CAIE A-Level Chemistry AS 5.1 Enthalpy Change H Questions

Practise interpreting ΔH signs, standard enthalpy definitions, pathway diagrams and energy calculations.

Exam points

distinguish exothermic negative ΔH from endothermic positive ΔH using system energy change
write standard enthalpy equations with correct stoichiometry, states and one mole of the defined product or process
read reaction profiles and calculate energy changes from bond energies or enthalpy data

Covered syllabus points

5.1.1—Chemical reactions are accompanied by enthalpy 5.1.2—Construct/interpret a reaction pathway diagram 5.1.3—Terms 5.1.4—Energy transfers occur during chemical 5.1.5—Bond energies and ΔHr 5.1.6—Some bond energies are exact and some bond 5.1.7—Enthalpy changes from appropriate experimental

[Maximum number: 1]

Which statement about enthalpy changes is correct?

Enthalpy changes of reaction are always negative.

Enthalpy changes of combustion are always positive.

Enthalpy changes of formation are always positive.

Enthalpy changes of neutralisation are always negative.

[Maximum number: 1]

For a particular reversible reaction the backward reaction is endothermic.
The activation energy of the backward reaction is $160 \mathrm{~kJ} \mathrm{~mol}^{-1}$ .
It can be assumed that the backward reaction proceeds by a mechanism that is the exact reverse of the mechanism for the forward reaction.

Which statement about the activation energy of the forward reaction is correct?

The activation energy of the forward reaction is equal to $-160 \mathrm{~kJ} \mathrm{~mol}^{-1}$ .

The activation energy of the forward reaction is $0 \mathrm{~kJ} \mathrm{~mol}^{-1}$ but less than $+160 \mathrm{~kJ} \mathrm{~mol}^{-1}$ .

The activation energy of the forward reaction is equal to $+160 \mathrm{~kJ} \mathrm{~mol}^{-1}$ .

The activation energy of the forward reaction is greater than $+160 \mathrm{~kJ} \mathrm{~mol}^{-1}$ .

[Maximum number: 1]

Which equation represents the reaction whose standard enthalpy change is the standard enthalpy change of formation of water?

$2 \mathrm{H}_{2}(\mathrm{~g})+\mathrm{O}_{2}(\mathrm{~g}) \rightarrow 2 \mathrm{H}_{2} \mathrm{O}(\mathrm{l})$

$\mathrm{H}_{2}(\mathrm{~g})+\frac{1}{2} \mathrm{O}_{2}(\mathrm{~g}) \rightarrow \mathrm{H}_{2} \mathrm{O}(\mathrm{l})$

$\mathrm{H}^{+}(\mathrm{aq})+\mathrm{OH}^{-}(\mathrm{aq}) \rightarrow \mathrm{H}_{2} \mathrm{O}(\mathrm{l})$

$2 \mathrm{H}(\mathrm{g})+\mathrm{O}(\mathrm{g}) \rightarrow \mathrm{H}_{2} \mathrm{O}(\mathrm{l})$

[Maximum number: 1]

At $550^{\circ} \mathrm{C}$ nitrogen dioxide reacts with unburnt hydrocarbon fragments, such as $\mathrm{CH}_{3}$ , in the catalytic converter of a motor vehicle.

4 \mathrm{CH}_{3}+7 \mathrm{NO}_{2} \rightarrow 3 \frac{1}{2} \mathrm{~N}_{2}+4 \mathrm{CO}_{2}+6 \mathrm{H}_{2} \mathrm{O}

Which row gives the energy change for this reaction and a possible reason for it?

energy change of reaction

reason why the reaction is endothermic or exothermic

endothermic

chemical energy is converted to heat energy

endothermic

the $\mathrm{N} \equiv \mathrm{N}$ bond energy is very high

exothermic

$\mathrm{CO}_{2}$ and $\mathrm{H}_{2} \mathrm{O}$ have negative $\Delta H_{\mathrm{f}}^{\ominus}$ values

exothermic

double bonds are broken in $\mathrm{NO}_{2}$

[Maximum number: 1]

Some bond energies are listed.

Table for Question 5 — CAIE A-Level Chemistry AS

One mole of hydrogen reacts with oxygen to give water vapour.

Using the bond energy data, what is the value for the enthalpy change of this reaction?

$+221 \mathrm{~kJ} \mathrm{~mol}^{-1}$

$+6 \mathrm{~kJ} \mathrm{~mol}^{-1}$

$-242 \mathrm{~kJ} \mathrm{~mol}^{-1}$

$-417 \mathrm{~kJ} \mathrm{~mol}^{-1}$

[Maximum number: 1]

The table shows bond energies for some diatomic molecules. Deuterium, D, is an isotope of hydrogen.

Table for Question 6 — CAIE A-Level Chemistry AS

Which statements are correct?

1 Diatomic molecules have exact values for their bond energies, which are always positive.

2 The trend in Group 7 bond energies can be explained by the variation in instantaneous dipole-induced dipole (id-id) forces.

3 A value for the enthalpy change for the reaction between deuterium and chlorine can be calculated using these data alone.

1 only

1 and 2 only

1 and 3 only

2 and 3 only

[Maximum number: 1]

The enthalpy change for neutralisation of $\mathrm{HNO}_{3}(\mathrm{aq})$ with NaOH(aq) is $-57.0 \mathrm{~kJ} \mathrm{~mol}^{-1}$ .
In an experiment, $20.0 \mathrm{~cm}^{3}$ of $4.00 \mathrm{moldm}^{-3} \mathrm{HNO}_{3}$ is mixed with $30.0 \mathrm{~cm}^{3}$ of $2.00 \mathrm{~mol} \mathrm{dm}^{-3} \mathrm{NaOH}$ in an insulated container. The initial temperature of both solutions is $25.0^{\circ} \mathrm{C}$ .

It can be assumed that the heat capacity of the product mixture is $4.2 \mathrm{~J} \mathrm{~cm}^{-3}{ }^{\circ} \mathrm{C}^{-1}$ and that there are no heat losses.

What is the maximum final temperature of the mixture?

$41.3^{\circ} \mathrm{C}$

$44.0^{\circ} \mathrm{C}$

$46.7^{\circ} \mathrm{C}$

$52.1^{\circ} \mathrm{C}$

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Covered syllabus points

Question 1

Question 2

Question 3

Question 4

Question 5

Question 6

Question 7