6.5 Enthalpy of Formation

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    Standard Enthalpy of Formation

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

    n = moles products     m = moles 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) 90.4kJ
    N2O(g) 81.6kJ
    NO2(g) 33.8kJ

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

    N2H4(g) + O2(g) → N2(g) + 2H2O(g)          ΔH°rxn = ?

    ΔHf° (kJ/mol)
    N2H4(g) +95.4kJ
    H2O(g) -242kJ

    Formation Reactions

    FORMATION REACTIONS
    1. Form 1 mole of a single product
    2. Reactants are elements in their standard state.
    STANDARD STATES
    Gases He, Ne, Ar, Kr, Xe, Rn, H2, N2, O2, F2, Cl2
    Liquids Br2, Hg
    Solids Everything Else
    Special C(s, graphite), S8

    Allotrope – one of multiple possible physical forms of an element

    Which of the following are formation reactions?

    2H2(g) + O2(g) → 2H2O(l)
    CO(g) + ½O2(g) → CO2(g)
    C(s, diamond) + O2(g) → CO2(g)
    C(s, graphite) + O2(g) → CO2(g)