The enthalpy changes referred to by equations: 1/2 N2(g)+3/2 H2(g)+ x H2O(l)>>NH

Question

The enthalpy changes referred to by equations:
1/2 N2(g)+3/2 H2(g)+ x H2O(l)>>NH3 (aq)
1/2 H2(g)+ 1/2 Cl2 (g)+y H2O(l)>> HCl (aq) are not strictly standard enthalpies of formation since the products are not in standard states(though the reactants are). the measured enthalpies of neutralization and dissolution are not standard enthalpies either. yet there is an equation states that the standard enthalpy of formation is calculated by summing for non-standard enthalpy changes. explain why this is so?

Explanation / Answer

This is as per Hess's law of constant heat summation. According to Hess's law enthalpy change of a multiple step reaction is equal to the sum of enthalpies of each step. It is because Enthalpy is a state function so change in enthalpy is independent of path followed and only depend on initial and final States.

So we can calculate standard Enthalpy of a reaction by summing non standard Enthalpy of various steps from which reaction can be obtained.

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