Heats of Formation

Sample Problem #2

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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 ionization of hydrogen bromide:

HBr(g) --> H⁺(aq) + Br^-(aq)

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

Remember, the heat of formation of H⁺ is zero. The equation becomes:

DH = DH_f Br^-(aq) - DH_f HBr(g)

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

DH = -120.9 kJ - (-36.2 kJ)

DH = -84.7 kJ

Answer

DH = -84.7 kJ

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