balanced equation below with state symbols.

CaCO3(__) + 2HCl(__) → CaCl2(__) + CO2(__) + H2O(__)

 5b. [1 mark] 

Neither method actually gives the initial rate. Outline a method that would allow the initial rate to be determined.

 5c. [1 mark] 

Deduce, giving a reason, which of the two methods would be least affected by the chips not having exactly the same mass when used with the different concentrations of acid.

 5d. [1 mark] 

State a factor, that has a significant effect on reaction rate, which could vary between marble chips of exactly the same mass.

 5e. [1 mark] 

A group recorded the following results with 1.00 mol dm−3 hydrochloric acid:

Justify why it is inappropriate to record the uncertainty of the mean as ±0.01 s.