4.3SPECIFIC LATENT HEAT

HEAT

SPECIFIC HEAT

CAPACITY

LATENT HEAT

SOLID LIQUIDS GAS

TEMPERATURE

THERMA EQUILIBRIUM

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

1) Latent heat fusion

2) 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

GAS

SOLID AND LIQUID

LIQUID AND GAS

• Molecules are held together tightly.

• When heated, it gain energy and vibrate about their fixed position at greater speeds and amplitudes.

• The liquid vaporizes completely into gas.

• The liquid temperature rises.

• Molecules begin to moves faster and randomly.

• The liquid starts to boil• Latent heat of vaporization is

absorbed the liquid.

• 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

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

Substance

Specific latent heat of fusion

ºC

Specific latent heat of vaporization

ºCkJ.kg-1

kJ.kg-1

Water 334 0 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. 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)

SolutionUse the energy equivalence method.Electric energy = Heat energy Pt = mL t = mL / P = 0.08 x 2.26 x 106/ 500 = 362 s

LATENT HEAT

• SOLID LIQUID

CHANGE IN STATE

SYMBOL: LSI UNITJOULE/KG

3 STATES OF MATTERSOLID

LIQUIDSGASES

LATENT HEAT OFVAPORIZATION

LATEN HEAT OF FUSION

LATENT HEAT IS ABSORBED

LATENT HEAT IS RELEASED

LATENT HEAT IS ABSORBED

LATENT HEAT IS RELEASED

• LIQUID GAS

Q=ML

Q=Pt ML=Pt