Enthalpy Change

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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); DH = -98.2 kJ

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

Solution

The thermochemical equation tells us that DH 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:

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

DH = -2.89 kJ

Answer

-2.89 kJ

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