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Specific Heat Capacity and Latent Heat Questions - GCSE standard

Q2. An electric kettle of thermal capacity 820 J/K contains 0.90 kg of water.

If it takes 9.0 min to raise the temperature of the kettle and its contents from 19°C to 100°C, how much longer should it take to boil away half the water?

(Specific heat capacity of water = 4200 J/kg K

Latent heat of vaporisation of water = 2.3 x 106 J/kg)

(5 marks)

Heat given to the water while bringing it to the boil:

Q = mass x specific heat capacity x T (from data sheet)

= 0.9 x 4200 x (100-19)

= 306,180 J

Heat given to the container as the water got hotter:

Q = heat capacity x T = 820 x 81 = 66,420 J

Total heat energy supplied in 8.0 minutes

Q = 306,180 + 66,420 = 372,600 J

Rate of heat supply per second = 372,600/(9.0 x 60)

= 2000 J/s

= 690 W

Heat required to boil away half of the water

Q = ml (from data sheet)

= (0.9/2) x 2.3 x 106

= 1,035,000 J

Time (in seconds) taken for the kettle to supply this = 1,035,000/690

= 1,500 s

= 25 minutes