AP Chemistry – Solutions

solution: homogeneous mixture; solid, liquid, or gas solvent: substance present in the greatest amount solute: also present;there might be more than one

O HH O HH + O HH O HH O HH O HH ++ ++ –– + – + – + – + –– + – + – + – + – + – + – + – + -- then solvation: solvent particles surround solute particles (if solvent is H 2 O, we say “hydration”) -- solvent particles “pull” solute ions from the crystal Intermolecular forces (IMFs) operate between solvent and solute particles. Solutions form when solute-solvent IMFs are comparable to solute-solute IMFs. S o l v a t i o n / h y d r a t i o n = “ g l o m m i n g ”

Energy Changes and Solution Formation (1) solute particles are separated from the crystal lattice; (2) solvent particles separate to make room for solute particles; (3) solvent and solute particles are attracted to each other;  H 1 > 0  H 2 > 0  H 3 < 0  =  H soln -- If  H soln is too (+) (i.e., too endo), soln won’t form. (3) above isn’t very (–) for oil and water and, with (1) and (2) requiring energy input, this doesn’t form a solution.

For polar/nonpolar, (3) isn’t (–) enough, and (1) and (2) lead to a  H soln that is too (+). Spontaneous processes tend to occur... (A) when  H soln is (–), (i.e., ____) (B) when the disorder of the system increases ** NOTE: (B) is MORE influential in determining whether or not soln forms exo The enthalpy of solution formation (i.e.,  H soln ) for ammonium nitrate is (+), but the system’s disorder increases from NH 4 NO 3 (s) to NH 4 + (aq) + NO 3 – (aq), and so solution formation is still spontaneous.

Sometimes it is not readily apparent whether a solution has been formed or whether a chemical reaction took place. To help you decide, consider the following: If the product is evaporated to dryness, a solution would give you what you started with. e.g., NaCl(s) + H 2 O(l)  NaCl(aq) Ni(s) + 2 HCl(aq)  NiCl 2 (aq) + H 2 (g) (SOLN) (RXN)

Solubility Crystallization is the opposite of the solution process. -- When the rates of solution and crystallization are equal, __________ is established. SOLUTE + SOLVENT SOLUTION solution crystallization A saturated solution of NaCl, in which the rates of solution and crystallization are equal. equilibrium