Reaction Enthalpy

Theory

The enthalpy change accompanying a reaction is called enthalpy of a reaction (ΔrH).

H = (Sum of enthalpies of products) - (Sum of enthalpies of reactants)

a_i and b_i are stoichiometric coefficients of products and reactants in a balanced chemical equation.

CH₄ (g) + 2O₂ (g) → CO₂ (g) + 2H₂O (l)

H

= [H_m(CO₂,g) + 2H_m(H₂O,l)]  -  [H_m(CH₄,g) + 2H_m(O₂,g)]

H_m = Molar enthalpy

Standard enthalpy of reactions (∆_rH^⊝):

Enthalpy of a reaction varies with temperature.

So, for comparison, it needs to be expressed at standard state conditions.

The enthalpy change for a reaction when all the participating substances are in their standard states is the standard enthalpy of reaction.

P = 1 bar, T = 298 K

‘Θ’ represents standard state.

Enthalpy changes during phase transformations:

Solid → Liquid, Liquid → Gas, Solid → Gas

Energy is supplied in these transformations, as intermolecular forces of attraction are broken.

Enthalpy of fusion (Δ_fusH^Θ): Enthalpy change that is accompanied when one mole of a solid substance undergoes melting in standard state.

H₂O (s) → H₂O (l), Δ_fusH^Θ = 6.00 kJ/mol

Enthalpy of vaporisation (Δ_vapH^Θ): Enthalpy change that is accompanied when one mole of a liquid substance at constant temperature (boiling point) and 1 bar pressure undergoes vaporisation.

H₂O (l) → H₂O (g), Δ_vapH^Θ = 40.79 kJ/mol

Enthalpy of sublimation (Δ_subH^Θ): Enthalpy change that is accompanied when one mole of a solid substance directly changes into gaseous state at constant temperature and 1 bar pressure.

CO₂ (s) → CO₂ (g), Δ_subH^Θ = 25.2 kJ/mol (at 195 K)

Standard enthalpy of formation:

Standard enthalpy of formation (Δ_fH^Θ) of a substance is the enthalpy change accompanied in formation of 1 mole of a compound from its elements in their most stable states or reference states.

Reference state:

Dihydrogen gas → H₂ (g)

Dioxygen → O₂ (g)

Carbon → C_graphite

Sulphur → S_rhombic

Enthalpy change of a reaction: 

Δ_rH^Θ = (Sum of standard enthalpies of formation of products) - (Sum of standard enthalpies of formation of reactants)

a_i and b_i are stoichiometric coefficients of products and reactants in a balanced chemical equation.

CaCO₃ (s) → CaO(s)+CO₂ (g)

Δ_rH^Θ=[∆_fH^⊖ [CaO(s)] + ∆_fH^⊖ [CO₂ (g)]] - [∆_f H^⊖ [CaCO₃ (s)]]

= [(-635.1 kJ/mol) + (-393.5 kJ/mol)] - (-1206.9 kJ/mol)

= 178.3 kJ/mol (endothermic reaction)