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enthalpy confusion
Josh C
#1 Posted : Wednesday, June 22, 2011 5:32:40 PM
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Q1) if change in internal energy is (heat added to system) + (work on system), why is the change in enthalpy = change in internal energy + pressure * change in volume- i,e, work? isnt this already accounted for by the change in internal energy term?

clarkeeuwes
#2 Posted : Thursday, June 23, 2011 12:21:57 AM
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Josh - you're absolutely right in recognizing these equations are closely related; if we plug them into one another (and keep our +/- signs consistent!), we see how the whole thing collapses into something we already know.


Remember that in ΔE = q + w, when the system does work on the surroundings (expands) we consider this negative, so w is equal to (-PΔV).

When we put that into our enthalpy equation ΔH = ΔE + PΔV, we see that ΔH = (q -PΔV) + PΔV, or ΔH = q. The mechanical work term completely drops out of the equation.

This means that a change in enthalpy depends on the heat input.
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