Solutions

Part I: The Solution Process

Solution:

Solutions:

Solute:

Solvent

Types of Solutions

The solution process

To dissolve a solute in a solvent, the intermolecular attractions between solute and solute must be strong enough to compete with solvent-solvent and solute-solute attractions.

Solvation

Hydration

Dissolving an ionic solid in water

The ion-dipole attractions between the salt and the solvent must be strong enough to overcome the lattice energy

“Steps” of the solution process

Step 1

“Steps” of the solution process

Step 1

Step 2

“Steps” of the solution process

Step 1

Step 2

Step 3

Endothermic vs. exothermic?

Why do processes occur?

Achieve a lower energy state Exothermic processes lead to lower enthalpy

Why do processes occur?

More randomness Greater “entropy” Can more than

compensate for increasing enthalpy

Did it dissolve, or did it react?

Just because a substance “disappears” into a solvent doesn’t mean that the substance dissolved!

Dissolving vs. reaction

Dissolving Physical change

Can recover the original substance by evaporating the solvent

Dissolving vs. reaction

Dissolving Physical change

Can recover the original substance by evaporating the solvent

Chemical change New substances form

Solutions

saturated unsaturated

Supersaturated solutions

Will this substance dissolve?

“Like dissolves like”

Non-polar substances tend to dissolve in non-polar solvents

“Like dissolves like”

Non-polar substances tend to dissolve in non-polar solvents

Polar substances tend to dissolve in polar solvents.

“Like dissolves like”

It comes down to having similar intermolecular attractions.

Gases in solution

Solubility depends on intermolecular attractions

Gases in solution

Solubility depends on intermolecular attractions London forces

As molar mass increases, dispersion forces are stronger

Henry’s Law

The solubility of a gas in a liquid is directly dependent on the gas pressure

Henry’s Law

The solubility of a gas in a liquid is directly dependent on the gas pressure

Pressure really doesn’t affect solubility of solids and liquids

Henry’s Law

Sg = kPg

Solubility and temperature

Usually, solubility of ionic solids increases with temperature

Gas solubility and temperature

Solubility of gases typically decreases with increasing temperature