[INeedSerotonin]

Hello

I was doing some exercises, and this one asked me to tick "true" or "false": "The chemical reaction of CaO with water is a process in which there is absortion of heat."

Is there a way to know it without having studied this specific reaction previously? The answer to the problem is "false", but I just don't know why.

Thank you in advance

[mjc123]

In general, you just have to know the reaction; there's no simple way of predicting. If you know about the bond energies of various chemical bonds, you can estimate whether there is an overall increase or decrease in bond energy, but in condensed phases you have to consider intermolecular interactions as well, and the approach doesn't really work for ionic compounds.

I'm not sure what level you're at (as you posted in the high school forum), but one approach in this case may be to say that as you go down from two molecules to one, there is probably a decrease in entropy, which means ΔH must be negative to ensure ΔG is negative.

[INeedSerotonin]

Thank you. I like your explanation that a decrease in the number of molecules is an evidence of an exothermic reaction. I found this a strange exercise; usually the enthalpy is given, but not this time.

[mjc123]

I like your explanation that a decrease in the number of molecules is an evidence of an exothermic reaction.

Not invariably. I would say "a hint" rather than "evidence". There are so many variables. And it's a bit of a cheat in this case - "CaO" is not really a molecule; calcium oxide is an ionic solid.

[Fish200398]

http://chemistry-reference.com/reaction.asp?rxnnum=183 it says exotherm right? release energy aka heat