The Student Room Group
Reply 1
Please
Reply 2
nas7232
A container holding 2.3 litres of milk at 15 degree's C is put into a freezer. Calculate the energy that must be removed from the milk tor eduece its temperature to the freezer temperature of -30 degrees C. Assume that milk behaves like water and ice.

SHC of water = 4.2
SHC of ice = 2.1
Specific latent heat of fusion = 330
density of water = 1kg litre-1

Gotto work out energy removed :s-smilie: [6 marks]


I think it will be (2.3 x 4.2 x 15) + (330 x 2.3) + (2.3 x 2.1 x 30) KJ
Reply 3
wouldn't it be

H=mc T H = Lm + H=mc T
((2.3 litre x 1kg litre-1) x 4.2 x 15) + (330 x 2.3) + (2.3 x 2.1 x 30) KJ

What happeerend to the delta T in the last two bits :s-smilie:

Also the last bit asks for:

It costs 8.2p per KWh to remove energy from the freezer. what is the cost of freezing the milk
Reply 4
you will have to reduce the milk to 0 *C

then you wil have to freeze it

then you will have to reduce the ice to -30 *C

mass of the milk/ice = 2.3 kg

=> E = (2.3 * 4.2 * 15) + (2.3 * 330) + (2.3 * 2.1 * 30) = 1048.8 KJ

1 kWh = 1000 * 60 * 60 = 3600 KJ

=> price = 1048.8/3600 = 29p
Reply 5
Thanks :smile: