Solutions Factors that affect solubility intermolecular...

Solutions

Colligative Properties •  vapor pressure lowering

Raoult’s Law: PA = XAP°A •  boiling point elevation

ΔTb = Kbm •  freezing point depression

ΔTf = Kfm •  osmotic pressure

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π =nV⎛

⎝ ⎜

⎞

⎠ ⎟ RT = M RT

Colloids 1

Factors that affect solubility •  intermolecular interactions

(like dissolves like) •  temperature •  pressure

Mary J. Bojan Chem 110

SOLUBILITY

FACTORS THAT AFFECT SOLUBILITY

Temperature –  Effect of Temperature on solubility of ionic solids –  Effect of temperature on solubility of gases

Type of intermolecular interactions and their strengths

(LIKE DISSOLVES LIKE)

Pressure 2 Mary J. Bojan Chem 110

How does solubility of ionic solids change with temperature?

3 Mary J. Bojan Chem 110

How does solubility of gases change with temperature?


In general: like dissolves like –  polar solvents dissolve polar (and ionic solutes) –  nonpolar solvents dissolve non-polar solutes

If intermolecular forces broken and formed are ~ the same, dissolution is probable.

How do intermolecular forces affect solubility?

Which one of the following will be most soluble in benzene (C6H6(l))?

1. H2O(l) 2. CH3OH(l) 3. HCl(l) 4. CH3CH2OH(l) 5. heptane


Like dissolves like Alcohol solubility in water

at 25°C (g/100g of H2O)

CH3OH total CH3CH2OH total

CH3CH2CH2OH total

CH3CH2CH2CH2OH 8.06

CH3CH2CH2CH2CH2OH 2.82

CH3CH2CH2CH2CH2CH2OH 0.62

6

As length of hydrocarbon chain increases, solubility of alcohol in water decreases.


Put these in order of increasing solubility in water


Which vitamin is soluble in fat? Which vitamin is water soluble?

8

Fat soluble vitamins can be stored in the body

Water soluble vitamins are not stored in appreciable amounts


Increase Psolute(gas); increase solubility Decrease Psolute(gas); decrease solubility

How does solubility of gases change with pressure?

amount of gas dissolved (Cg) is proportional to the partial pressure of that gas (Pg) above the solution.

k = Henry’s Law constant

9

HENRY’S Law Cg = k Pg

solubility Cg partial pressure Pg


Deep Sea Diving

Solubility increases with increasing P

− amount of N2 in blood increases as depth increases.

As divers rise, N2 bubbles out of blood causing painful “bends”.

Solution: use He/O2 mixture when diving deep. He is not as soluble in blood.


Ammonia Fountain

H2O + phenolphthalein (pink in base)


Test yourself To increase the solubility of N2 in water

1. increase T 2. decrease T 3. increase P 4. decrease P

How will the solubility of KClO3 be effected by an increase in pressure?

1. increase solubility 2. decrease solubility 3. no effect on solubility


COLLIGATIVE PROPERTIES

Properties that depend on the number of solute particles (collections) in solution but not on the identity of the solute.


Examples of Colligative properties •  vapor pressure lowering •  boiling point elevation •  freezing point depression •  osmotic pressure

Example: What is the total particle concentration in a 0.2M solution of Pb(NO3)2 ? (salt, strong electrolyte) [Pb+2]= [NO3

−]=

total concentration of particles =

COLLIGATIVE PROPERTIES


What is the total particle concentration 0.2M solution of CH3COOH (acetic acid)? (weak acid, weak electrolyte)

CH3COOH(aq) H+(aq) + CH3COO−(aq)

VAPOR PRESSURE LOWERING

Raoult’s Law: PA = XAP°A

PA ≡ vapor pressure of solution XA ≡ mole fraction of solvent P°A ≡ vapor pressure of pure solvent


Vapor pressure lowering is a colligative property It depends on the concentration but not on the nature of the solute.

rate of evaporation ↓ vapor pressure of SOLVENT ↓

VAPOR PRESSURE LOWERING


BOILING POINT ELEVATION FREEZING POINT DEPRESSION

ΔTb = Kbm Kb = molal boiling point elevation constant

ΔTf = Kfm Kf = molal freezing point depression constant

m ≡ molality of the solution


Kb and Kf are tabulated for different solvents

Eg. water Kb= 0.52°C/m Kf= 1.86°C/m benzene Kb= 2.53°C/m Kf= 5.12°C/m

Example

What is the freezing point of seawater assuming all of the salinity* is due to NaCl?

(MW of NaCl = 58.5 g/mol)

*ocean salinity is ca. (35 g salt)/(1 kg seawater) mainly: Cl–, Na+, SO4

2-, Mg2+, Ca2+, K+ 18 Mary J. Bojan Chem 110

Osmosis flow of molecules through a membrane

Osmotic pressure: π Pressure needed to stop the flow of molecules through a membrane

€

π = nV⎛

⎝ ⎜

⎞

⎠ ⎟ RT = MRT

units of atm molarity


Osmosis through a membrane

Hypertonic: solution: water moves out of cell

Hypotonic solution: water moves into cell; can cause cell to rupture.

Isotonic: solution that has same osmotic pressure as cell. 20 Mary J. Bojan Chem 110

Reverse Osmosis exert pressure on a concentrated solution, forcing solvent molecules through a membrane. Solute molecules are trapped in concentrated portion.)

Used to purify water.


An aqueous solution contains 3.50 mg of protein dissolved in sufficient water to form 5.00 mL of solution. The osmotic pressure of the solution at 25°C was found to be 1.54 torr.

Calculate the molar mass of the protein.


COLLOIDS

true colloidal mixture solution dispersion uniform uniform non-uniform molecules 20-2000Å sedimentary

particles particles

NaCl milk (fat particles) silt H2O fog (water droplets)

small large Particle size


Light scattering Tyndall effect: light is scattered when λ of light ≅ particle size

For colloids (size~ 200nm) λ is in visible region: Light passes through colloidal suspensions with scattering (milk, fog).


For molecules (~5Å) λ in X-ray region •  visible light passes through solutions without scattering •  Atmosphere scatters small λ light

Explains why the sky is blue

Test Question Which of the following aqueous solutions has the greatest total concentration of ions?

1.  0.2M NH4NO3 2.  0.2M Pb(NO3)2 3.  0.2M Na2SO4 4.  0.2M AlPO4 5.  0.2M AlBr3 6.  0.2M CH3COOH (acetic acid)


Solutions Factors that affect solubility intermolecular...

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