Exothermic and Endothermic Reactions: Practice Questions

These are original questions written for Marlbridge, in the style and at the standard of the examination. They are not reproduced past-paper questions — examination boards hold copyright in their own papers. Use these alongside the official past papers available free from your board.

Related: Exothermic and Endothermic Reactions revision notes

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Section A

1. Define exothermic and endothermic, stating the sign of ΔH and the direction of energy transfer in each case. [4]

2. Give two everyday or laboratory examples of an exothermic process, and two of an endothermic process. [4]

Section B

3. Explain, in terms of bond breaking and bond making, why a reaction is exothermic. [3]

4. For the reaction H₂ + Cl₂ → 2HCl, the bond energies in kJ mol⁻¹ are H–H 436, Cl–Cl 242, H–Cl 431.

(a) Calculate the energy needed to break the bonds in the reactants. [1] (b) Calculate the energy released in forming the bonds in the products. [1] (c) Calculate ΔH and state whether the reaction is exothermic or endothermic. [3]

5. Sketch or describe an energy profile diagram for an endothermic reaction, labelling reactants, products, activation energy and ΔH. [4]

Not on the 0620/5070 specification. Questions 6 and 8 use calorimetry and Q = mcΔT, which are not part of either syllabus’s chemical energetics content — the only quantitative energetics outcome required at this level is the bond-energy calculation (question 4). They’re included here only as useful background for progression to AS Chemistry (9701), and won’t be examined at IGCSE/O Level.

6. 50.0 cm³ of acid is neutralised by 50.0 cm³ of alkali (equal volumes); the temperature rises by 6.8 °C. Take the specific heat capacity as 4.18 J g⁻¹ °C⁻¹ and the density as 1.00 g cm⁻³.

(a) Calculate the energy released in joules. [2] (b) Give two reasons why the experimental value is lower in magnitude than the accepted value. [2]

7. A catalyst is added to a slow reaction.

(a) Explain, with reference to activation energy, how the catalyst increases the rate of reaction. [2] (b) State what happens to ΔH when a catalyst is used, explaining why. [2] (c) Describe how the catalysed pathway would be drawn on the same energy profile axes as the uncatalysed pathway. [2]

8. 100 cm³ of dilute acid (density 1.00 g cm⁻³) is neutralised by 100 cm³ of alkali (equal volumes), and the temperature rises by 5.5 °C. Take c = 4.18 J g⁻¹ °C⁻¹.

(a) Calculate the energy released, in joules. [2] (b) Give one reason for the difference between this experimental value and the accepted data-book value, other than heat loss to the surroundings. [1]

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Answers

1. Exothermic — energy is transferred to the surroundings, so the surroundings get warmer [1]; ΔH is negative [1]. Endothermic — energy is taken in from the surroundings, which get colder [1]; ΔH is positive [1].

2. Exothermic: combustion, neutralisation, respiration, oxidation [1] [1]. Endothermic: thermal decomposition, photosynthesis, the reaction of citric acid with sodium hydrogencarbonate, dissolving ammonium nitrate [1] [1].

3. Bond breaking is endothermic and bond making is exothermic [1]. In an exothermic reaction, more energy is released making the bonds in the products than is absorbed breaking the bonds in the reactants [1], so there is a net release of energy to the surroundings [1].

4. (a) 436 + 242 = 678 kJ mol⁻¹ [1]. (b) 2 × 431 = 862 kJ mol⁻¹ [1]. (c) ΔH = bonds broken − bonds made = 678 − 862 [1] = −184 kJ mol⁻¹ [1]; the negative value shows the reaction is exothermic [1].

5. Reactants drawn lower than the products on the energy axis [1]; a hump between them with activation energy measured from the reactants up to the peak [1] [1]; ΔH measured from the reactants up to the products, arrow pointing upwards and labelled as positive [1].

6. (a) Total volume = 50.0 + 50.0 = 100.0 cm³ (acid + alkali, both warmed), so mass = 100.0 g; Q = mcΔT = 100.0 × 4.18 × 6.8 [1] = 2842 J (2.84 kJ) [1]. (b) Heat is lost to the surroundings, the thermometer and the container during the reaction, so less energy ends up warming the measured solution than the reaction actually released [1]; a simple polystyrene cup is not fully insulated, so it loses further heat in the same direction [1]. (The assumption that the solution has the specific heat capacity of pure water is a separate simplifying assumption — it does not, by itself, explain why the value is lower.)

7. (a) The catalyst provides an alternative reaction pathway with a lower activation energy [1], so a greater proportion of colliding particles have enough energy to react, increasing the frequency of successful collisions [1]. (b) ΔH is unchanged [1], because the catalyst only affects the pathway between reactants and products, not their relative energy levels, which is what ΔH actually measures [1]. (c) A second, lower curve drawn between the same two starting and ending energy levels as the uncatalysed pathway [1], with a smaller hump representing the reduced activation energy [1].

8. (a) Total volume = 100 + 100 = 200 cm³ (acid + alkali, both warmed), so mass = 200 g; Q = mcΔT = 200 × 4.18 × 5.5 [1] = 4598 J (4.60 kJ) [1]. (b) The specific heat capacity used (4.18 J g⁻¹ °C⁻¹) is that of pure water, not the actual dilute acid/salt solution, which differs slightly [1].

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Where marks are usually lost

• Giving ΔH the wrong sign.
• Reversing “bonds broken − bonds made”.
• Measuring activation energy from the products instead of the reactants.
• Using the mass of the solute rather than the total solution mass in Q = mcΔT.
• Saying a catalyst changes ΔH, or drawing the catalysed curve starting/ending at different energy levels from the uncatalysed one — only the height of the hump should change.
• Explaining “increases the rate” without linking it back to activation energy and the proportion of particles with enough energy to react.

Question 7 draws on the catalyst section of the Exothermic and Endothermic Reactions revision notes; questions 6 and 8 draw on the calorimetry section instead, since question 8 is itself a calorimetry question and has nothing to do with catalysts.