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# Enthalpy Change Example Problem

## Enthalpy Change of a Reaction

Enthalpy is a measure of a system's energy.

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• Enthalpy Review

You may wish to review the Laws of Thermochemistry and Endothermic and Exothermic Reactions before you begin.

• Problem

Hydrogen peroxide decomposes according to the following thermochemical reaction:

H2O2(l) → H2O(l) + 1/2 O2(g); ΔH = -98.2 kJ

Calculate the change in enthalpy, ΔH, when 1.00 g of hydrogen peroxide decomposes.

• Solution

The thermochemical equation tells us that ΔH for the decomposition of 1 mole of H2O2 is -98.2 kJ, so this relationship can be used as a conversion factor. Using the Periodic Table, the molecular mass of H2O2 is 34.0, which means that 1 mol H2O2 = 34.0 g H2O2.

Using these values:

ΔH = 1.00 g H2O2 x 1 mol H2O2 / 34.0 g H2O2 x -98.2 kJ / 1 mol H2O2

ΔH = -2.89 kJ

The change in enthalpy, ΔH, when 1.00 g of hydrogen peroxide decomposes = -2.89 kJ

Anne Marie Helmenstine, Ph.D.