Question

In the following overall chemical reaction, aluminum displaces chromium from chromium(III) oxide and forms aluminum oxide.
2 Al(s) + Cr2O3(s) → 2 Cr(s) + Al2O3(s); ΔH = ?

Find the change in enthalpy for this reaction, using Hess' law and the enthalpy changes for the reactions given below.
(1a) 4 Al(s) + 3 O2(g) → 2 Al2O3(s); ΔH = −3351.4 kJ
(2a) 4 Cr(s) + 3 O2(g) → 2 Cr2O3(s); ΔH = −2279.4 kJ

Answer 1

Answer:

Change in enthalpy for the reaction is -536 kJ

Explanation:

• Overall chemical reaction can be represented a summation of two given elementary steps with slight modification.
• Take reaction (1a) and divide stoichiometric coefficients by 2
• Take reverse reaction (2a) and divide stoichiometric coefficient by 2
• Then add these two modified elementary steps to get overall chemical reaction
• $\Delta H$ is an additive property. hence value of $\Delta H$ will be changed in accordance with modification

$2Al+\frac{3}{2}O_{2}\rightarrow Al_{2}O_{3}.....\Delta H_{1}=\frac{-3351.4}{2}kJ=-1675.7kJ$

$Cr_{2}O_{3}\rightarrow 2Cr+\frac{3}{2}O_{2}....\Delta H_{2}=\frac{2279.4}{2}kJ=1139.7kJ$

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$2Al+Cr_{2}O_{3}\rightarrow 2Cr+Al_{2}O_{3}....\Delta H=\Delta H_{1}+\Delta H_{2}=-1675.7+1139.7kJ=-536kJ$