Specific Latent Heat

Learning Objectives

Specific latent heat including units and symbols, with reference to water supplies of the relatively high latent heat of vaporisation of water

Summary Notes

Latent heat is the energy required to change a fixed amount of substance into a different state. ie. solid to liquid

Over the period of time that latent heat is being absorbed, the temperature of a substance will not change as the energy is being used to change the state

There are 2 kinds of latent heat:

Latent heat of fusion - heat needed to change 1 mole of the substance from a solid to a liquid at its melting point
Latent heat of vaporisation - heat needed to change 1 mole of the substance from a liquid to a gas at its boiling point

At point A to B, the temperature of the substance is increasing as energy is added. The phase of the substance remains as a solid

At point B to C, the temperature remains constant as the phase of the substance changes from solid to liquid

At point C to D, the temperature increases as energy is added. The phase of the substance remains as a liquid

At point D to E, the temperature remains constant as the phase of the substance changes from liquid to gas

At point E to F, the temperature increases as energy is added. The phase of the substance remains as a gas

q = heat energy (kJ) n = amount of substance in moles (mol) L = latent heat (kJ/mol)

Eg. Calculate the heat energy required to vapourise 50 g of water at boiling point. Specific latent heat of vaporisation of water is 44 kJ/mol.

q = nL

n = m / M q = nL
n = 50 / 18 q = 2.78 x 44
n = 2.78 mol q = 122.22 kJ/mol

Practice Material

Specific Latent Heat Quiz including solutions

Note that in VCE the equation is represented by q=nL. This website uses E=mL - where m is mass. Consider how you might change the values to reflect the appropriate units on your exam

Teacher's Tip:

Take note of the units that are being used in a SLH question. Sometimes, conversion to different units are necessary.

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