Chemistry 9701 · AS & A Level · Reaction kinetics

Reaction kinetics — practice question

In aqueous solution, iodide ions react with acidified hydrogen peroxide, as shown in reaction 1. reaction 1: $2\text{I}^- + \text{H}_2\text{O}_2 + 2\text{H}^+ \rightarrow \text{I}_2 + 2\text{H}_2\text{O}$ The rate law for reaction 1 is given below. $\text{rate} = k[\text{I}^-][\text{H}_2\text{O}_2]$
(a(i))[1]

Explain what is meant by order of reaction.

(a(ii))[2]

Complete Table 4.1 by giving the order of reaction for $[\text{H}^+]$, $[\text{I}^-]$, and $[\text{H}_2\text{O}_2]$, and therefore the overall order of the reaction.

(a(iii))[1]

On Fig. 4.1, sketch a line that shows how $[\text{I}^-]$ changes with time.

(b(i))[1]

Two experiments are used to measure the rate of reaction 2. In the first experiment, the initial rate is measured with known concentrations of $\text{NO}_2$ and $\text{O}_3$. In the second experiment, the concentrations of $\text{NO}_2$ and $\text{O}_3$ are both multiplied by four. Predict how the initial rate for reaction 2 changes.

(b(ii))[1]

The mechanism for reaction 2 contains two steps. Define rate-determining step.

(b(iii))[2]

Suggest equations for the two steps in the mechanism for reaction 2.

(c(i))[1]

From Fig. 4.2, calculate a value for the rate constant, $k$, for this decomposition.

(c(ii))[1]

Using your answer to c(i), calculate the half-life, $t_{\frac{1}{2}}$, in seconds, for this decomposition.

Worked solution & mark scheme

This 10-mark question has a full step-by-step worked solution and mark scheme. One marking point: Order is the exponent to which concentration is raised in the rate equation

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