Factors That Affect Solubilityand Rate of Dissolving

Factors That Affect Rate of Dissolving

• Temperature, more specifically . . .

as T ↑, rate of dissolving ↑.

• Agitation

Shaking, stirring

• Surface area of solids.

How would you use this information to prepare orange juice from the frozen concentrate as expeditiously as possible?

• defrost (liquefy) the concentrate;

• shake or stir when water added.

Review of Intermolecular Forces

What Does Dissolving Mean at the Molecular Level?

• Dissolving is related to solute-solvent attraction.

• When attractive forces between different particles in mixture are greater than the attractive forces between like particles …

a solution forms.

• Strength of attraction governs

the amount of solute that dissolves.

The Process of Dissolving

1. Intermolecular forces between solute particles need to be broken—true for both ionic and molecular solutes. This is endothermic.

2. Some intermolecular forces in solvent (liquid) also need to be broken. This is endothermic.

3. Attraction between solute and solvent particles. This is

exothermic.

NaCl dissolving in water

What simple rule governs solubilty?

• Like dissolves like

• Polar dissolves polar (and

ionic).

eg. CH3OH (methanol) dissolves in

H2O.

NaCl dissolves in

water.

Name something polar that doesn’t dissolve in water.

A polar bear.

Take Note

• Not all ionic compounds are water soluble.

Some ionic compounds—with small, high-charge ions—have very strong lattice forces.

This explains poor solubility of some ionic compounds.

NaCl(aq) + AgNO3(aq) AgCl(s) + NaNO3(aq)

NB. Binary ionic compounds with a small electronegativity difference tend not to be water soluble.

eg. HgS, mercury (II) sulfide

↑ ↑

1.9 2.5 electronegativity values

∆eneg = 0.6 very small; HgS likely not water soluble.

eg. NaCl, ∆eneg = 2.1 large, NaCl soluble

How to tell if an ionic compound is water soluble?

Refer to a solubility chart.

• Non-polar solutes dissolve in non-polar solvents.

eg. I2 dissolves in hexane, CH3(CH2)4CH3.

Which of the following is/are polar molecules?

Solubility of Alcohols in H2O

CH3OH, miscible in all proportions

CH3CH2OH, miscible in all proportions

CH3CH2CH2OH, miscible in all proportions

CH3(CH2)3OH, reduced solubility

CH3(CH2)4OH, very reduced solubility

CH3(CH2)5OH, very,very reduced solubility

CH3(CH2)7OH, very, very,very reduced sol.. . . You get the idea . . .

With water, the –OH group in alcohols can form

hydrogen bonds.

The hydrocarbon chain in an alcohol isnon-polar.

As the length of the non-polar hydrocarbon chain increases, water solubility of the alcohol

decreases.

Effect of Temperature on Solubility

• For most solids in water: As T ↑,

solubility ↑

• Solubility of most liquids is not greatly affected by temperature. Why?

The liquid-liquid intermolecular forces are not as strong as the intermolecular forces between solid solute

particles with the solvent.

Solubility of Gases in Liquid

Solubility of gas in liquid

α 1/T

- or –

As T of liquid ↑, solubility of gas ↓

Heat Pollution of Water

Many industries return warm water to a river or lake.

What effect would this have?

Dissolved O2 ↓;

marine life can suffer.

At which temperature will you get a fizzier glass of pop—cold or warm?

Warm—why?

Solubility of CO2 ↓ as T ↑

Solubility of Gas and Pressure

As P of a gas above a liquid ↑, solubility

of gas ↑.

A potential problem for SCUBA divers is

narcosis, aka

“the bends.”

N2 dissolves in blood at high P under surface of water.

If diver ascends too fast, N2 bubbles out of the blood, much like CO2 in pop.

Can be painful—sometimes fatal.

Review• Factors that Affect Rate of Dissolving:

temperature

agitation,

particle size.

Also:

• Like dissolves like

• Forces between solute and solvent particles

• Solubility of gas in liquid (T and P)

HW

P 370 RQs #2, 3, 4, 5, 6, 8, 9, 10, 11, 12, 13, 14, 15, 16