SPECIFIC LATENT HEAT

Objective At the end of this lesson, you should be able to :State that transfer of heat during a change of phase does not cause a change in temperatureDefine specific latent heatState that

Determine the specific latent heat of fusion and vaporisationSolve problems involving specific latent heat.

Introduction The heat absorbed or the heat released at constant temperature during a change of phase is known as latent heat.

This transfer of heat does not causes a change in the kinetic energy of the molecules.The four main changes of phase:Melting BoilingCondensationSolidification

3 common characteristics in the 4 processes of change of phase:A substance undergoes a change of phase at a particular temperature.Heat energy is transferred during change of phase.During change of phase the temperature remains constant even though there is transfer of heat.

The amount of heat required to change the phase of substance depends on the mass and the type of material.

What is specific latent heat?The specific latent heat of a substance is the amount of heat required to change the phase of 1 kg of the substance at a constant temperature.

Formula

Example

The specific latent heat of fusionLatent heat of fusion is the heat absorbed by a melting solid.

The specific latent heat of fusion of a substance is defined as the amount of heat required to change 1 kg of the substance from the solid to liquid phase without a change in temperature.

The amount of heat required to melt the ice is the same as the amount of heat release when solidifies to become ice.

Formula

Example

Latent heat of vaporisation is the heat absorbed during boiling.

The specific latent heat of vaporisation of a substance is defined as the amount of heat required to change 1 kg of the substance from the liquid to gaseous phase without a change in temperature.

The specific latent heat of vaporisation

Formula

Example 18 kJ of heat energy is required to convert 8.0g of water to steam at its boiling point. What is the specific latent heat of vaporisation of water?

lv = Q/m = 18 kJ / 0.008 = 2.25 x 106 J kg-1

Heating CurveABCDEF

Cooling CurveABCDEF

Problem solving

A spoon of mass 50g is at temperature of 200C. This spoon is used to stir tea which is at a temperature of 900C . After stirring the final temperature reached by the spoon and tea is 890C. The mass of the tea is 200g.How much heat is absorbed by the spoon?

Calculate the specific hear capacity of the tea (specific capacity of the spoon=0.23 kJKg-10C-1)