Calculate the enthalpy of formation of OH-ions at 25°C from the following thermochemical data:
`H _(2) O (l) to H ^(+) (aq) + OH ^(-) (aq) , Delta H ^(@) = 57.3 kJ`
`H _(2) (g) + 1//2 O _(2) (g) to H _(2) O (l) , Delta H ^(@) =- 258.9 kJ`

Explanation

On addition of the above equations,
`H_(2) (g) + 1// 2O _(2) (g) to H ^(+) (aq) + OH ^(-) (aq) , Delta H ^(@) =- 2.28 6 kJ`
`Delta H ^(@) =- 228.6 kJ = 0 + Delta H _(f) ^(@) [ OH ^(-) (aq)] - (0+0) = - 228.6 kJ mol ^(-1)`
By convention, `Delta H _(f) ^(@) [H^(+) (aq) ] = 0`