CAIE A-Level Chemistry 5.1 Enthalpy Change

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]

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: 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 5.1 Enthalpy change, ΔH question 2 — CAIE A-Level Chemistry AS

[Maximum number: 2]

The Group 17 elements are oxidising agents.

$\mathrm{Cl}_{2}$ 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.

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

All question bank results loaded

Covered syllabus points

5.1 Enthalpy change, ΔH question 1

5.1 Enthalpy change, ΔH question 2

5.1 Enthalpy change, ΔH question 3