Enthalpy of Combustion:
The enthalpy change when one mole of a substance is completely burnt in excess of oxygen or air is called enthalpy of combustion. It is denoted by ∆Hc. Some examples of combustion reactions are given below:
i) Combustion of carbon,
C (s) + O2 (g) ——→ CO2 (g) ∆Hc = – 393.5 kJ
ii) Combustion of methane,
CH4 (g) + 2O2 (g) ——→ CO2 (g) + 2H2O (l) ∆Hc = – 890.3 kJ
iii) Combustion of benzene,
C6H6 (l) + 15/2O2 (g) ——→ 6CO2 (g) + 3H2O (l) ∆Hc = – 3268 kJ
Enthalpy of Formation:
The enthalpy change when one mole of a substance is formed from its elements in there most stable form is called enthalpy of formation. It is denoted by ∆Hf. Some examples of formation reactions are given below:
i) Formation of carbon dioxide,
C (s) + O2 (g) ——→ CO2 (g) ∆Hf = – 393.5 kJ
ii) Formation of methane,
C (s) + 2H2 (g) ——→ CH4 (g) ∆Hf = – 74.8 kJ
Enthalpy of Sublimation:
The enthalpy change when one mole of a solid is directly converted into its gaseous state at a temperature below its melting point is called enthalpy of sublimation or molar enthalpy of sublimation. It is denoted by ∆Hsub. For examples, when one mole of solid iodine sublimes, the heat change is 62.4 kJ and is represented as:
I2 (s) ——→ I2 (g) ∆Hsub = + 62.4 kJ
Enthalpy of Fusion:
The enthalpy change when one mole of a solid substance is converted into its liquid state at its melting point is called enthalpy of fusion or molar enthalpy of fusion. It is denoted by ∆Hfus. For examples, when one mole of ice melts at 0C, the heat absorbed is 6.0 kJ and is represented as:
H2O (s) ——→ H2O (l) ∆Hfus = 6.0 kJ/mol
(ice) (water)
Enthalpy of Vaporisation:
The enthalpy change when one mole of a liquid is converted into gaseous state at its boiling point is called enthalpy of vaporisation or molar enthalpy of vaporisation. It is denoted by ∆Hvap. For examples, the heat change accompanying the vaporisation of one mole of water (liquid) into gas (steam) at 100C, is 40.6 kJ and is represented as:
H2O (l) ——→ H2O (g) ∆Hvap= 40.6kJ /mol
(Water) (Steam)