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

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 2Take reverse reaction (2a) and divide stoichiometric coefficient by 2Then add these two modified elementary steps to get overall chemical reaction is an additive property. hence value of will be changed in accordance with modification

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