Question

The enthalpy of formation of water from hydrogen and oxygen is $-286.0kJ$ ${mol}^{-1}$. The enthalpy of decomposition of water into hydrogen and oxygen is ________ $kJ$ ${mol}^{-1}$:

• A. $-286.0$
• B. $-143.0$
• C. $286.0$
• D. $143.0$

Answer 1

The correct option is C $286.0$

Since, formation reaction is the reaction involving formation of 1 mole of that species from its constituent elements while decomposition reaction is breaking down of 1 mole of any species into its constituent species.
i.e for water
Formation reaction => $H_2+\frac{1}{2}{O}_2\to H_{2}O$
Decomposition reaction => $H_{2}O\to H_2+\frac{1}{2}{O}_2$
Hence, the formation reaction and decomposition reaction are reverse reactions of each other.
Hence, the enthalpy of decomposition reaction will be negative of enthalpy of formation reaction i.e,
$\Delta H_{d\left(H_{2}O\right)}=-\Delta H_{f\left(H_{2}O\right)}=-(-286.0)=286.0$
Hence answer is option C.