5.5 Standard Enthalpies of Formation

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• Standard enthalpy of formation of a compound is the change in enthalpy that occurs when 1 mol of a compound is formed from its elements in their standard states   * Standard State: a substance in its most stable state at SATP (25°C, 100 kPa) – state on the periodic table     * Standard enthalpies of formation for Al2O3(s) is -1675.5 kJ/mol     * Standard Enthalpy of formation for aluminum chloride is -704.2 kJ/mol

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Example

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Four Ways to Calculate Enthalpy Change

• Experimentally   * measuring the heat of the surroundings (q)   * hess’ law
• Theoretically   * bond energies   * standard enthalpies of formation

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