Chapter 11

Properties of solutions:

concentration of solutions

solution process

Solubility and equilibrium process

Solute: electrolytes and non electrolytes

The raoult's law

colligative properties of solutions

Solutions are: Homogeneous mixtures composed by Solvent (the major component) Solute (the other components)

solvent and solute can be:
Solid
Gas
Liquid

Solid solution are called alloys can be: substitutional alloy (Zn + Cu = Brass) or interstitial alloy (C + Fe = carbon Steel)

  • m/V --> g/L grams of solute per liters of solutions
  • ppm --> g/1*10^6g grams of solute per tons of solution
  • Xi = ni/ntot (Σni=1) Mole fraction
  • % w/w grams of solute in 100 grams of solution
  • % v/v mL of solute in 100 mL of solution
  • Molarity --> mol/L moles of solute per liter of solution
  • molality --> mol/kg moles of solute per kilos of solution

Liquid solutions

"like dissolves like"

Henry's Law
[solutions of a gas into a liquid solvent]
S(g) = k(H) * P(gas)

  • S(gas) is the solubility of the gas
  • P(gas) is the partial pressure of the gas
  • kH is the Henry's constant (mol / L*atm)

Solubitlity of a gas increase as Pressure increases and as Temperature decreases.

the gas dissolution is Exothermic (dh<0)

[solutions of a solid or liquid into a liquid solvent]:

  • Salt + Water
  • Polar Solute + Polar Solvent
  • Alcohol + Water
  • Apolar solute + Apolar solvent

Short chain alcohols dissolves very well in water (both polar), but for long chain the idrophobic group R is an obstacle. (The latter are more soluble in Hexane, which is apolar)

Gibbs free Energy change ΔG
is a state function related to potential energy of a chemical/phisical process

  • ΔG < 0 (increase of Universe entropy ΔS increse)
  • ΔG > 0 process is not spontaneous
  • ΔG = 0 process is at equilibrium

Gibbs-Helmholtz equation
ΔG = ΔH - T*ΔS
[ΔS change in entropy = Heat at constant pressure - disorder in a mixture]

for Solutions:
ΔG(sn) = ΔH(sn) - T*ΔS(sn)

ΔG < 0
the solution froms spontaneously:

  • ΔS > 0 increase the molecular disorder
  • ΔH = 0 it is an ideal solution (ΔG=-TΔS)
  • ΔH < 0 exothermic (ΔG = -ΔH-TΔS <<0)
  • ΔH endothermic |ΔH|<|TΔS|

Heat of solution: ΔH(sn)
solute and solvent separate: ΔHso>0, ΔHsv>0
solute and solvent mix ΔHmix<0
ΔHsn =ΔHso+ΔHsv+ΔHmix
ΔHsn can be 0, <0, >0

CaCl2 + H2O is exothermic (anti-freezing)
NH4NO3 +H2O is endothermic (ice pack)
NaCl + H2O is endothermic
NaOH + H2O is exothermic

Water as a Sovent causes Hydratation:
this means ions in water are never alone, they're always surrounded by H2O molecules (called Hydratation shells) (8 - octahedral shape to minimize repulsion)

Heats of Hydration
solvatation of ions by water is always exothermic
ΔHsn = ΔHlatttic + ΔHhydr

Solubilityis the max amount of solute that can be dissolved in a solvent at a given temperature.
[usually are more soluble at high T (but not CaCO3)]

Electrolytes and non-Electrolytes
Electrolyte is a solute that forms a conductor solution.

  • Strong electrolytes (completely dissociated) α = 1
  • Weak electrolytes (partially dissciated) 0<α<1
    [α = num. diss.molecules / tot. diss. molecules]

Non electrolytes do not fors ions so are not conductors
[sugar dissolves in H2O but is not a conductor]

Colligative's property of solutions
[do not depends on number of particles]
vapour pressure depression:
ΔP = P0(sv) - P(sv) = X(so) * P0(sv)
[non-volatile solute makes the vapour pressure of the solvent decrease]

Boiling Point Elevation and Freezing point depression:
ΔTb = kb*m*i ....................ΔTf = kf*m*i

Osmotic pressure:
π = M*R*T*i
M molarity
R = 0.0821
T temperature in K

The Raoult's Law
ΔP = P0(sv) - P(sv) = Xso P0(sv)
P(sv) = X(sv) P0(sv)


This is true for ideal solutions ΔHsn = 0

for electrolytes one need the Van't Hoff factor "i"
i = [1 + (v - 1) * α]


ΔP = (n(so) * i(so) / n(so) * i(so) + n(sv)) * P0(sv)

Deviation from Ideal Solutions (ΔH(sn) = 0)

  • solute - solvent interactions are weak => positive deviation (evaporates easily)
  • solute - solvent interactions are strong => negative deviation (evaporates difficultly)

see phase diagram of pure vs solution of H2O

Osmosis and Osmotic Pressure
π = M*R*T*i
π = n(so)/V(sn)*R*T*i

osmotic pressure is the applied hydrostatic pressure required to prevent the net flow of the solvent against % gradient

Substances can be:

  • isotonic
  • ipotonic
  • ipertonic

PAG 37 Lesso 11

Saturated soltions
are in a dynamic equilibrium
solute dissolves =solute undissolves

  • saturated
  • unsaturated
  • supersaturated

equilibrium Contant for saturated solutions:
K = [Mr+]^n * [Xs-]^m / [Mn * Xn]
K = [Mr+]^n * [Xs-]^m [for solid concentration]


Mr+ cation
Xs- anion