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    Assuming that you have the ∆G*f for the components of the combustion of Ethanol and you need to figure out the ∆G* of the entire reaction. How do I do it?

    O2 (g) = 0 kJ/mol
    C2H5OH (l) = -174.8 kJ/mol
    H2O (l) = -237.2 kJ/mol
    CO2 (g) = -394.4 kJ/mol

    P.S. If I can have a better way to format equations, that would be appriciated.

    You need to use hess's law: the energy for a process is the same regardless of the route taken.

    Taking the balance equation:

    C2H5OH + (7/2)O2 ----> 2CO2 + 3H2O

    One mole of ethanol produces 2 moles of CO2 (2 x -394.4 kJ/mol) and 3 moles of H2O (3 x -237.2 kJ/mol) - so the energy at the end is -1500.4 kJ/mol. At the start ∆G of formation for elements (such as oxygen) is zero so that leaves -174.8 kJ/mol.

    ∆G = ∆G(products) - ∆G(reactants)

    so ∆G = -1500.4 - (-174.8) = -1325.6 kJ/mol

    p.s. I see why you asked now :p:

    I'd assume it's the same as enthalpy change; write the equation for the combustion of ethanol, write the elements beneath them, and draw 2 arrows up from the elements to products and reactants. Put in the values for these, then add some things together :p: (Remember it's negative the value on the arrow going to ethanol)
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Updated: November 12, 2008

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