4.3 SPECIFIC LATENT HEAT

SPECIFIC HEAT CAPACITY TEMPERATURE THERMA EQUILIBRIUM LATENT HEAT SPECIFIC HEAT CAPACITY SOLID LIQUIDS GAS

What is latent heat ? Latent heat is defined as the heat absorbed or released when a substance changes its physical state completely at constant temperature

2 different type of latent heat Latent heat fusion Latent heat of vaporization

What is latent heat of fusion ? Latent heat fusion is the heat absorbed when a solid melt at constant temperature

During melting, the heat absorbed is used to break up the bonds between the particles. The particles are free from their fixed positions and are able to vibrate and move among each other .

What is latent heat of vaporization ? Latent heat of vaporization is the heat absorbed when a liquid changes into vapor at constant temperature

When liquid boils, the heat absorbed is used to completely break the bonds of the particles and also to do work against atmospheric pressure when the gaseous vapor expands into the atmosphere.

LIQUID AND GAS GAS SOLID LIQUID SOLID AND LIQUID The liquid starts to boil Latent heat of vaporization is absorbed the liquid. Molecules are held together tightly. When heated, it gain energy and vibrate about their fixed position at greater speeds and amplitudes. GAS The liquid vaporizes completely into gas. SOLID LIQUID The liquid temperature rises. Molecules begin to moves faster and randomly. The solid begin to melt. Latent heat of fusion is absorbed by the solid. The molecules are freed from their fixed positions and are able to move about. SOLID AND LIQUID

What is specific latent heat? Specific latent heat, L, of a substance defined as the amount of heat required to changed the phase of 1 kg of the substance at a constant temperature. It can be calculated as; L = Q/m

where; Q = latent heat absorbed or released by the substance m = mass of the substance The S.I unit joule per kilogram (JKg-1) The amount of heat required to change the phase of substance depends on the mass and type of material of the substance

Specific latent heat of fusion Specific latent heat of vaporization Substance Specific latent heat of fusion ºC Specific latent heat of vaporization kJ.kg-1 Water 334 2258 100 Ethanol 109 -114 838 78 Ethanoic acid 192 17 395 118 Chloroform 74 -64 254 62 Mercury 11 -39 294 357 Sulphur 54 115 1406 445 Hydrogen 60 -259 449 -253 Oxygen 14 -219 213 -183 Nitrogen 25 -210 199 -196

What is specific latent heat of fusion ? Specific latent heat of fusion of a substance defined as the amount of heat required to change 1 Kg of the amount from solid to liquid phase at constant temperature

What is specific latent heat of vaporization ? Specific latent heat of vaporization 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 S.I unit is joule per kilogram (JKg-1) Heat needed to change one phase to another phase ; Q = mL If the heat is supplied by immersion electric heater; Q = Pt Where P = power t = time Pt = mL

EXAMPLES The specific latent heat of fusion of ice is 336 000 J/kg. What is the quantity of heat required to melt 1.5 kg of ice at 0oC. Solution Heat required, Q = ml =1.5 x 336 000 = 504 KJ

A 500 W electric heater is used to boil water A 500 W electric heater is used to boil water. What is the time required to reduce the mass of water by 80 g after the water has reached its boiling point? ( Specific latent heat of vaporization of water = 2.26 x 106 J/Kg) Solution Use the energy equivalence method. Electric energy = Heat energy Pt = mL t = mL / P = 0.08 x 2.26 x 106/ 500 = 362 s

CONCLUSION

LATENT HEAT 3 STATES OF MATTER CHANGE IN STATE SOLID LIQUIDS GASES LATEN HEAT OF FUSION CHANGE IN STATE LATENT HEAT OF VAPORIZATION LATENT HEAT IS ABSORBED LATENT HEAT IS ABSORBED SYMBOL: L SI UNIT JOULE/KG SOLID LIQUID LIQUID GAS LATENT HEAT IS RELEASED LATENT HEAT IS RELEASED Q=ML Q=Pt ML=Pt