Thermodynamics Question

[90210]

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Okay, so recently I encountered something that confused me.

First of all, the internal energy of a system is defined by this equation:
deltaE = q + w (E = Energy; q = heat; w=work)

Where "E" is considered a state function (but q and w are not).

However, reading TPR Biology, I saw this equation (which I vaguely remember from undergrad). Of the prep materials I used, I don't think any book presented this:

deltaH = deltaE - PdeltaV

If you rearrange this equation:

deltaE = deltaH + PdeltaV

It would imply that deltaH = q.

I know they are both forms of energy, but how do you explain the discrepancy between enthalpy being a state function and q (heat) not ...yet this equation is implying they are the same thing.

 

[MT Headed]

You are implying that w=PdeltaV. That is only true when pressure is a constant.

When pressure is a constant, PdeltaV (the amount of mechanical work) is also a state function. It only depends on the initial and final values of P and V, and not what path you took to get there.

 

[movax]

It would imply that deltaH = q.

I know they are both forms of energy, but how do you explain the discrepancy between enthalpy being a state function and q (heat) not ...yet this equation is implying they are the same thing.

At constant pressure, delta H is heat. This has a high chance of appearing on the test, IMHO. Think about the coffee cup calorimeter vs. the bomb calorimeter, for instance. The former is constant pressure, the latter is constant volume.