Home / iGCSE Chemistry (0620) Theory (Core):5.1 Exothermic and endothermic reactions: Exam Style Questions Paper 3

iGCSE Chemistry (0620) Theory (Core):5.1 Exothermic and endothermic reactions: Exam Style Questions Paper 3- New Syllabus

iGCSE Chemistry Theory (Core) Paper 3 - All Chapters

Question

Excess dilute citric acid, \( \mathrm{C}_6\mathrm{H}_8\mathrm{O}_7 \), is added to aqueous sodium hydrogencarbonate to form aqueous sodium citrate, \( \mathrm{Na}_3\mathrm{C}_6\mathrm{H}_5\mathrm{O}_7 \), carbon dioxide and water.
\( \mathrm{C}_6\mathrm{H}_8\mathrm{O}_7(\mathrm{aq}) + 3\mathrm{NaHCO}_3(\mathrm{aq}) \rightarrow \mathrm{Na}_3\mathrm{C}_6\mathrm{H}_5\mathrm{O}_7(\mathrm{aq}) + 3\mathrm{CO}_2(\mathrm{g}) + 3\mathrm{H}_2\mathrm{O}(\mathrm{l}) \)
(a) Give the meaning of the symbol (l) in the equation.
(b) Fig. 8.1 shows the total volume of carbon dioxide gas, \( \mathrm{CO}_2 \), produced as the reaction proceeds.
 
Use Fig. 8.1 to determine the total volume of \( \mathrm{CO}_2 \) produced at 24 s.
(c) The reaction is repeated using excess citric acid of a lower concentration.
All other conditions stay the same.
State the effect, if any, on:
(i) the time taken for the reaction to finish
(ii) the total volume of \( \mathrm{CO}_2 \) produced when the reaction is complete.
(d) Fig. 8.2 shows the reaction pathway diagram for the reaction of dilute citric acid with aqueous sodium hydrogencarbonate.
Deduce the type of energy change shown in the diagram in Fig. 8.2.
Explain your answer.

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))

▶️ Answer/Explanation

(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.

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