Home / iGCSE Chemistry (0620) Theory (Core):6.3 Reversible reactions and equilibrium: Exam Style Questions Paper 3
(a) Select two processes that show a chemical change.
Tick (✓) two boxes.
(b) The reaction of hydrogen with nitrogen is a reversible reaction.
Write the symbol that shows a reversible reaction.
(c) Table 7.1 shows the results of four experiments.
(i) State which experiment shows the greatest temperature change.
(ii) State which experiment is the most endothermic.
(iii) In experiment 4, magnesium was added to dilute hydrochloric acid.
Complete the symbol equation.
$$\mathrm{Mg} + \dots \mathrm{HCl} \rightarrow \mathrm{MgCl}_2 + \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots$$
(iv) Fig. 7.1 shows an incomplete reaction pathway diagram for an endothermic reaction.
Complete Fig. 7.1 by labelling:
Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):
• Topic 6.1 — Physical and chemical changes (Part (a))
• Topic 6.3 — Reversible reactions and equilibrium (Part (b))
• Topic 5.1 — Exothermic and endothermic reactions (Parts (c)(i), (c)(ii), (c)(iv))
• Topic 3.1 — Formulae (Part (c)(iii))
(a) burning methane (second box down ticked) and rusting of iron (fifth box down ticked).
Explanation: A chemical change results in the formation of new chemical substances with different properties. Burning methane produces carbon dioxide and water ($\mathrm{CH}_4 + 2\mathrm{O}_2 \to \mathrm{CO}_2 + 2\mathrm{H}_2\mathrm{O}$). Rusting iron produces hydrated iron(III) oxide ($\mathrm{Fe}_2\mathrm{O}_3 \cdot x\mathrm{H}_2\mathrm{O}$). Boiling water, dissolving salt, and mixing ink are reversible physical changes where no new substance is formed.
(b) $\rightleftharpoons$
(c)(i) (experiment) 4
Explanation: The magnitude of temperature change is calculated as $|T_\text{final} – T_\text{initial}|$.
Exp 1: $|23-18| = 5\,^{\circ}\mathrm{C}$; Exp 2: $|17-19| = 2\,^{\circ}\mathrm{C}$; Exp 3: $|14-18| = 4\,^{\circ}\mathrm{C}$; Exp 4: $|29-19| = 10\,^{\circ}\mathrm{C}$. Experiment 4 shows the largest change at $10\,^{\circ}\mathrm{C}$.
(c)(ii) (experiment) 3
Explanation: An endothermic reaction absorbs thermal energy from the surroundings, resulting in a decrease in temperature. Experiment 3 is the only one showing a temperature decrease of the highest magnitude ($4\,^{\circ}\mathrm{C}$ drop), making it the most endothermic.
(c)(iii) $\mathrm{Mg} + 2\mathrm{HCl} \rightarrow \mathrm{MgCl}_2 + \mathrm{H}_2$
Explanation: Magnesium reacts with hydrochloric acid in a single displacement reaction. Magnesium displaces hydrogen. Two molecules of $\mathrm{HCl}$ are required to provide 2 chloride ions to form the neutral salt, $\mathrm{MgCl}_2$, and release one molecule of hydrogen gas.
(c)(iv) energy (on the y-axis); reactants on left hand line and products on right hand line on the graph.
Explanation: For an endothermic reaction, energy is absorbed, so the energy of the products is higher than the energy of the reactants. The vertical axis is therefore labelled ‘energy’ (or enthalpy). Reactants are at the lower energy level on the left, and products at the higher energy level on the right.