Thermochemistry

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My question is more conceptual. It is expressed in a series of two problems:

What energy change occurs when 1.2 moles of methane combust with an enthalpppy change of -434 KJ?
CH4+2O2 TO CO2+2H20
I understand it is simply the moles of mthane times the enthalpy change yet I am curious if a stoichiometry set-up exists which our teacher neglegected to teacch us. That would manifest if given moles of oxygen. I would infer you either convert to methane, as 434 is per mol of methane. Or set up stoich and end up dividing by 4 somewhere.

Another example would be:

The heat of formation of Fe2O3 is -826 KJ/mol. Calc heat if 30 g of iron reacts.
4Fe+3O2 TO 2Fe2O3

As the heat is for the product the moles of iron must be converted to iron oxide, Hence:

30 g * 1 mole Fe/55.35 g * 2 mole Fe2O3/4 mole Fe * -826 kJ/1 mole Fe2O3

Any help would be greatly appreciated.

  • Thermochemistry -

    a. Enthalpy for the reaction is 434 kJ and that is for 16 g CH4 (1 mol).
    So 434 x 1.2/1 = ? or you can do
    1.2 mol CH4 = 1.2*16 = 19.2 and
    434 kJ x 19.2/16 = ?

    b. dHf for 1 mol = 826 kJ; therefore, dH rxn as written is 826/mol x 2 mol = 1652 kJ.

    1652 kJ x (30g/4*atomic mass Fe) = ? kJ.

  • Thermochemistry -

    Thanks!

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