5.3 Hess's Law and Enthalpies of Formation

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    Hess’s Law

    3NO(g) ➔ N2O(g) + NO2(g)           ΔH°rxn = ?

    THERMODYNAMIC DATA
    2NO2(g) ➔ 2NO(g) + O2(g) ΔH° = +113kJ
    2N2(g) + O2(g) ➔ 2N2O(g) ΔH° = +163kJ
    N2(g) + O2(g) ➔ 2NO(g) ΔH° = +181kJ

    C2H4(g) + 6F2(g) ➔ 2CF4(g) + 4HF(g)          ΔH°rxn = ?

    THERMODYNAMIC DATA
    H2(g) + F2(g) ➔ 2HF(g) ΔH° = -537kJ
    C(s) + 2F2(g) ➔ CF4(g) ΔH° = -680kJ
    2C(s) + 2H2(g) ➔ C2H4(g) ΔH° = +52.3kJ

    C3H8(g) + 5O2(g) ➔ 3CO2(g) + 4H2O(l)     ΔH°rxn = ?

    THERMODYNAMIC DATA
    2C3H8(g) + 7O2(g) ➔ 6CO(g) + 8H2O(g) ΔH° = -2390kJ
    CO(g) + ½O2(g) ➔ CO2(g) ΔH° = -283kJ
    H2O(l) ➔ H2O(g) ΔH° = +40.7kJ

    Enthalpies of Formation

    ΔH°rxn = ∑nΔH°products - ∑mΔH°reactants

     

    Given the following standard enthalpy of formation values, calculate the ΔH°rxn of the following reaction:

    3NO(g) ➔ N2O(g) + NO2(g)          ΔH°rxn = ?

    ΔHf°    (kJ/mol)
    NO(g) 90.4
    N2O(g) 81.6
    NO2(g) 33.8

    What is a Formation Reaction?

    1) Form 1 mole of a single product

    2) Reactants are elements in their standard states.

    STANDARD STATES
    Gases He, Ne, Ar, Kr, Xe, Rn, H2, N2, O2, F2, Cl2
    Liquids Br2, Hg
    Solids Everything Else
    Special C(graphite), S8