Chemistry 9701/42 — February/March 2019

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

questions

100

marks

120

minutes

Topics Equilibria · Group 2 · Chemical Energetics · Transition Elements · Organic Synthesis · Carboxylic Acids and Derivatives · +8 more

Q1Group 2Chemical EnergeticsTransition ElementsEquilibriaReaction KineticsFree sample

(a)

State one natural and one man-made occurrence of oxides of nitrogen.

(b)

Under conditions of high pressure and a catalyst, nitrogen monoxide, $\text{NO}$ , forms two other oxides of nitrogen, dinitrogen monoxide, $\text{N}_2\text{O}$ , and dinitrogen trioxide, $\text{N}_2\text{O}_3$ .

...... \text{NO}(\text{g}) \rightarrow ...... \text{N}_2\text{O}(\text{g}) + ...... \text{N}_2\text{O}_3(\text{g}) \quad \Delta H^\ominus = -195.2 \text{ kJ mol}^{-1}, \quad \Delta G^\ominus = -102.8 \text{ kJ mol}^{-1}

(i)

Balance the equation above for the formation of $\text{N}_2\text{O}$ and $\text{N}_2\text{O}_3$ from $\text{NO}$ .

(ii)

State how the oxidation number of nitrogen changes during this reaction.

$\text{NO} \rightarrow \text{N}_2\text{O}$ from ............................... to ...............................

$\text{NO} \rightarrow \text{N}_2\text{O}_3$ from ............................... to ...............................

(iii)

Calculate the entropy change for the reaction at $298\text{ K}$ . Include the units in your answer.

\Delta S^\ominus = \text{..............................}

\text{units} = \text{..............................}

(iv)

State whether the sign of $\Delta S^\ominus$ calculated in (iii) agrees with that predicted from your balanced equation in (i). Explain your answer.

(c)

At room temperature $\text{N}_2\text{O}_3$ dissociates.

\text{N}_2\text{O}_3(\text{g}) \rightleftharpoons \text{NO}(\text{g}) + \text{NO}_2(\text{g})

(i)

Write the expression for $K_p$ for this equilibrium. Include the units in your answer.

K_p = \text{..............................}

\text{units} = \text{..............................}

(ii)

A $1.00\text{ dm}^3$ flask at $25^\circ\text{C}$ is filled with pure $\text{N}_2\text{O}_3(\text{g})$ at an initial pressure of $0.60\text{ atm}$ .
At equilibrium, the partial pressure of $\text{NO}_2(\text{g})$ is $0.48\text{ atm}$ .

Calculate the partial pressures of $\text{NO}(\text{g})$ and $\text{N}_2\text{O}_3(\text{g})$ at equilibrium. Hence calculate the value of $K_p$ at $25^\circ\text{C}$ .

p(\text{NO}(\text{g})) = \text{.........................................}

p(\text{N}_2\text{O}_3(\text{g})) = \text{.........................................}

K_p = \text{.........................................}

(d)

NO reacts readily with oxygen.

2\text{NO}(\text{g}) + \text{O}_2(\text{g}) \rightarrow 2\text{NO}_2(\text{g})

The table shows how the initial rate of this reaction at $25^\circ\text{C}$ depends on the initial concentrations of the reactants.

$[\text{NO}(\text{g})]$ / $\text{mol dm}^{-3}$	$[\text{O}_2(\text{g})]$ / $\text{mol dm}^{-3}$	initial rate / $\text{mol dm}^{-3}\text{ s}^{-1}$
0.100	0.0500	3.50
0.0500	0.100	1.75
0.0500	0.0500	0.875

(i)

Deduce the order of reaction with respect to each reactant. Explain your reasoning.

order with respect to $[\text{NO}(\text{g})]$ : ..............................................................................................

order with respect to $[\text{O}_2(\text{g})]$ : ................................................................................................

(ii)

State the rate equation for this reaction. Use the rate equation to calculate the rate constant. Include the units for the rate constant in your answer.

\text{rate} = \text{..............................}

\text{rate constant, } k = \text{..............................}

\text{units of } k = \text{..............................}

(e)

NO reacts with iron pentacarbonyl, $\text{Fe}(\text{CO})_5$ , as shown. NO and CO are both monodentate ligands.

\text{Fe}(\text{CO})_5 + 2\text{NO} \rightarrow \text{Fe}(\text{CO})_2(\text{NO})_2 + 3\text{CO}

During this reaction the co-ordination number of the iron changes.

(i)

State what is meant by the term co-ordination number.

(ii)

Describe how the co-ordination number of the iron changes during this reaction.

from ............................................................... to ...............................................................

(iii)

Only one stereoisomer of $\text{Fe}(\text{CO})_2(\text{NO})_2$ exists.

Use this information to suggest the geometry of the complex.

(f)

The complex $\text{Ru}(\text{NO})\text{L}_2\text{Cl}_3$ exists in three isomeric forms. L represents the monodentate ligand $\text{C}_6\text{H}_5\text{P}(\text{CH}_3)_2$ .

(i)

Complete the three-dimensional diagrams to show the three isomers of $\text{Ru}(\text{NO})\text{L}_2\text{Cl}_3$ .

Fig. 1.1

(ii)

Suggest the type of isomerism shown.

The rest of this paper

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