Energetics – IGCSE Chemistry exam questions with mark scheme

1Grade 1–3Triple only4 marks

The diagram shows the energy profile for a reaction.

(a) State which arrow, A or B, represents the activation energy. [1]

(b) State which arrow represents the enthalpy change, ΔH. [1]

(a) A(1)
(b) B(1)
(c) exothermic(1)
(c) products have less energy than the reactants / energy is given out to the surroundings / ΔH is negative (allow arrow B points downwards)(1)
Answer: (a) A (b) B (c) exothermic – products lower in energy than reactants
Examiner tip: Activation energy is measured from the REACTANTS to the peak, not from the products.

2Grade 6–7Triple only4 marks

50.0 cm³ of 1.00 mol/dm³ hydrochloric acid is mixed with 50.0 cm³ of 1.00 mol/dm³ sodium hydroxide solution in a polystyrene cup. The temperature rises by 6.5 °C.

Calculate the molar enthalpy change, ΔH, for the reaction in kJ/mol.
[Assume the specific heat capacity of the solution is 4.18 J/g/°C and that 1.00 cm³ of solution has a mass of 1.00 g.] [4]

heat energy change $Q = mc\Delta T$ = 100 × 4.18 × 6.5 = 2717 (J) (accept 2.717 kJ)(1)
moles of acid / water = 1.00 × 50.0 ÷ 1000 = 0.0500 (mol)(1)
2.717 ÷ 0.0500 = 54.3 (kJ/mol) (allow 54 or 54.34; allow ecf from their Q and moles)(1)
negative sign: ΔH = −54.3 kJ/mol (sign mark dependent on numerical working)(1)
Answer: ΔH = −54.3 kJ/mol
Examiner tip: Use the TOTAL mass of solution (100 g), not 50 g, and remember the minus sign for an exothermic reaction.

3Grade 6–7Triple only4 marks

Methane burns in oxygen. $$\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}$$

Bond	C–H	O=O	C=O	O–H
Bond energy in kJ/mol	412	496	743	463

Use the bond energies to calculate the enthalpy change, ΔH, for this reaction. [4]

energy needed to break bonds = (4 × 412) + (2 × 496) = 2640 (kJ)(1)
energy released making bonds = (2 × 743) + (4 × 463) = 3338 (kJ)(1)
2640 − 3338(1)
ΔH = −698 (kJ/mol) (+698 scores 3)(1)
Answer: ΔH = −698 kJ/mol
Examiner tip: Count every bond: 4 C–H, 2 O=O, 2 C=O and 4 O–H (2 per water molecule).

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