How can you tell if a double displacement reaction will occur?

Question

One of my classmates told me that if at least one of the products of the reaction (if it occurred) is a solid, liquid, or gas, that the reaction will happen, but that if all of the products are aqueous, then it won't occur. However, she also said she wasn't completely sure. Was she right?

Answer 1

She is right. Formation of a solid or gas takes product out of the reaction. Either it drops to the bottom or passes off into the air. This drives the reaction to completion. By liquid, I think she means formation of water or another only slightly ionized liquid like acetic acid. That also takes product out of the mixture, even though it's still soluble. By "aqueous," I think she means that the ions stay as ions and thus stay in the mixture. In this, she is also right.

Answer 2

That is mostly correct. Technically speaking, there is no such thing as a double replacement reaction. If everything is aqueous then those ions will just be floating around in solution, they won't form an actual compound.

To figure out if a reaction will occur, you need much more theory. For starters there's the HSAB theory (my least favorite of any chemistry theory) which states that there are "hard" and "soft" acids and bases. soft bases will want to bond to soft acids, hard acids will want to bond with hard bases.
check this out if you want to hear about it more:
http://en.wikipedia.org/wiki/Hsab

But to see if a reaction is spontaneous (will occur) you need to calculate the free energy of the reaction.

G= H - TS
where G is the free energy, H is the enthalpy, T is the temperature, and S is the entropy.

if G is negative, then the reaction is spontaneous and the reaction will proceed to the right.