The standard enthalpies of formation of $\mathrm{Al}_{2}\mathrm{O}_{3}$ and CaO are $-1675~\mathrm{kJ~mol}^{-1}$ and $-635~\mathrm{kJ~mol}^{-1}$ respectively. For the reaction $3\mathrm{CaO}+2\mathrm{Al} \rightarrow 3\mathrm{Ca}+\mathrm{Al}_{2}\mathrm{O}_{3}$ the standard reaction enthalpy $\Delta_{\mathrm{r}}\mathrm{H}^{0}=$ ______ kJ. (Round off to the nearest integer).

JEE Main · 2021 · 17 Mar 2021 · Shift 1 · Chemical Thermodynamics

The standard enthalpies of formation of

Al_{2} O_{3}

and CaO are

- 1675 kJ mol^{- 1}

and

- 635 kJ mol^{- 1}

respectively.
For the reaction

3 CaO + 2 Al \to 3 Ca + Al_{2} O_{3}

the standard reaction enthalpy

Δ_{r} H^{0} =

______ kJ.
(Round off to the nearest integer).

Answer: 230

Calcium and aluminium appear as free elements in their standard states, so their enthalpies of formation are zero and only the two oxides contribute:

Δ_{r} H^{\circ} = Δ_{f} H^{\circ} (A l_{2} O_{3}) - 3 Δ_{f} H^{\circ} (CaO)

Substituting the given values,

Δ_{r} H^{\circ} = - 1675 - 3 (- 635) = - 1675 + 1905

, so the standard reaction enthalpy is

230

kJ.

JEE Main · 2021 · 17 Mar 2021 · Shift 1 · Integer answer