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