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Enthalpy Change Example Problem

Enthalpy Change of a Reaction

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  • 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

  • Answer

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