Suggest how the change of iodine concentration could be followed.

A student produced these results with [H⁺] = 0.15 mol\(\,\)dm⁻³. Propanone and acid were in excess and iodine was the limiting reagent.

Determine the relative rate of reaction when [H⁺] = 0.15 mol\(\,\)dm⁻³.

The student then carried out the experiment at other acid concentrations with all other conditions remaining unchanged.

State and explain the relationship between the rate of reaction and the concentration of acid.

use a colorimeter/monitor the change in colour
OR
take samples AND quench AND titrate «with thiosulfate»

Accept change in pH.
Accept change in conductivity.
Accept other suitable methods.
Method must imply “change”.

[1 mark]

best fit line

relative rate of reaction = «\(\frac{{ - \Delta y}}{{\Delta x}} = \frac{{ - \left( {0.43 - 0.80} \right)}}{{50}}\) =» 0.0074/7.4 x 10⁻³

Best fit line required for M1.

M2 is independent of M1.

Accept range from 0.0070 to 0.0080.

[2 marks]

Relationship:
rate of reaction is «directly» proportional to [H⁺]
OR
rate of reaction \(\alpha \) [H⁺] 

Explanation:
more frequent collisions/more collisions per unit of time «at greater concentration»

Accept "doubling the concentration doubles the rate".

Do not accept “rate increases as concentration increases”.

Do not accept collisions more likely.

[2 marks]