**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