[Corvettaholic]

OK, I'm trying to self-teach myself chemisty because I can't afford tuition at ASU. I think I got entropy down, which is the tendency of energy to disperse as much as possible, and everything wants to be at max entropy. Luckily, chemical bonds prevent that from happening (I don't want to be mush).

Enthalpy... what exactly is that? The impression I get is that it has something to do with an energy change after a reaction? Does it tell you if a reaction is endothermic or exothermic?

[nemzy]

enthalpy, H, is the quantity U+PV.  U being the sum of the kinetic and potential energies of the particles making up a system.  While p = pressure and v = volume

If change in H is -, it is exothermic

If change in H is +, it is endothermic

[nemzy]

if it is endo..heat is evolved

and if it is exo..heat is absorbed

[Corvettaholic]

So you compare the enthalpy of the reactants with the enthalpy of the product(s), then you can figure out endo or exo, right? Sounds easy enough. If the product is solid and the reactants gas, that means the kinetic energy of the atoms goes way down, so that would be a pretty good cue its endothermic. If the volume greatly increases, maybe it melted or sublimed, and therefore likely exo. For calculations, assume pressure is at 1 atm?