SOLUBILITY AS AN EQUILIBRIUM PHENOMENA

Equilibrium in Solution

solute (undissolved) solute (dissolved)

Solubility

  A saturated solution contains the maximum amount of solute that will dissolve in a given solvent at a specific temperature

  An unsaturated solution contains less solute than the solvent has the capacity to dissolve at a specific temperature

  A supersatured solution contains more solute than is present in a saturated solution at a specific temperature

Crystallization

  Sodium acetate crystals rapidly form when a seed crystal is added to a supersaturated solution of sodium acetate

Ways of Expressing Concentration

  The concentration is the amount of solute present in a given quantity of solution or solvent.

  Parts by mass  Percent by mass (parts per hundred)  Parts per million (ppm)  Parts per billion (ppb)

  Mole fraction   Molarity   Molality

Ways of Expressing Concentration

Mole Fraction (X)

XA = moles of A

sum of moles of all components

M = moles of solute

liters of solution

Molarity (M)

Molality (m)

m = moles of solute

mass of solvent (kg) Converting between

molarity (M) and molality (m) requires density.

Ways of Expressing Concentration

Percent by Mass

% by mass = x 100% mass of solute mass of solute + mass of solvent

= x 100% mass of solute mass of solution

Parts per milion

ppm = x 106 mass of solute mass of solution

Ways of Expressing Concentration

Parts per milion

For dilute solutions (in the ppm and ppb level), density is almost equal to 1 g/mL

ppm = x 106 mass of solute mass of solution

ppm = gsolute gsolution

1 gsolution mLsolution

1000 mLsolution

1 Lsolution

1000 mgsolute

1 gsolute

x x x

ppm = mgsolute L solution

Exercise

1) A solution is made by dissolving 13.5 g of glucose, C6H12O6, in 0.100 kg of water. What is the mass percentage of solute in this solution?

2) A 2.5 g sample of ground water was found to contain 5.4 μg of Zn2+. What is the concentration of Zn2+ in parts per million?

3) The solubility of MnSO4∙H2O in water at 20°C is 70 g per 100 mL of water. Is a 1.22 M solution of MnSO4∙H2O in water at 20°C saturated, supersaturated or unsaturated?

Exercise

4) What is the mass percentage of NaCl in a solution containing 1.50 g of NaCl in 50.0 g of water?

5) What is the molality of a 5.86 M ethanol (C2H5OH) solution whose density is 0.927 g/mL?

6) What is the mole fraction of HCl in a solution of hydrochloric acid that has 36% HCl by mass? What is its molality? What is its molarity? The density of the acid solution is 1.305 g/mL

Exercise

7) Calculate the molality, molarity and mole fraction of FeCl3 in a 28.8 mass % aqueous solution (with density of 1.280 g/mL)

8) An automobile antifreeze mixture is made by mixing equal volumes of ethylene glycol (d=1.114 g/mL; MW = 62.07 g/mol) and water (d=1.00 g/mL) at 20°C. The density of the resulting mixture is 1.070 g/mL. a) % volume c) molarity e) mole fraction b) % mass d) molality

Intermolecular Forces Temperature Pressure

Factors Affecting Solubility

Intermolecular Forces

  Substances with similar intermolecular forces tend to be soluble in one another.

  The stronger the attractions between solute and solvent, the greater the solubility.

  Liquid-Liquid Interactions: “Miscible” – when two liquids mix in all proportions, e.g. H2O, C2H5OH “Immiscible” – when two liquids do not dissolve significantly in one another, e.g. H2O, C6H14

Temperature on Ionic Salts

  The change in enthalpy of salt solutions (ΔHsol‘n) can either be positive or negative

solute + solvent + heat solution

solute + solvent solution + heat

Temperature is a measure of internal energy in a system. It is a state…

Heat is the transfer of energy due to differences in energy of interacting systems. It is a path taken… (which for a path that is constant in pressure is equal to the ENTHALPY, ΔH)

Temperature on Ionic Salts

Solubility is the amount of solute needed to form a saturated solution in a given quantity of solvent

Solids:

↑T → ↑Solubility

Thermodynamics of different processes

  For the any process,

  Gibbs free energy(G) = the energy that’s free to do work (other than expansion work!)

  The higher the G, the more work a system can do

Thermodynamics of different processes

  When Gfinal is higher than Ginitial, the energy to do work of a system increase, then you need an input of energy (from the surrounding!) this process does not happen spontaneously

Thermodynamics of different processes

  When Gfinal is less than Ginitial, the energy to do work of a system decreases… the system loses energy, it has less energy to do further work this process happens spontaneously

Thermodynamics of different processes

  When Gfinal is equal to Ginitial, there is no change in energy in the system, and we call it a state of equilibrium.

Thermodynamics of solution process

  For solutions to occur, ∆Gsol’n<0   How is ∆G related to enthalpy (∆H), entropy (∆S)

and Temperature?

Temp

(+) (+/-) (+)

Negative x positive x positive = negative

Temperature on Gases

Why don’t carbonated beverages taste as good when they’re no longer cold?

Gases:

↑T → ↓Solubility

Thermodynamics of solution process

  For solutions to occur, ∆Gsol’n<0

(-) (-)

Negative x positive x negative = positive

(+)

Pressure

Sg = kHPg

The solubility of a gas in a liquid is proportional to its partial pressure over the solution (Henry’s law).

Carbonated beverages are bottled under PCO2 > 1 atm. As the bottle is opened, PCO2 decreases and the solubility of CO2 decreases. Therefore, bubbles of CO2 escape from solution.

Crystallization – playing with Solubility

  Lower temp low solubility   Higher temp high solubility

Exercise

  For a saturated aqueous solution of each of the following at 20°C and 1 atm, will be the solubility increase, decrease or the same?  He (g), decrease T  RbI (s), increase P

  Caffeine is about 10 times soluble in hot water as in cold water. A chemist puts a hot-water extract of caffeine into an ice bath and some caffeine crystallizes. Is the remaining solution saturated, unsaturated, or supersaturated?