LATENT HEAT
•Latent heat is the energy required to change a fixed amount of substance (such as 1 mole) 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 (this is since the energy is being used to change state).
•Latent heat values are given the symbol L and have the unit kilojoules per mole, kJmol-1.
•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.
•Latent heat of fusion of water is 6.0 kJmol-1 - This energy is used to break some of the hydrogen bonding between water molecules in the ice lattice.
•Latent heat of vaporisation of water is 44.0 kJmol-1 -This energy is used to completely break the hydrogen bonds between water molecules.
Calculation involving latent heat:
•The heat energy required for a state change is given by the formula:
q= n x L
Where q is the heat energy (kJ)
n is the amount of substance (mol)
L is the latent heat (kJmol-1)
•Calculate the heat energy, in kJ, required to evaporate 100g of water at 100oC.
n= m/M, n= 100/18 = 5.56 mol.
L = 44.0 kJmol-1 (heat of vaporisation- since temperature is 100oC- boiling point of water)
q = n x L
q = 5.56 x 44.0 = 244kJ.
•Latent heat is the energy required to change a fixed amount of substance (such as 1 mole) 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 (this is since the energy is being used to change state).
•Latent heat values are given the symbol L and have the unit kilojoules per mole, kJmol-1.
•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.
•Latent heat of fusion of water is 6.0 kJmol-1 - This energy is used to break some of the hydrogen bonding between water molecules in the ice lattice.
•Latent heat of vaporisation of water is 44.0 kJmol-1 -This energy is used to completely break the hydrogen bonds between water molecules.
Calculation involving latent heat:
•The heat energy required for a state change is given by the formula:
q= n x L
Where q is the heat energy (kJ)
n is the amount of substance (mol)
L is the latent heat (kJmol-1)
•Calculate the heat energy, in kJ, required to evaporate 100g of water at 100oC.
n= m/M, n= 100/18 = 5.56 mol.
L = 44.0 kJmol-1 (heat of vaporisation- since temperature is 100oC- boiling point of water)
q = n x L
q = 5.56 x 44.0 = 244kJ.