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Name: Andy
Status: student
Grade: other
Location: N/A
Date: September 2008


Question:
What is enthalpy? I mean, what does it really mean? Is there any physical interpretation? Perhaps a more tangible one? I have been reading about it being the sum of thermodynamic properties H = U + PV. But is it really just that? Is it only a sum of terms? Is enthalpy only defined by a mathematical expression? All of these is really confusing. I am looking for a real physical definition like when you say longitude is a distance between point A and B. So it is easy to understand that a certain piece of paper has 11 inches. But what does it imply to say, water at this temperature and pressure has an enthalpy value of X?



Replies:
Andy,

This is a good question, but it does have a somewhat long answer, so here goes:

The first law of thermodynamics states that energy is conserved. This means that it any transfer, the amount of energy in the closed system does not change. This also means that any increase or decrease in the energy of the system must be because energy came into or out of (respectively) the system.

The result of this is that we can *track* energy (imagine an auditor or an accountant keeping tabs of how much energy went in or out). So we can *invent* a tracking term called "internal energy". Notice that internal energy is always written as a "delta" E - this is because we want to emphasize that all we are doing is monitoring the changes in the energy of the system (whether something went in or out).

It is important to remember that we can not speak of "E" (without the delta) only. This is because we have no access to how much energy is in the system. We can only know how much *change* happened.

Enthalpy is derived from internal energy. Enthalpy is another invented term and is another bookkeeping term or a way of tracking energy changes. We talk of enthalpy as the change in internal energy when the only changes to speak of is heat going in an out of the system.

In this sense we can talk of how much heat went into a mole of substance in a particular process. For example: when 18g of ice (one mole of water) melts, we can talk of the enthalpy of melting - how much heat had to have gotten into the system in order for 1 mole of ice to change to 1 mole of liquid water.

So - enthalpy is a measure of changes in heat into or out of a system. As such, it is not as tangible as 11 inches of paper, because we are talking about a change, the amount of heat that caused a particular change.

I hope that helped.

Greg (Roberto Gregorius)


It may be easier to see what enthalpy "really is" if the definition: H = E + PV is expressed as a difference, since all thermodynamic properties except entropy (but that is a different story, the Third Law of Thermodynamics). So call dH the "change in enthalpy", dU the "change in energy" and the product d(PV) = P x dV + V x dP. Combining all this we get: dH = dE + VdP +PdV. Now most chemical reactions are run at constant pressure -- usually the atmosphere -- so P is constant or equivalently dP = 0. This leaves: dH = dU + PdV. The second term PdV is the work (It's basically a force x distance) done by the process (for example a chemical reaction). If we rearrange the equation: dH = dU + PdV to read: dU = dH - PdV. So the energy change dU is the enthalpy change dH with the energy that goes into doing work by the process against the atmosphere (or the other way around, the work done by the atmosphere on the process). If the process does work against the atmosphere, pushing the atmosphere "out of the way" then the energy change is less "than it should be". That is the physical meaning of enthalpy. If you have to ponder this a bit, do not feel bad. It took the noted physical chemist/physicist J. Willard Gibbs in 1875 -- historically not that long ago -- to fully unravel the utility of enthalpy. My only further caution is do not be too sure you understand what energy "really IS". That is one of the slippery concepts that we become used to because "everybody knows what it IS", until you try to put a precise definition on it. Then you end up with a lot of examples, but a circular definition.

Keep digging,

Vince Calder



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