Question

At a given set of conditions, 246.2 kJ is given off when 1 mol of H2O(g) forms from its elements. Under the same conditions, 284.6 kJ is given off when 1 mol of H2O(l) forms from its elements. Find ΔH for the vaporization of water at these conditions.

Answer 1

The ΔH for the vaporization of water at these conditions is 38.4 kJ/mol.

Step-by-step explanation:

Given:

`H_2(g)+(1)/(2)O_2(g)\rightarrow H_2O(g),\Delta H^o_(1)=-246.2 kJ/mol`..[1]

`H_2(g)+(1)/(2)O_2(g)\rightarrow H_2O(l),\Delta H^o_(2)=-284.6 kJ/mol`..[2]

To find: ΔH for the vaporization of water at these conditions:

`H_2O(l)\rightarrow H_2O(g),\Delta H^o_(vap)=?`...[3]

Solution:

[1] - [2] = [3] (using Hess's law)

`\Delta H^o_(vap)=\Delta H^o_(1)-\Delta H^o_(2)`

`=-246.2 kJ/mol -(-284.6 kJ/mol) =38.4 kJ/mol`

The ΔH for the vaporization of water at these conditions is 38.4 kJ/mol.