Enthalpy and how it differs from internal energy, heat capacity and calorimetry, standard enthalpies of reaction, formation and combustion, bond enthalpies, and Hess's law.
3 lessons, about 1 h 20 min3 simulations11 problems4 practice questionsPython in every lesson
Created by StemPy LabMost reactions run in open vessels at constant pressure. The heat they take in there is the enthalpy change, H = U + pV. For gases it differs from the internal energy change by the change in moles of gas times RT. Heat capacity links heat to a temperature change, and calorimeters use it to measure both quantities.
Enthalpy changes are compared under standard conditions: pure substances at 1 bar. A thermochemical equation gives the enthalpy change for the amounts written. Melting, boiling and subliming have their own enthalpies. Standard enthalpies of formation give any reaction enthalpy as products minus reactants. Combustion, atomisation and solution enthalpies are other useful kinds.
Hess's law says the enthalpy change of a reaction is the same whether it happens in one step or many. So thermochemical equations can be added like algebra to find enthalpies that cannot be measured directly. Bond enthalpies give another route: for gases, the bonds broken minus the bonds formed estimates the reaction enthalpy.
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