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9701/43

Chemistry 9701/43 — October/November 2020

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

10
questions
100
marks
120
minutes

Topics Transition Elements · Carboxylic Acids and Derivatives · Analytical Techniques · Reaction Kinetics · Chemical Energetics · Group 2 · +7 more

Q1Reaction KineticsFree sample

Nitrogen monoxide, NO, reacts with oxygen to form nitrogen dioxide, NO2\text{NO}_2.

2NO(g)+O2(g)⇌2NO2(g)2\text{NO(g)} + \text{O}_2\text{(g)} \rightleftharpoons 2\text{NO}_2\text{(g)}

The rate equation for the forward reaction is shown.

rate=k[NO]2[O2]\text{rate} = k[\text{NO}]^2[\text{O}_2]
(a)

Complete the following table.

the order of reaction with respect to [NO]
the order of reaction with respect to [O₂]
the overall order of reaction
1M
(b)

Two separate experiments are carried out at 30°C to determine the rate of the forward reaction.

experiment[NO] / mol dm−3\text{mol dm}^{-3}[O₂] / mol dm−3\text{mol dm}^{-3}rate / mol dm−3 s−1\text{mol dm}^{-3}\text{ s}^{-1}
10.003000.002001.51×10−41.51 \times 10^{-4}
20.005006.05×10−56.05 \times 10^{-5}
(i)

Use the data for experiment 1 to calculate the value of the rate constant, kk. State the units of kk.

2M
(ii)

Calculate the value of [NO] in experiment 2.

1M
(c)

Define the term rate-determining step.

1M
(d)

Peroxodisulfate ions, S2O82−\text{S}_2\text{O}_8^{2-}, react with iodide ions, I−\text{I}^-.

S2O82−+2I−→2SO42−+I2\text{S}_2\text{O}_8^{2-} + 2\text{I}^- \rightarrow 2\text{SO}_4^{2-} + \text{I}_2

The rate equation for the reaction in the absence of any catalyst is shown.

rate=k[S2O82−][I−]\text{rate} = k[\text{S}_2\text{O}_8^{2-}][\text{I}^-]
(i)

Suggest equations for a two-step mechanism for this reaction, stating which of the two steps is the rate-determining step.

2M
(ii)

A large excess of peroxodisulfate ions is mixed with iodide ions. Immediately after mixing, [I−]=0.00780 mol dm−3[\text{I}^-] = 0.00780 \text{ mol dm}^{-3}. Under the conditions used, the half-life of [I−][\text{I}^-] is 48 seconds.

Calculate the iodide ion concentration 192 seconds after the peroxodisulfate and iodide ions are mixed.

1M

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