Solutions-homogeneous mixtures of two or more pure substances

2 Parts of a Solution

Solute- is dissolved (usually the lesser amount)

Solvent- dissolves the solute (usually the greater amount)

Solute Solvent Example

solid solid Alloys (brass, steel)

solid liquid Salt water

gas solid Air bubbles in ice cubes

liquid liquid “suicides” (mixed drinks)

gas liquid Soft drinks

gas gas Air

The solvent pulls solute particles apart and surrounds or solvates

them

Solubility- if a compound is soluble it must overcome attraction

forces between:

1. Solvent molecules

2. Solute molecules

3. New interactions between solute and solvent

Lattice energy- the energy

required to completely

separate a mole of a solid

ionic compound into its

gaseous ions.

If an ionic salt is soluble in water, it is because the ion-dipole

interactions are strong enough to overcome the lattice energy of

the salt crystal.

The energy change of the overall process depends on H for each

of these steps.

NaCH3CO2 (s) + heat → Na+(aq) + CH3CO2-(aq)

NaCH3CO2 (s) + heat → Na+(aq) + CH3CO2-(aq)

WARNING! Just because a substance disappears when it

comes in contact with a solvent, it doesn’t mean the substance

dissolved.

• 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.

Factors Affecting Solubility

“like dissolves like”

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

increasing mass.

• Larger molecules have stronger dispersion forces.

• The solubility of liquids and solids does not change appreciably

with pressure.

• The solubility of a gas in a liquid is directly proportional to its

pressure.

Sg = kPg

Sg = the solubility of the gas;

k = Henry’s law constant for that gas in that solvent;

Pg = the partial pressure of the gas above the liquid.

Generally, the solubility

of solid solutes in liquid

solvents increases with

increasing temperature.

Generally, the solubility

of solid solutes in liquid

solvents increases with

increasing temperature.

Warm tea vs ice tea

Generally, the solubility

of gas solutes in liquid

solvents decreases with

decreasing temperature.

Ex. Carbonated soft drinks are more

“bubbly” if stored in the refrigerator.

Warm lakes have less O2 dissolved in

them than cool lakes.

Types of Solutions

Saturated

Solvent holds as much solute as is possible at that

temperature.

Dissolved solute is in dynamic equilibrium with solid

solute particles.

Unsaturated

Less than the maximum amount of solute for that

temperature is dissolved in the solvent.

Supersaturated

Solvent holds more solute than is normally possible at that

temperature.

These solutions are unstable; crystallization can usually be

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

the flask.

An IDEAL SOLUTION is one where the properties depend only on

the concentration of solute.

Need conc. units to tell us the number of solute particles per

solvent particle.

The unit “molarity” does not do this! 𝑀 = 𝑚𝑜𝑙 𝑜𝑓 𝑠𝑜𝑙𝑢𝑡𝑒

𝐿 𝑜𝑓 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛

Mole Fraction, X 𝑋𝐴 =𝑚𝑜𝑙 𝐴

𝑚𝑜𝑙 𝐴 + 𝑚𝑜𝑙 𝐵

Molality, m 𝑚 𝑜𝑓 𝑠𝑜𝑙𝑢𝑡𝑒 =𝑚𝑜𝑙 𝑠𝑜𝑙𝑢𝑡𝑒

𝑘𝑖𝑙𝑜𝑔𝑟𝑎𝑚𝑠 𝑠𝑜𝑙𝑣𝑒𝑛𝑡

Weight % 𝑚𝑎𝑠𝑠 % 𝑜𝑓 𝐴 = 𝑚𝑎𝑠𝑠 𝑜𝑓 𝐴

𝑡𝑜𝑡𝑎𝑙 𝑚𝑎𝑠𝑠 𝑥 100

Colligative Properties-depend only on the NUMBER of solute

particles relative to solvent particles, not on the KIND of solute

particles.