Question

The enthalpy of formation of $H_2O\left(l\right) is -285.77\, kJ\, mol^{-1}$ and enthalpy of neutralisation of strong acid and strong base is $-56.07 \,kJ\, mol^{-1}$, what is the enthalpy of formation of $OH^{-}$ ion

• A. $+229.70\, kJ$
• B. $-229.70\, kJ$
• C. $+226.70\, kJ$
• D. $-22.670\, kJ$

Answer 1

Answer: (B)

$H^{+}\left(aq\right)+OH^{-}\left(aq\right)\to H_{2}O\left(l\right);\Delta H=-56.07 kJ$
$\Delta H=\Delta H_{f}\left(H_{2}O\right)-\left[\Delta H_{f}\left(H^{+}\right)+\Delta H_{f}\left(OH^{-}\right)\right]$
$\left[\because\Delta H_{f}\left(H^{+}\right)=0\right]-56.07=-285.77-\left(0+x\right)$
$\therefore x=-285.77+56.07=-229.70 kJ$