How much energy is released when water freezes 1g?

1. Heat:

(i) Heat is a form of energy, in transit, that flows from a body at higher temperature to a body at lower temperature.

(ii) SI unit of heat is joule. Its CGS unit is calorie.

1 calorie = 4.186 J.

2. Thermal Equilibrium:

When two or more bodies are at different temperature are brought into thermal contact, then net heat lost by the hot bodies is equal to net heat gained by the cold bodies until they attain thermal equilibrium.

3. Temperature and Kinetic Energy:

The average kinetic energy of the molecules is directly proportional to the absolute temperature.

4. Specific Heat:

The specific heat of a substance is the amount of heat required to raise the temperature of unit mass of the substance by one unit.

S=QmΔt

5. Evaporation:

The process of escaping of molecules from the surface of a liquid at any temperature is called evaporation and it is a cooling process. 

6. Condensation and Boiling:

(i) Condensation is the reverse process of evaporation.

(ii) Boiling is the process in which the liquid phase changes to gaseous phase at a constant temperature and constant pressure.

7. Latent Heat:

(i) The heat energy is used to change the state of water from liquid to vapour is called Latent heat of vapourization.

(ii) The heat energy required to convert 1 gm of solid completely into liquid at a constant temperature is called latent heat of fusion.

Heat of Fusion-the amount of heat required to convert unit mass of a solid into the liquid without a change in temperature. (or released for freezing)

For water at its normal freezing point of 0 ºC, the specific heat of Fusion 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

Note- The Heat of Fusion equation is used only at the melting/freezing transition, where the temperature remains the same only and that is why there is no temperature change (DT) in this formula. It stays at 0 Celsius for water.

Note #2-Energy is required to melt and released when it freezes

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 temperatures above 100 ºC, affects the temperature of the sample. E: Steam absorbs heat and thus increases its temperature. D: Water boils and absorbs latent heat of vaporization. C: Rise in temperature as liquid water absorbs heat. B: Absorption of latent heat of fusion. A: Rise in temperature as ice absorbs heat. from-http://www.physchem.co.za/Heat/Latent.htm