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Math Help - Enthalpy using Hess's law (Chem)...

  1. #1
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    Enthalpy using Hess's law (Chem)...

    This is the question:


    Now, I've gotten as far as this:

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    Don't know if it's right or wrong, but solve and post all the steps..
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  2. #2
    MHF Contributor Unknown008's Avatar
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    Not all the equations show up... =/

    But I suppose that's what are given and this is how I work them out:



    You are already given \Delta H_1 which is -91.8\ kJ/mol. (and this I think is the enthalpy of reaction and not formation and hence, you do not divide by 2)

    Then, \Delta H_2 = (3\times -241.8)\ kJ/mol (since you have 3 mol of water being formed from my Hess Cycle)

    Then, \Delta H_3 = (2\times -906.2)\ kJ/mol (I think they gave you the standard enthalpy of combustion of ammonia. So, for each mol of ammonia burnt, you get 906.2 kJ but since you burn 2 moles, it's twice)

    Then, \Delta H_4 = ? kJ/mol This is what you need to find I think.

    Then, from Hess' Law, we find that by following the arrows:

    \Delta H_2 + \Delta H_4 = \Delta H_1 + \Delta H_3

    This will give you the answer.
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