AP Chemistry – Solutions
solution: homogeneous mixture; solid, liquid, or gassolvent: substance present in the greatest amount
solute: also present; there might be more than one
OH H
OH
H +O
H HO
HHO
HHOH H
d+d+
d–
+ – + – + – + – – + –+ – + – +– + – + – +– +
-- then solvation: solvent particles surround soluteparticles (if solvent is H2O, we say “hydration”)
-- solvent particles“pull” solute ionsfrom the crystal
Intermolecular forces (IMFs) operate betweensolvent and solute particles. Solutions formwhen solute-solvent IMFs are comparableto solute-solute IMFs.
Solvation/hydration
= “glomming”
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;
DH1 > 0
DH2 > 0
DH3 < 0
S = DHsoln
-- If DHsoln is too (+)(i.e., too endo),soln won’t form.
(3) above isn’t very (–) foroil and water and, with (1)and (2) requiring energyinput, this doesn’t form a solution.
For polar/nonpolar, (3) isn’t (–) enough, and(1) and (2) lead to a DHsoln that is too (+).
Spontaneous processes tend to occur...
(A) when DHsoln is (–), (i.e., ____)
(B) when the disorder of the system increases
** NOTE: (B) is MORE influential indetermining whether or notsoln forms
exo
The enthalpy of solution formation (i.e., DHsoln) forammonium nitrate is (+), but the system’s disorderincreases from NH4NO3(s) to NH4
+(aq) + NO3–(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, considerthe following: If the product is evaporated to dryness, a solution
would give you what you started with.
e.g.,
NaCl(s) + H2O(l) NaCl(aq)
Ni(s) + 2 HCl(aq) NiCl2(aq) + H2(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 SOLUTIONsolution
crystallization
A saturated solution of NaCl,in which the rates of solutionand crystallization are equal.
equilibrium