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 q­3 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.



870_Hess’s law.JPG

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

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 O(g) —> H2O(l)  ΔH = – 890.36 kJ / mole

                        ∴ ((ΔHf))H2O  = –890.36 kJ / mole


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

      C + 1/2 O2 —> 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.

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