SPECIFIC LATENT HEAT Objective n At the end

SPECIFIC LATENT HEAT

Objective n At the end of this lesson, you should be able to : n State that transfer of heat during a change of phase does not cause a change in temperature n Define specific latent heat n State that n Determine the specific latent heat of fusion and vaporisation n Solve problems involving specific latent

Introduction n The heat absorbed or the heat released at constant temperature during a change of phase is known as latent heat. n This transfer of heat does not causes a change in the kinetic energy of the molecules. n The four main changes of phase: n Melting n Boiling n Condensation n Solidification

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

The specific latent heat of vaporisation n Latent heat of vaporisation is the heat absorbed during boiling. n 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.

Formula

Example n 18 k. J of heat energy is required to convert 8. 0 g of water to steam at its boiling point. What is the specific latent heat of vaporisation of water? n lv = Q/m = 18 k. J / 0. 008 = 2. 25 x 106 J kg-1

Problem solving