CAIE A-Level Chemistry 5. Chemical energetics Question Bank

Practise using ΔH signs, equations, diagrams, calorimetry, bond energies and Hess cycles to interpret and calculate energy changes.

Covered syllabus points

5.1 Enthalpy change, ΔH 5.2 Hess’s law

[Maximum number: 1]

One commercially available 'heat pad' contains iron, activated carbon and water. The 'heat pad' is activated by air. This causes the pad to get hotter.

Which statement describes the chemical reaction occurring in the 'heat pad' when it is exposed to air?

The reaction is endothermic and iron gains electrons.

The reaction is endothermic and iron loses electrons.

The reaction is exothermic and iron gains electrons.

The reaction is exothermic and iron loses electrons.

[Maximum number: 11]

For many compounds the enthalpy change of formation cannot be calculated directly. An indirect method based on enthalpy changes of combustion can be used.

The enthalpy change of combustion can be found by a calorimetry experiment in which the heat energy given off during combustion is used to heat a known mass of water and the temperature change recorded.

(a)

Explain the meaning of the term standard enthalpy change of combustion.

[ 3 ]

(b)

In an experiment to determine the enthalpy change of combustion of ethanol, 0.23 g of ethanol was burned and the heat given off raised the temperature of 100 g of water by $16.3^{\circ} \mathrm{C}$ .

[ 4 ]

(i)

Calculate the heat energy change, q, during the combustion of 0.23 g of ethanol.

q=\ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots . . . . \mathrm{J}

[ 1 ]

(ii)

Calculate the enthalpy change on burning 1 mole of ethanol. Include a sign in your answer.

\Delta H=\ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots . . . \mathrm{kJ} \mathrm{~mol}^{-1}

[ 1 ]

(iii)

Suggest two reasons why the value for the enthalpy change of combustion of ethanol determined by a simple laboratory calorimetry experiment is likely to be lower than the true value.

[ 2 ]

(c)

The table gives some enthalpy change of combustion values.

Table for Question (c) — CAIE A-Level Chemistry AS

[ 4 ]

(i)

Construct a labelled energy cycle to show how these values could be used to calculate the enthalpy change of formation of $\mathrm{C}_{3} \mathrm{H}_{7} \mathrm{OH}(\mathrm{I}), \Delta H_{\mathrm{f}}$ .

3 \mathrm{C}(\mathrm{~s})+4 \mathrm{H}_{2}(\mathrm{~g})+\frac{1}{2} \mathrm{O}_{2}(\mathrm{~g}) \xrightarrow{\Delta H_{\mathrm{f}}} \mathrm{C}_{3} \mathrm{H}_{7} \mathrm{OH}(\mathrm{l})

[ 3 ]

(ii)

Calculate the enthalpy change of formation, $\Delta H_{\mathrm{f}}$ , of $\mathrm{C}_{3} \mathrm{H}_{7} \mathrm{OH}(\mathrm{I})$ .

\Delta H_{\mathrm{f}}=\ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots . . \mathrm{kJ} \mathrm{~mol}^{-1}

[ 1 ]

$\begin{aligned} \Delta H_{f}+(-2021.0)=3(-393.5)+4(-285.8) \Delta H_{f}=-302.7\left(\mathrm{~kJ} \mathrm{~mol}^{-1}\right) \end{aligned}$

12 1

Total:

[Maximum number: 3]

Calcium, magnesium and radium are Group 2 elements. Radium follows the same trends as the other members of Group 2.

Magnesium, Mg , burns in oxygen, $\mathrm{O}_{2}$ .

The activation energy, $E_{\mathrm{a}}$ , for this reaction is $+148 \mathrm{~kJ} \mathrm{~mol}^{-1}$ .

On Fig. 1.1:
- sketch a reaction pathway diagram for the reaction that occurs when Mg burns in $\mathrm{O}_{2}$
- label the diagram to show the enthalpy change, $\Delta H$ , and the activation energy, $E_{\mathrm{a}}$ , for the reaction.

Fig. 1.1

Correct exothermic reaction pathway diagram is shown in the attached official markscheme figure.

Marking points:
M1 sketch shows an exothermic reaction with a hump, labelled reactants $\mathrm{Mg}+\mathrm{O}_{2}$ and product MgO.
M2 arrow from reactants / $\mathrm{Mg}+\mathrm{O}_{2}$ to product / MgO shown as $\Delta H$ .
M3 arrow showing activation energy / $E_{\mathrm{a}}$ / (+)148.

Figure for Answer to Question 5. Chemical energetics question 3 — CAIE A-Level Chemistry AS

[Maximum number: 4]

The Group 17 elements are oxidising agents.

(a)

$\mathrm{Cl}_{2}$ and $\mathrm{Br}_{2}$ can each react with $\mathrm{NH}_{3}$ to give $\mathrm{N}_{2}$ and a hydrogen halide, HX .

3 \mathrm{X}_{2}+2 \mathrm{NH}_{3} \rightarrow \mathrm{~N}_{2}+6 \mathrm{HX}

X = Cl or Br

The relative bond strengths of X-X and H-X determine the difference in enthalpy change of the two reactions.

[ 4 ]

(i)

Define enthalpy change of formation, $\Delta H_{\mathrm{f}}$ .

[ 2 ]

(ii)

Table 1.1 gives data relevant to the reaction of $\mathrm{Cl}_{2}(\mathrm{~g})$ with $\mathrm{NH}_{3}(\mathrm{~g})$ .

Use the data in Table 1.1 to calculate the enthalpy change of the reaction of $\mathrm{Cl}_{2}(\mathrm{~g})$ with $\mathrm{NH}_{3}(\mathrm{~g})$ .

Table 1.1

[ 2 ]

M1 $(6 \times -92) - (2 \times -46)=$

M2 $-460\,\mathrm{kJ\,mol}^{-1}$

All question bank results loaded

Covered syllabus points

5. Chemical energetics question 1

5. Chemical energetics question 2

Question (a)

Question (b)

Question (i)

Question (ii)

Question (iii)

Question (c)

Question (i)

Question (ii)

5. Chemical energetics question 3

5. Chemical energetics question 4

Question (a)

Question (i)

Question (ii)