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Heat of Fusion-the
amount of heat required to convert unit mass of a solid into the
liquid without a change in temperature.
For
water at its normal boiling point of 0 ºC, the specific heat
of vaporization is 334 J g-1. This means that to
convert 1 g of ice at 0 ºC to 1 g of water at 0 ºC, 334
J of heat must be absorbed by the water. Conversely, when 1 g of
water at 0 ºC freezes to give 1 g of ice at 0 ºC, 334 J
of heat will be released to the surroundings.
Heat
of Fusion of Water Hf = 334 J /g q=
m Hf
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The diagram on the
left shows the uptake of heat by 1 kg of water, as it
passes from ice at -50 ºC to steam at temperaturesabove
100 ºC, affects the temperature of the sample.
A:
Rise in temperature as ice absorbs heat.
B:
Absorption of latent heat of fusion.
C:
Rise in temperature as liquid water absorbs heat.
D:
Water boils and absorbs latent heat of vaporization.
E:
Steam absorbs heat and thus increases its temperature.
from-http://www.physchem.co.za/Heat/Latent.htm |
Note- Heat of Fusion
questions occur in section B only and there is no
temperature change. It stays at 0
Celsius.
| Sample
Questions |
Answers |
| 1. How much energy
is required to melt 10.g of ice at its melting point? |
q= m Hf
q = 10.g x 334
J/g = 3340J or 3.34kJ |
| 2. How much energy is released when 20. g
of water is frozen at 0oC? |
q= m Hf
q = 20.g x 334
J/g = 6680j or 6.68kJ |
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