Thermochemistry

The branch of chemistry which deals with energy changes involved in chemical reactions is called thermochemistry. The energy change that occurs in a chemical reactions is largely due to change of bond energy.

Change of internal energy in a chemical reaction

Let us consider a chemical reaction taking place at constant temperature and at constant volume. In such a case, w = 0 and hence from the first law

ΔU = qv

Where qv is the heat exchanged at constant volume, or heat or enthalpy of reaction at constant volume.

Change of Enthalpy in a chemical reaction

Let qP be the heat exchanged in the chemical reaction taking place at constant pressure, Then evidently,

ΔH = qP = Heat or Enthalpy of reaction at constant pressure.

Exothermic and Endothermic reaction

Reaction that give out heat, i.e. which are accompanied by evolution of heat, are called exothermic reaction. In such reactions ΔH is negative. On the other hand, reaction that intake heat, i.e. which are accompanied by absorption of heat are called endothermic reactions. In these reactions ΔH is positive.

Enthalpy of reaction

It is the enthalpy change taking place during the reaction when the number of moles of reactants and products are same as the stoichiometric coefficient indicates in the balanced chemical equation. The enthalpy change of the reaction depends upon the conditions like temperature, pressure etc under which the chemical reaction is carried out. Therefore, it is necessary to select the standard state conditions. According to thermodynamics conventions, the standard state refers to 1 bar pressure and 298 K temperature. The enthalpy change of a reaction at this standard state conditions is called standard enthalpy of the reaction (ΔH°)

Different types of enthalpy

(i) Enthalpy of formation: Enthalpy change when one mole of a given compound is formed from its elements.

H ₂₍ g) + 1/2O ₂ (g) → 2H ₂ O(l), ΔH = –890.36 kJ / mol

(ii) Enthalpy of combustion: Enthalpy change when one mole of a substance is burnt in oxygen.

CH ₄ + 2O ₂ (g) → CO ₂ + 2H ₂ O(l), ΔH = –890.36 kJ / mol

(iii) Enthalpy of Neutralization: Enthalpy change when one equivalent of an acid is neutralized by a base or vice – versa in dilute solution. This is constant and its value is –13.7 kcal for neutralization of any strong acid by a base since in dilute solutions they completely dissociate into ions.

H+ (aq) + OH– (aq) → H ₂ O(l), ΔH = –13.7 kcal

For weak acids and bases, heat of neutralization is different because they are not dissociated completely and during dissociation some heat is absorbed. So total heat evolved during neutralization will be less.

e.g. HCN + NaOH → NaCN + H ₂ O, ΔH = –2.9 kcal

Heat of ionization in this reaction is equal to (–2.9 + 13.7) kcal = 10.8 kcal

(iv) Enthalpy of hydration: Enthalpy of hydration of a given anhydrous or partially hydrated salt is the enthalpy change when it combines with the requisite no.of mole of water to form a specific hydrate. For example, the hydration of anhydrous copper sulphate is represented by

CuSO ₄ (s) + 5H ₂ O (l) → CuSO ₄ 5H ₂ O(s), ΔH° = –18.69 kcal
(v) Enthalpy of Transition: Enthalpy change when one mole of a substance is transformed from one allotropic form to another allotropic form.

C (graphite) → C(diamond), ΔH° = 1.9 kJ/mol