Chemical Thermodynamics Chapter 4 Chemistry Class 12 Textbook Solution

1. In a chemical reaction, chemical bonds are both broken and formed.
2. The enthalpies of reactions involving substances with covalent bonds can be determined by knowing the bond enthalpies of the reactants and products.
3. These calculations assume that all bonds of the same type within the molecules are identical.
4. The enthalpy change of a reaction can be calculated using the following expression:
   Δ_rH° = Σ ΔH° (reactant bonds) - Σ ΔH° (product bonds)
   For example, consider the reaction: H_2(g) + I_2(g) → 2HI_(g)
   The enthalpy change is given by:
   Δ_rH° = [ΔH° (H - H) + ΔH° (I - I)] - [2ΔH° (H - I)]

Note:
i. When the reactants and products are diatomic molecules, the above equation provides accurate results. Bond enthalpies are precisely known for these molecules.
ii. For reactions involving polyatomic molecules, the enthalpies calculated using the above equation are approximate and represent average bond enthalpies.