The Process of Dissolution

• The process of dissolving a solute in a solvent is a PHYSICAL CHANGE• The intermolecular forces between the solvent and the solute are sufficient

to overcome the intermolecular forces between solute particles– Remember the video of NaCl dissolving in water

• A TRUE SOLUTION IS A HOMOGENEOUS MIXTURE

• Suspensions of colloids (tiny particles) are not true solutions though they sometimes look like it. The TYNDALL EFFECT will distinguish between a true solution and a heterogeneous suspension

True solution

Heterogeneous mixture showingTyndall effect

Factors that affect Dissolution

Stirring: Continuous stirring or agitation will increase the rate of dissolving.

Temperature: Increasing the temperature of the solvent will increase the rate of dissolving.

Surface Area: Increasing the surface area of the solute will increase the rate of dissolving.

How fast a solute can dissolve depends upon….

How fast a substance dissolves GOES UP when…

Stirring or shaking is applied

The substance is powdery vs. chunky (surface area)

Temperature is increased

SOLUBILITY

• SOLUBILITY is defined by HOW MUCH solute dissolves, not how fast

• Reported as grams of solute in 100 grams of solvent at a given temperature

REMEMBER: LIKE DISSOLVES LIKE

Immiscible liquids

Miscibleliquids

Non-polar in polar Polar in polar

• UNSATURATED

• SATURATED

• SUPER SATURATED

DEGREE OF SATURATION

Solute completely dissolved andthe solvent can hold more

Some solute remainsinsoluble-solvent can’t dissolve more

Unstable solution createdby the dissolution of more solute than the solventcan theoretically dissolve

QUANTIFYING SOLUBILITYTHE SOLUBILITY CURVE

Super saturated

saturated

unsaturated

If the solute isdissolved…

Sol

ubi

lity

g/10

0 g

wat

er

Temperature oC

Using the Solubility Curve

Sol

ubi

lity

g/10

0 g

wat

er

Temperature oC

Read At 60 oC, the solution is saturated at 116 g/100 g water

Read80 g/100 g water will makea saturated solution at 17 oC

ReadA solution of 80 g/100 g wateris unsaturated at 40 oC

ReadA solution of 120 g/100 g wateris supersaturated at 40 oC

Other Features

Gases display a “negative”solubility with temperature…in other words, as the solution temperature increases the solubility decreases

Most solutes display a “positive”solubility with temperature…in otherwords, as the solution temperatureincreases the solubility increases

Henry’s Law-PRESSURE VS. SOLUBILITY FOR A GAS

• Henry’s Law states that the solubility of a gas is directly proportional to the partial pressure of that gas on the surface of the liquid

INCREASE P above the liquid

INCREASE solubility of the gas in the liquid

http://hogan.chem.lsu.edu/matter/chap26/animate2/an26_033.mov

TEMPERATURE VS. SOLUBILITY FOR A GAS• Increased temperature causes an increase in kinetic energy• There doesn’t need to be much energy input to cause gas SOLUTE intermolecular forces• The higher kinetic energy causes more motion in molecules which break intermolecular forces of attraction (gas solute to solvent) and the gas can escape from solution.

Why doesn’t this apply to most solids?

• Ionization-occurs when an ionic compound dissociates during dissolution (dissolving)

• NaCl (s) Na+ (aq) + Cl – (aq)100%

• HF (q) H+ (aq) + F- (aq) 5-10%

• NaCl is a better CONDUCTOR of electricity than HFbecause it has a higher concentration of ions in solution

Electrolytes-Solutions (or melts) of IONS that CONDUCT ELECTRICITY

NaCl is a strong electrolyteHF is a weak electrolyte

Is sugar an electrolyte?Is distilled (pure) water an electrolyte?Will tap water conduct electricity?

• The word “colligative” refers to “counting” or “number of particles”• Colligative properties are those properties that DEPEND on the NUMBER of

dissolved particles

• When a sugar cube is dissolved in water, 1 particle (molecule) of sugar is dissolved for each one present in the cube

– 1 mole of sugar will give 1 mole of sugar molecules in solution

– C6H12O6 (s) C6H12O6 (aq)

• When a NaCl crystal is dissolved in water, 2 particles (ions) are dissolved for each formula unit of NaCl in the crystal

– 1 mole of NaCl produces 2 moles of ions in solution

– NaCl (s) Na+ (aq) + Cl- (aq) 2 moles of ions per mole of formula unit

• THE MORE PARTICLES IN SOLUTION (PER MASS OF SOLVENT) THE GREATER THE COLLIGATIVE AFFECT

• The boiling point (bp) of a liquid GOES UP • The freezing point (fp) of a liquid GOES DOWN

Colligative Properties

1) WHICH WILL GIVE THE BIGGEST INCREASE IN bp, 1 MOLE OF SUGAR OR 1 MOLE OF SALT IN 1000g OF WATER?

2) WHICH WILL GIVE THE BIGGESST DECREASE IN fp, 1 MOLE OF SALT IN 1 LITER OF WATER OR 1 MOLE OF SALT IN 500 g OF WATER