Question

State and explain Hess’s law of constant heat summation.

Answer 1

The law states that, “Overall, the enthalpy change for a reaction is equal to the sum of enthalpy changes of individual steps in the reaction”.

The enthalpy change for a chemical reaction is the same regardless of the pathway taken during the reaction. Hess’s law is a direct result of the principle that enthalpy is a state function. The enthalpy change of a reaction depends only upon the initial and final states, independent of the reaction path.

To determine the overall reaction equation, the reactants and products from the different steps are combined or subtracted as algebraic entities.

Consider the synthesis of NH₃:

i. \[\ce{\phantom{...}2H2_{(g)} + N2_{(g)} -> N2H4_{(g)}, \Delta_rH^0_1 = +95.4 kJ}\]
ii. \[\ce{N2H4_{(g)} + H2_{(g)} -> 2NH3_{(g)}, \Delta_rH^0_2 = -187.6 kJ}\]
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\[\ce{\phantom{.....}3H2_{(g)} + N2_{(g)} -> 2NH3_{(g)}}\], Δ_rH⁰ = −92.2 kJ

The sum of the enthalpy changes for steps (i) and (ii) is equal to the enthalpy change for the overall reaction.