Properties of Solutions

15.2

Solvents and Solutes

–An aqueous solution is water that contains dissolved substances.–In a solution, the dissolving medium

is the solvent.–In a solution, the dissolved particles

are the solute.

Solutions

• Solutions are homogeneous mixtures of two or more pure substances.

• In a solution, the solute is dispersed uniformly throughout the solvent.

Dissolution vs reaction

• Dissolution is a physical change—you can get back the original solute by evaporating the solvent.

• If you can’t, the substance didn’t dissolve, it reacted.

Ni(s) + HCl(aq) NiCl2(aq) + H2(g) NiCl2(s)dry

Why does a solution form?

•LIKE DISSOLVES LIKE–Polar solvents such as water

dissolve ionic compounds and polar compounds. –Nonpolar solvents such as

gasoline dissolve nonpolar compounds.

15.2

• Vitamin A is soluble in nonpolar compounds (like fats).

• Vitamin C is soluble in water.

Which vitamin is water-soluble and which is fat-soluble?

Why does a solution form?Energy Changes in Solution

To determine the enthalpy change, we divide the process into 3 steps.

1. Separation of solute particles.

2. Separation of solvent particles to make ‘holes’.

3. Formation of new interactions between solute and solvent.

Enthalpy Is Only Part of the Picture

Entropy is a measure of: • Dispersal of energy in the

system.• Number of microstates

(arrangements) in the system.

b. has greater entropy, is the favored state

Assessing Entropy ChangeIn the process illustrated below, water vapor reacts with excess solid sodium sulfate to form the hydrated form of the salt. The chemical reaction is

Does the entropy of the system increase or decrease?

Solution formation

How does a solid dissolve into a liquid?

What ‘drives’ the dissolution process?

What are the energetics of dissolution?

How Does a Solution Form?1. Solvent molecules attracted to surface ions.2. Each ion is surrounded by solvent molecules.3. Enthalpy (DH) changes with each interaction broken or

formed.

Ionic solid dissolving in water

How Does a Solution Form

The ions are solvated (surrounded by solvent).

If the solvent is water, the ions are hydrated.

The intermolecular force here is ion-dipole.

How Does a Solution Form?1. Solvent molecules attracted to surface ions.2. Each ion is surrounded by solvent molecules.3. Enthalpy (DH) changes with each interaction broken or

formed.

Solution Formation

• The compositions of the solvent and the solute determine IF a substance will dissolve. • The factors that determine the RATE which a

substance dissolves are:»stirring (agitation)»temperature»the surface area of the dissolving

particles

16.1

Solubility

–The solubility of a substance is the amount of solute that dissolves in a given quantity of a solvent at a specified temperature and pressure to produce a saturated solution.–Solubility is often expressed in

grams of solute per 100 g of solvent.

16.1

Solubility

• Some liquids combine in all proportions, while others don’t mix at all. –Two liquids are miscible if they

dissolve in each other in all proportions. –Two liquids are immiscible if they

are insoluble in each other.

16.1

Solubility

•A saturated solution contains the maximum amount of solute for a given quantity of solvent at a given temperature and pressure. •An unsaturated solution contains

less solute than a saturated solution at a given temperature and pressure.

16.1

Solubility• In a saturated

solution, the rate of dissolving equals the rate of crystallization, so the total amount of dissolved solute remains constant.

16.1

Factors Affecting Solubility

The stronger the intermolecular attractions between solute and solvent, the more likely the solute will dissolve.Example: ethanol in water

Ethanol = CH3CH2OH

Intermolecular forces = H-bonds; dipole-dipole; dispersion

Ions in water also have ion-dipole forces.

Degree of saturation

• Unsaturated Solution Less than the maximum

amount of solute for that temperature is dissolved in the solvent.

No solid remains in flask.

Degree of saturation

• Supersaturated Solvent holds more solute than is normally possible at

that temperature.These solutions are unstable; crystallization can often

be stimulated by adding a “seed crystal” or scratching the side of the flask.

Factors Affecting Rate of Solubility

–Temperature –The solubility of most solid

substances increases as the temperature of the solvent increases. –The solubilities of most gases are

greater in cold water than in hot.

16.1

Temperature

Generally, the solubility of solid solutes in liquid solvents increases with increasing temperature.

Factors Affecting Solubility

– Pressure• Changes in pressure have little effect on the solubility

of solids and liquids, but pressure strongly influences the solubility of gases. • Gas solubility increases as the partial pressure of the

gas above the solution increases.

16.1

Factors Affecting the Solubility of Gases

• Solubility of most gases decreases as temperature increases

• Gases tend to have weak intermolecular forces– Ex: N2 and O2 form weak dipole-induced dipole forces

and weak dispersion forces with water • As the kinetic energy of particles within a solution

increases the gas particles break free from weak attractions and re-enter the gas phase

Factors Affecting Solubility of Gases

• Real life:– Warm soda goes flat faster than cold soda– Fishing– Thermal pollution – The Bends

Gases in Solution

• In general, the solubility of gases in water increases with increasing mass.

Why?• Larger molecules have

stronger dispersion forces.

Gases in Solution

Gases in Solution

• The solubility of liquids and solids does not change appreciably with pressure.

• But, the solubility of a gas in a liquid is directly proportional to its pressure.

Increasing pressure above solution forces more gas to dissolve.

Factors Affecting Solubility of Gases

• 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) of the gas above the liquid.

16.1

Temperature

Carbonated soft drinks are more “bubbly” if stored in the refrigerator.

Warm lakes have less O2 dissolved in them than cool lakes.

Factors Affecting Solubility16.1