# Energy vs. Enthalpy

1. Sep 9, 2009

### pzona

I'm currently working on problems comparing $$\Delta$$E to $$\Delta$$H (in reaction systems and physical processes) and I'm having a bit of a hard time understanding the actual difference between the two. I know what each is, and I realize that they're completely different (although dependent of each other). I can recite their definitions but I'm not sure how to think of one as opposed to the other in terms of real life application. I was wondering if anyone could explain this difference better than my textbook. Any help is appreciated.

2. Sep 9, 2009

### Mapes

I don't know if this helps, but the enthalpy of a system is its energy plus the work needed to move the surroundings out of the way.

For condensed systems at constant pressure, $\Delta E$ and $\Delta H$ are relatively close, since volume changes in these systems are small compared to gaseous systems.

Another way to look at enthalpy is that it's the quantity that's spontaneously minimized in an adiabatic (thermally insulated) system at constant pressure.

3. Sep 10, 2009

### Topher925

Do you understand the differences between exergy and gibbs free energy? Exergy and gibbs free energy is analogous to energy and enthalpy.

4. Sep 11, 2009

### alxm

There's no specific relationship between energy and enthalpy. Enthalpy is a certain form of energy.

If you have a change in energy, $$\Delta E$$, then how that relates to $$\Delta H$$, if it relates at all, depends entirely on which change in energy $$\Delta E$$ is supposedly measuring.

5. Sep 13, 2009

### pzona

I'm looking at potential and kinetic energies for E (in this case, whichever energy applies to the system, depending on the question).

Another related question: E=(3/2)RT can be used only for an ideal gas, correct? I had a problem in which I used this for the combustion products of decane, but I don't remember hearing anything else in class for "real" gases, so I used this since I knew the temperature change for the system. Would this give me an answer close enough to the real value?

6. Sep 14, 2009

### mazinse

Yeah the real gas equation is big and long and a headche to use, I dont think it's used at all in academic problems. And yes whenever energy is mentioned for the overall reaction, use the E. The enthalpy is related to energy in terms of the gibbs free energy equation.

So in a physics problem you would have Kinetic + potential = delta E, beware of the + and - signs.

7. Sep 15, 2009

### Mapes

H = E + PV? Seems pretty specific to me.

8. Sep 15, 2009

### pzona

That's the equation we've been using for a lot of the problems. The hardest part is learning to think of each variable in terms of real things. I'm getting better at it, but I guess it's like everything in chem, it just takes a lot of practice