Standard enthalpy change of neutralisation

The standard enthalpy change of neutralisation, ΔHn o, is the change in enthalpy when one mole of water is formed from an acid reacting with an alkali, in their most stable forms, under standard conditions, e.g.

\frac{1}{2}H_2SO_4(aq)+NaOH(aq)\rightarrow \frac{1}{2}Na_2SO_4(aq)+H_2O(l)\; \; \; \Delta H_n^{\: o}=-57.1\: kJmol^{-1}

or in the ionic form,

H^+(aq)+OH^-(aq)\rightarrow H_2O(l)\; \; \; \; \; \; \Delta H_n^{\: o}=-57.1\: kJmol^{-1}

Since acids and alkalis, by definition, are in the aqueous state, their most stable form is the aqueous form. The standard enthalpy change of neutralisation is also the standard enthalpy change of reaction between sulphuric acid and sodium hydroxide to give sodium sulphate and water, i.e. ΔHn o =ΔHr o.

 

Question

Is the standard enthalpy change of neutralisation, H+(aq) + OH(aq)  H2O(l), the same as the standard enthalpy change of formation of water?

Answer

No. The definition of the standard enthalpy change of formation of water requires the reactants to be in their standard states, i.e. H2(g) and O2(g).

 

 

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