HESS’S LAW

This law states that the amount of heat evolved or absorbed in a process, including a chemical change is the same whether the process takes place in one or several steps.

Suppose in a process the system changes from state A to state B in one step and the heat exchanged in this change is q. Now suppose the system changes from state A to state B in three steps involving a change from A to C, C to D and finally from D to B. If q₁, q₂ and q_­3 are the heats exchanged in the first, second and third step, respectively then according to Hess’s law

            q₁ + q₂ + q₃ = q

Hess’s law is simply a corollary of the first law of thermodynamics. It implies that enthalpy change of a reaction depends on the initial and final state and is independent of the manner by which the change is brought about.

Sample problem.

In this case express ΔH in terms of ΔH₁, ΔH₂, ΔH₃ .

Solution:       ΔH = ΔH₁ + ΔH₂ + ΔH₃

Sample problem.    H₂O (l) ® H₂(g) + O₂(g)    DH = + 890.36 kJ / mole

What is DH for H₂O (l) from its constituent elements

Solution:       H₂O(l) —> H₂(g) + 1/2 O₂ (g)     ΔH = + 890.36 kJ / mole

                        H₂(s) + 1/2 O₂ (g) —> H₂O(l)  ΔH = – 890.36 kJ / mole

                        ∴ ((ΔH_f))_H2O  = –890.36 kJ / mole

Exercise: 

(i) C + 1/2 O₂ —> CO₂(g)   ΔH = –94 Kcals

      C + 1/2 O_{2 —>} CO(s)    ΔH = –26.4 Kcals

     CO + 1/2 O₂ —> CO₂(g)    ΔH =?

(ii) What is heat evolved using neutralisation of HCN by a strong base? Heat of ionization of HCN is 10.8 Kcal.