[Chemistry Class Notes] on Specific Latent Heat of Fusion Pdf for Exam

Heat of Fusion and Heat of Vaporization

The latent heat of fusion is also called as the enthalpy of fusion. It refers to the amount of the energy which needs to be supplied to a solid, generally in the heat form. This aids in triggering a change in the physical state and, in turn, converts it to a liquid. Consider, for example, the specific heat of fusion of water of 1 kg, which refers to the amount of the heat energy which needs to be supplied for converting 1 kg ice without a change in the temperature is known to be 333.55 kJ.

The heat of solidification is known to be the opposite of the latent heat of fusion. The value of the heat of solidification for a substance is equal in the magnitude of the specific latent heat of fusion but has an opposite sign. Consider, for example, that the amount of the energy which is absorbed by the ice for changing to water is always equal to the amount of the energy which is liberated by water for turning into ice.

The latent heat of fusion for a substance is also regarded as the energy which is needed for accommodating an increase in the volume of the given substance after it undergoes a change in its physical change. Melting point is known as the temperature at which any given substance tends to undergo a transition in its phase. When the heat of solidification is considered, this temperature is also called as the freezing point of the given substance. The environmental pressure is assumed to be always 1 unless it is specified.

Specific Heat of Fusion and Molar Heat of Fusion and Vaporization of Water

The specific heat of fusion is similar to the latent heat of fusion since the melting of a given solid at normal pressure generally needs heat energy. Consider, for example, specific latent heat of ice. It is the change in the heat of ice when it changes from solid to liquid, that is, ice to water.

If one unit mass of the given substance is taken into consideration, the energy which is needed for converting it to the liquid state when pressure is kept constant, it is known as the specific latent heat of fusion for that particular substance. However, if the change in the heat is to be calculated per mole basis, this latent heat of fusion is called the molar heat of fusion of that substance. 

Since liquids tend to possess a higher amount of internal energy which is associated with them when compared to the solids, some amount of energy has to be supplied to the solids for facilitating its melting. Similarly, some energy gets released by the liquids when they turn into solids. The particles or the molecules which make up the liquids have higher potential energy because they are held together by weaker intermolecular forces of attraction. Hence, the energy which is needed for dissociating the intermolecular forces between the liquid particles is lower when compared to that of the solids.

One such example of the latent heat of freezing is observed when water is cooled. When it is initially in the solid-state, it cools to the temperatures below 0 ⁰C and the temperature of water steadily drops till it reaches 0 ⁰C. However, at this point water crystallizes and forms ice. Once it turns into ice and is entirely frozen, the temperature of ice drops below 0 ⁰C as it cools further.

The specific latent heat of fusion is always a positive value, except for the case in helium. Below 0.3 K temperature, which is almost absolute zero, the isotope of helium, helium-3 contains a negative value of the specific latent heat of fusion.