Heats of Formation

Sample Problem #1

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Review

You may wish to review the Laws of Thermochemistry and Endothermic and Exothermic Reactions before you begin. Tables are available for heats of formation of common compounds and ions in aqueous solution.

Problem

Calculate DH for the following reaction:

8 Al(s) + 3 Fe₃O₄(s) --> 4 Al₂O₃(s) + 9 Fe(s)

Solution

DH for a reaction is equal to the sum of the heats of formation of the product compounds minus the sum of the heats of formation of the reactant compounds:

DH = S DH_f products - S DH_f reactants

Omitting terms for the elements, the equation becomes:

DH = 4 DH_f Al₂O₃(s) - 3 DH_f Fe₃O₄(s)

The values for DH_f may be found in the Heats of Formation of Compounds table. Plugging in these numbers:

DH = 4(-1669.8 kJ) - 3(-1120.9 kJ)

DH = -3316.5 kJ

Answer

DH = -3316.5 kJ

Next page > Sample Problem #2 > Page 1, 2

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