Chemistry 9701/43 — October/November 2017

Cambridge A-Level · A Level Structured Questions · worked solutions for every part, with the mark scheme

questions

100

marks

120

minutes

Topics Transition Elements · Electrochemistry · Equilibria · Introduction to A Level Organic Chemistry · Nitrogen Compounds · Reaction Kinetics · +5 more

Q1ElectrochemistryTransition ElementsReaction KineticsFree sample

The compound nitrosyl bromide, NOBr, can be formed by the reaction shown.

2\text{NO} + \text{Br}_2 \rightleftharpoons 2\text{NOBr}

(a)

Using oxidation numbers, explain why this reaction is a redox reaction.

(b)

Nitrosyl bromide contains a trivalent nitrogen atom.

Draw the ‘dot-and-cross’ diagram for NOBr. Show outer electrons only.

(c)

The rate of the reaction was measured at various concentrations of the two reactants, NO and $\text{Br}_2$ , and the following results were obtained.

experiment	[NO] / $\text{mol dm}^{-3}$	[ $\text{Br}_2$ ] / $\text{mol dm}^{-3}$	initial rate / $\text{mol dm}^{-3}\text{ s}^{-1}$
1	0.03	0.02	$3.4 \times 10^{-3}$
2	0.03	0.04	$6.8 \times 10^{-3}$
3	0.09	0.04	$6.1 \times 10^{-2}$
4	0.12	0.06	to be calculated

The general form of the rate equation for this reaction is as follows.

\text{rate} = k[\text{NO}]^a[\text{Br}_2]^b

(i)

What is meant by the term order of reaction with respect to a particular reagent?

(ii)

Use the data in the table to deduce the values of $a$ and $b$ in the rate equation.
Show your reasoning.

(iii)

Use the data in the table to calculate the initial rate for experiment 4.

(iv)

Use the results of experiment 1 to calculate the rate constant, $k$ , for this reaction.
Include the units of $k$ .

(v)

By considering the rate equation, explain why the rate decreases with decreasing temperature.

(d)

The reaction between X and Y was studied.

2\text{X} + \text{Y} \rightarrow \text{Z}

The following sequence of steps is a proposed mechanism for the reaction.

\begin{aligned} \text{step 1} &\quad 2\text{X} \rightarrow \text{V} \\ \text{step 2} &\quad \text{V} + \text{Y} \rightarrow \text{Z} \end{aligned}

The general form of the rate equation for this reaction is as follows.

\text{rate} = k[\text{X}]^m[\text{Y}]^n

Step 1 is the slower step in the mechanism.

Deduce the values of $m$ and $n$ in the rate equation.

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