Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):
• Topic 3.1 — Formulae (Part (a))
• Topic 6.2 — Rate of reaction (Parts (b), (c)(i), (c)(ii))
• Topic 5.1 — Exothermic and endothermic reactions (Part (d))
(a)
Liquid
The state symbol (l) represents the physical state: liquid.
(b)
55 (cm³)
From the graph (Fig. 8.1), when the reaction time is 24 s, the corresponding point on the curve reads a total volume of 55 cm³ of carbon dioxide produced.
(c)(i)
Increases / takes longer
Using a lower concentration of citric acid means there are fewer acid particles per unit volume. This reduces the frequency of successful collisions, so the rate of reaction decreases and it takes a longer time to finish.
(c)(ii)
No change
The total volume of \( \mathrm{CO}_2 \) produced depends on the amount of the limiting reactant. The question states that excess acid is still used in both cases; the amount of sodium hydrogencarbonate is the same. Therefore, the total volume of gas produced eventually will be the same.
(d)
Endothermic
Explanation: Energy (level) of products above that of reactants (ORA).
The reaction pathway diagram shows the products at a higher energy level than the reactants. This positive overall enthalpy change (\( \Delta H \)) indicates an endothermic reaction where thermal energy is absorbed from the surroundings.
