(a)

The iodination of benzene requires the presence of nitric acid.

(i)

Using bond enthalpies from the Data Booklet, calculate the enthalpy change for the following reaction.

(a)

(i)

What is meant by the term lattice energy?

(b)

The apparatus shown in the diagram can be used to measure the enthalpy change of formation of magnesium oxide, $\Delta H_{\mathrm{f}}^{\ominus}(\mathrm{MgO})$ .

List the measurements you would need to make using this apparatus in order to calculate $\Delta H_{\mathrm{f}}^{\ominus}(\mathrm{MgO})$ .

[ 3 ]

[Maximum number: 5]

Sulfuric acid is manufactured by the Contact process.
One stage in this process is the conversion of sulfur dioxide into sulfur trioxide in the presence of a heterogeneous catalyst of vanadium(V) oxide, $\mathrm{V}_{2} \mathrm{O}_{5}$ .

(a)

Some bond energies are given.

Use the data, and the enthalpy change for the conversion of sulfur dioxide into sulfur trioxide, to calculate a value for the S=O bond energy in $\mathrm{SO}_{3}$ .
S=O bond energy in $\mathrm{SO}_{3}=$ $\mathrm{kJ} \mathrm{mol}^{-1}$

The Contact process is usually carried out at a temperature of about $400^{\circ} \mathrm{C}$ and a pressure just above atmospheric pressure. Using a higher or lower temperature and pressure would affect both the rate of production of sulfur trioxide and the yield of sulfur trioxide.

[ 2 ]

(b)

A reaction pathway diagram for both the catalysed and uncatalysed reactions between $\mathrm{SO}_{2}$ and $\mathrm{O}_{2}$ is shown.

The letters A-E represent energy changes.
Complete the table by stating which letter, A-E, represents the energy change described.

The equation for this stage of the Contact Process is shown.

2 \mathrm{SO}_{2}(\mathrm{~g})+\mathrm{O}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{SO}_{3}(\mathrm{~g}) \quad \Delta H=-196 \mathrm{~kJ} \mathrm{~mol}^{-1}

[ 3 ]

(a)

When $\mathrm{SiCl}_{4}$ vapour is passed over Si at red heat, $\mathrm{Si}_{2} \mathrm{C} l_{6}$ is formed. $\mathrm{Si}_{2} \mathrm{C} l_{6}$ contains a Si - Si bond.
The reaction of $\mathrm{Si}_{2} \mathrm{Cl}_{6}$ and $\mathrm{Cl}_{2}$ re-forms $\mathrm{SiCl}_{4}$ .

\mathrm{Si}_{2} \mathrm{C} l_{6}(\mathrm{~g})+\mathrm{Cl}_{2}(\mathrm{~g}) \rightarrow 2 \mathrm{SiCl}_{4}(\mathrm{~g})

Use bond energy data from the Data Booklet to calculate $\Delta H^{\ominus}$ for this reaction.

\Delta H^{\ominus}=

[ 2 ]

[Maximum number: 22]

Ammonia, $\mathrm{NH}_{3}$ , is manufactured from nitrogen and hydrogen by the Haber process.

\mathrm{N}_{2}(\mathrm{~g})+3 \mathrm{H}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{NH}_{3}(\mathrm{~g}) \quad \Delta H=-92 \mathrm{~kJ} \mathrm{~mol}^{-1}

(a)

Some bond energies are given.
$\mathrm{N} \equiv \mathrm{N}=944 \mathrm{~kJ} \mathrm{~mol}^{-1}$ $\mathrm{H}-\mathrm{H}=436 \mathrm{~kJ} \mathrm{~mol}^{-1}$

(i)

Use the data to calculate a value for the N-H bond energy.

You must show your working.

\mathrm{N}-\mathrm{H} \text { bond energy }=\ldots \ldots \ldots \ldots \ldots \ldots . . . . . . . . . . . . . . . . k J ~ m o l-1

[Maximum number: 22]

Ammonia, $\mathrm{NH}_{3}$ , is manufactured from nitrogen and hydrogen by the Haber process.

\mathrm{N}_{2}(\mathrm{~g})+3 \mathrm{H}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{NH}_{3}(\mathrm{~g}) \quad \Delta H=-92 \mathrm{~kJ} \mathrm{~mol}^{-1}

(a)

Some bond energies are given.
$\mathrm{N} \equiv \mathrm{N}=944 \mathrm{~kJ} \mathrm{~mol}^{-1}$ $\mathrm{H}-\mathrm{H}=436 \mathrm{~kJ} \mathrm{~mol}^{-1}$

(i)

Use the data to calculate a value for the N-H bond energy.

You must show your working.

\mathrm{N}-\mathrm{H} \text { bond energy }=\ldots \ldots \ldots \ldots \ldots \ldots . . . . . . . . . . . . . . . . k J ~ m o l-1

[Maximum number: 1]

Enthalpy changes, $\Delta H$ , can be positive or negative.
Which row is correct?

$\Delta H$ positive

$\Delta H$ negative

atomisation

bond breaking

neutralisation

bond making

combustion

bond making

[Maximum number: 2]

The rate of the reaction $\mathrm{H}_{2}(\mathrm{~g})+\mathrm{I}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{HI}(\mathrm{g})$ is studied.

(a)

At 400 K the rate constant for the forward reaction is approximately 1000 times greater than the rate constant for the backward reaction. The overall orders of the forward and backward reactions are the same.

[ 2 ]

(i)

At 700 K the rate constant for the forward reaction is approximately 50 times greater than the rate constant for the backward reaction.

Use this information and the information in (c)(i) to deduce the signs of the $\Delta H$ values of the forward and backward reactions. Explain your answer.

[ 2 ]

(a)

When sulfur is heated under pressure with chlorine, the major product is $\mathrm{SCl}_{2}(\mathrm{C} l-\mathrm{S}-\mathrm{Cl})$ .

\mathrm{S}_{8}(\mathrm{~g})+8 \mathrm{Cl}_{2}(\mathrm{~g}) \rightarrow 8 \mathrm{SCl}_{2}(\mathrm{~g})

Use data from the Data Booklet to calculate the enthalpy change, $\Delta H$ , for this reaction. The eight sulfur atoms in the $\mathrm{S}_{8}$ molecule are all joined in a single ring by single bonds.

\Delta H=

$\mathrm{kJ} \mathrm{mol}^{-1}$

[ 2 ]

[Maximum number: 1]

Which statement about enthalpy changes is correct?

A

Enthalpy changes of atomisation are always negative.

B

Enthalpy changes of combustion are always positive.

C

Enthalpy changes of formation are always positive.

D

Enthalpy changes of neutralisation are always negative.

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