Edexcel A-Level Chemistry A2 11.6 How to I Obtain Data to Calculate the Order with Respect to the Reactants and the Hydrogen Ion in the Acid Catalysed Questions

Keep propanone and acid in excess when isolating iodine, test linear iodine loss for zero order and then use rate dependence on propanone and H⁺ to constrain the mechanism.

Exam points

Demonstrate zero order in iodine from a linear iodine-concentration–time relationship.
Deduce propanone and H⁺ orders from controlled initial-rate comparisons.
Use rate = k[propanone][H⁺] to identify species required before or in the slow step.

[Maximum number: 1]

Propanone reacts with iodine in both acidic and alkaline conditions.

The rate equation for the reaction between propanone and iodine in acidic conditions is

\text { rate }=\mathrm{k}\left[\mathrm{H}^{+}\right]\left[\mathrm{CH}_{3} \mathrm{COCH}_{3}\right]

The reaction was carried out at two different pH values, all other conditions remaining unchanged.

In the first reaction pH = 2.0
In the second reaction the rate was found to be $\frac{1}{3}$ of the original value.
What was the pH of the second reaction, to 1 decimal place?

0.7

1.5

2.5

2.7

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Edexcel A-Level Chemistry A2 11.6 How to I Obtain Data to Calculate the Order with Respect to the Reactants and the Hydrogen Ion in the Acid Catalysed Questions question 1