6.3 Enthalpy

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    Enthalpy Change (ΔH)

    ΔH = qp


    Enthalpy change is equal to the heat change of process/reaction when carried out at constant pressure.  Enthalpy change is also a state function.  A state function is a function that depends only upon the initial and final states of the system being independent of path.

    Endothermic vs. Exothermic Reactions

    Endothermic (ΔH > 0) reactions absorb heat.

    Exothermic (ΔH < 0) reactions release heat.


    Fusion (a.k.a. melting, the conversion of solid to liquid), vaporization (a.k.a. boiling, the conversion of liquid to gas), and sublimation (the conversion of solid to gas) are all endothermic phase changes requiring the absorption of heat.


    Freezing (the conversion of liquid to solid), condensation (the conversion of gas to liquid), and deposition (a.k.a. vapor deposition, the conversion of gas to solid) are all exothermic phase changes releasing heat as they occur.

    phase changes

    Enthalpy and Stoichiometry

    2H2(g) + O2(g) → 2H2O(l)       ΔH°rxn = -572kJ

    What is the enthalpy change when 3 moles of O2 are consumed in the reaction above?



    What is the enthalpy change when 5 moles of H2 are consumed in the reaction above?



     What is the enthalpy change when 9g H2O(l) are produced in the reaction above?