Solutions

Ch 15 & 16

What is a solution? A solution is uniform mixture that may contain

solids, liquids, or gases.

Known as a homogenous mixture

Solution = solute + solvent Solvent – The substance in greater abundance in

the solution

A solvent dissolves the solute. (dissolving medium)

Solute – The substance dissolved in the solvent

Characteristics of Solutions Soluble – The solute’s ability to dissolve in

a solvent

Insoluble – The solute is not able to dissolve in a solvent

Immiscible – Two liquids that can be mixed together, but separate shortly after you stop mixing them

Miscible – Two liquids that are soluble in each other

Solvation in Aqueous Solutions Solvation – The process of surrounding solute

particles with solvent particles to form a solution

Aqueous solution – A solute dissolved in water

Caused by constant, random molecular motion of solute and solvent particles. (Remember KMT)

In an aqueous solution each ion is surrounded by water molecules

Individual solute ions break away from the crystal and the charged ions become surrounded by solvent molecules (H2O) and the ionic crystal dissolves

Solvation Process

Rules for Solvation Like dissolves like Polar molecules dissolve polar molecules Non-polar molecules dissolve non-polar

molecules

Polar Molecule

Non-polar Molecule

Equal distribution of electrons

CO O O

H H

H2OCO2

Factors that Affect Rate of Solvation (How fast something dissolves) Need to increase collisions between solute

and solvent molecules1.Agitating the mixture (Stirring)

New collisions between solute and solvent will occur

2.Increasing the surface area of the solute (Crushing the solute) Greater surface area allows more collisions to

occur

3.Increasing the temperature of the solvent Increases kinetic energy of molecules and

more frequent collisions occur4.Increasing the pressure (For gases)

Increases the rate of collision between particles.

Solubility (How much is dissolved) Solubility – The maximum amount of solute

that will dissolve in a given amount of solvent at a specified temperature and pressure.

Solubility is usually expressed in grams of solute per 100 g of solvent or in grams/liter.

Solubility cont. . . Saturated solution contains the maximum amount of

dissolved solute for a given amount of solvent at a specific pressure and temperature.

In a saturated solution, solvation and crystallization are in equilibrium (They are happening at equal rates at the same time.)

Unsaturated solution contains less dissolved solute for a given temperature and pressure than a saturated solution

Supersaturated solution contains more dissolved solute than a saturated solution at the same temperature

A supersaturated solution is made at high temperatures, cooled slowly, and is unstable

Factors That Affect Solubility (How much is dissolved)

1. Temperature Most substances as temperature increases

solubility increases

Gases are the exception and solubility tends to decrease as temperature increases, because they are moving quickly escaping the solvent

2. Pressure (For Gases Only) The solubility of a gas in any solvent increases

as the pressure above the solution increases, keeping the gas from escaping in the solvent

Henry’s Law (Pressure and Solubility of a Gas) Henry’s Law states that at a given

temperature the solubility (S) of a gas in a liquid is directly proportional to the pressure (P)

2

2

1

1

P

S

P

S

Describing Solutions - Electrolytes

Electrolytes are compounds that dissociate or ionize in water to form a solution that conducts an electric current

To dissociate or ionize means to break apart into individual ions

Ionic compounds are electrolytes because they dissociate into ions

When NaCl is dissolved in water it breaks up into Na+ and Cl- ions.

Electrolytes dissociate into individual ions and conduct an electric current

Strong Electrolyte

Weak Electrolyte Nonelectrolyte

Conducts a strong current.NaCl

Conducts a weak current.HgCl2

Does not conduct a current.Sugar water

Strong and Weak Electrolytes

A strong electrolyte conducts a strong current and the compound has completely dissociated into ions NaCl

A weak electrolyte conducts a poor current because only part of the solute exists as ions. NH3 (Ammonia)

A nonelectrolyte does not dissociate or form ions, thus does not conduct a current. Most molecular compounds are

nonelectrolytes Sucrose (sugar)

Solubility Example 1

If the solubility of NaCl at 25˚C is 36.2 g/100 g H2O, what is the maximum mass of NaCl that can be

dissolved in 200.0 g of H2O at the same temperature?

Solubility Example 2If the solubility of KNO3 at 20˚C is 79 grams in 250

grams of water, what is the solubility in grams per liter?

Henry’s Law Example

2

2

1

1

P

S

P

S

The solubility of a gas is 0.35 g/L at 25.0 kPa of pressure. What is the solubility when the pressure is

increased to 115 kPa?