A-LevelPhysicsThermodynamics9702/43 · Oct/Nov 2019 · Q310 Marks

(a) State what is meant by specific latent heat. (b) A student determines the specific latent heat of vaporisation of a liquid using the apparatus illustrated in Fig. 3.1. [Figure 3.1] The heater is switched on. When the liquid is boiling at a constant rate, the balance reading is noted at 2.0 minute intervals. After 10 minutes, the current in the heater is reduced and the balance readings are taken for a further 12 minutes.

(a)State what is meant by specific latent heat.[2]
(b)(i)From time 0 to time 10.0 minutes, the mass of liquid evaporated is 56 g. Use Fig. 3.3 to determine the mass of liquid evaporated from time 12.0 minutes to time 22.0 minutes.[1]
(b)(ii)Explain why, although the power of the heater is changed, the rate of loss of thermal energy to the surroundings may be assumed to be constant.[1]
(b)(iii)Determine a value for the specific latent heat of vaporisation L of the liquid.[4]
(b)(iv)Calculate the rate at which thermal energy is transferred to the surroundings.[2]

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Mark scheme

Marking points from the official mark scheme (Oct/Nov 2019). The mark or mark type is shown in brackets.

[B1] (a) (thermal) energy per (unit) mass (to change state)
[B1] (a) (heat transfer during) change of state at constant temperature
[A1] (b)(i) 32 g
[B1] (b)(ii) temperature difference (between liquid and surroundings) does not change
[C1] (b)(iii) VIt = mL
[C1] (b)(iii) 230 x 1.2 × 60 × 10 = (56 × L) + H
[C1] (b)(iii) 190 x 1.0 × 60 × 10 = (32 x L) + H
[C1] (b)(iii) 86 × 600 = (56 – 32) × L
[A1] (b)(iii) L = 2200 J g⁻¹
[C1] (b)(iv) 230 × 1.2 × 600 = (56 × 2150) + H or 190 x 1.0 × 600 = (32 × 2150) + H (accept both or either for C1)
[A1] (b)(iv) = 75 W

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