About entalpy change

[yifan]

Does anybody know why the entalpy change for a reaction differs in different pressure and temperature?

[shrumeo]

enthalpy is a state property of a substance, like temperature and pressure
it also varies with temperature and pressure via H=U+PV
U is the internal energy, and you know what P and V are

so at different temps and pressures, a substance will have different enthalpies
this will change the deltaH in a reaction between two substances

[GCT]

Enthalpy pertains to reactions in open systems. To be truthful, enthalpy does not directly pertain to the energy of a chemical reaction, the PV which shrumeo mentioned relates to a separate work function...that is when a exothermic chemical reaction occurs, the energy can be used to change the internal energy U as well as expand against the atmosphere PV.

The energy of a specific chemical reaction is constant, and in most cases PV is considered negligible. Think bond energies.

Thus the enthalpy of the reaction does not change, rather different amounts of energy is released at different temperature and pressure since these two factors influence how many of such individual reactions take place.

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