Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):
• Topic 6.2 — Rate of reaction (Parts a, b)
• Topic 5.1 — Exothermic and endothermic reactions (Part c)
(a) For the correct answer:
total volume of \(\text{H}_2\text{S}\) = 132.5 \(\text{cm}^3\)
Explanation: The graph in Fig. 8.1 shows the volume of hydrogen sulfide gas increasing over time until it flattens into a horizontal plateau. This plateau represents the completion of the reaction. Reading the value from the vertical axis gives 132.5 cm³.
(b)(i) For the correct answer:
decreases / less (time)
Explanation: Increasing the concentration of hydrochloric acid increases the number of reactant particles per unit volume. According to collision theory, this increases the frequency of successful collisions between reactants, leading to a faster rate of reaction. Therefore, the reaction reaches completion in a shorter time.
(b)(ii) For the correct answer:
no change
Explanation: The total volume of hydrogen sulfide produced is determined by the amount of the limiting reactant, which is sodium sulfide (\(\text{Na}_2\text{S}\)). The question states that “excess” hydrochloric acid is used, so sodium sulfide is the limiting reactant. Since the amount of sodium sulfide is the same in both experiments, the total volume of \(\text{H}_2\text{S}\) produced remains unchanged.
(c) For the correct answers:
exothermic (1)
energy (level) of products below that of reactants (1) (or equivalent reverse argument)
Explanation: In a reaction pathway diagram, the vertical axis represents the energy of the chemical system. If the products are at a lower energy level than the reactants, energy has been released to the surroundings, and the reaction is exothermic. The diagram in Fig. 8.2 shows this arrangement, confirming an exothermic energy change.
