Question

On the basis of the following thermochemical data $: \left(\Delta f G^{o}H^{+}_{\left(aq\right)}=0\right)$ $H_{2}O\left(\ell\right) \rightarrow H^{+}\left(aq\right)+OH^{-}\left(aq\right); \Delta H=57.32kj$ $H_{2}\left(g\right)+\frac{1}{2}O_{2}\left(g\right) \rightarrow H_{2}O\left(\ell\right); \Delta H=-286.20kj$ The value of enthalpy of formation of $OH^-$ ion at $25^o\,C$ is :

• A. $-22.88\, kJ$
• B. $-228.88\, kJ$
• C. $+228.88\, kJ$
• D. $-343.52\, kJ$

Answer 1

Answer: (B)

$-228.88\, kJ$

Answer 2

By adding the two given equations, we have $H_{2\left(g\right)}+\frac{1}{2}O_{2\left(g\right)} \rightarrow H^{+}_{\left(aq\right)}+OH^{-}_{\left(aq\right)}; \Delta H=-228.88\,Kj$ Here $\Delta H^{o}_{f} of H^{+}_{\left(aq\right)}=0$ $\therefore \Delta H^{o}_{f} of OH^{-}=-228.88\,kj$