
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:
H_{2}O_{2}(l) → H_{2}O(l) + 1/2 O_{2}(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 H_{2}O_{2} is 98.2 kJ, so this relationship can be used as a conversion factor. Using the Periodic Table, the molecular mass of H_{2}O_{2} is 34.0, which means that 1 mol H_{2}O_{2} = 34.0 g H_{2}O_{2}.
Using these values:
ΔH = 1.00 g H_{2}O_{2} x 1 mol H_{2}O_{2} / 34.0 g H_{2}O_{2} x 98.2 kJ / 1 mol H_{2}O_{2}
ΔH = 2.89 kJ

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