C3 2018

Question 1

Explain why the use of a large excess of H2O2 and I- means that the rate of reaction at a fixed temp depends only on the concentration of H+

Answer 1

Conc of H2O2 and I- are negligible / constant
So is zero order

Question 2

Samples of reaction mixture are removed at timed intervals and titrated with alkali to determine the conc of H+
State and explain what must be done to each sample before it is titrated with alkali

Answer 2

Quench by dilution

Question 3

In aq solution A B C and D are all colourless but E is dark blue
X is available that reacts rapidly with E

Answer 3

Measure some reagents
Measure X using colorimeter
Into separate container

Start clock timer at point of mixing
Time recorded for appearance of blue colour
Use of same conc of B and C
Samp temp
Repeat with diff conc of A

1/t taken as measure of rate
Plot 1/time against volumes of A
Interpret of time vs conc graph

Question 4

Suggest and explain another change to the experiment that would decrease the percentage uncertainty in the use of the same thermometer

Answer 4

Increase conc of acid so that its larger
Increase magnitude of temp change