unit 4: solutions and solubility

Solution:

homogeneous mixture

Solute:

substance being dissolved, part of the solution present in the smaller amount

Solvent:

part of the solution present in the greatest amount, does the dissolving

Miscible:

liquids that mix completely

Immiscible:

unable to mix

SOLUBILITY AND FORCES BETWEEN PARTICLES

• In order to dissolve:

  • the solute particles must be attracted to the solvent particles

  • the intermolecular forces between solute and between solvent  particles need to be broken

1. Types of Solute & Solvent Particles

• Ionic and polar solutes will dissolve in polar solvents because particles of both are charged

• Polar solutes will dissolve in polar solvents

• Non-polar solvents dissolve non-polar solutes due to similar intermolecular forces (no full or partial charges) 

• Recall: ∆EN of non-polar molecules is < 0.5 (0.4 and below), polar molecules is between 1.6-0.5, and  ionic compounds is 1.7 and above

This relationship is summarized in the expression: 

“Like dissolves like”

“Like dissolves like”

2. Temperature

For Solids:

• Increases in temperature causes increased solubility as a higher in temperature causes:

  • Spaces between particles increase resulting in more space for particles of solute to dissolve

  • Solvent particles have greater kinetic energy which results in more frequent and energetic collisions with the solute

temperature for gases

• Increases in temperature causes decreased solubility 

  • Molecules in gaseous state have higher kinetic energy than those dissolved in the solvent

  • Increasing temperature provides energy for gas molecules to escape solution

3. Pressure

• Pressure is force per unit area

• No effect on solubility of solids or liquids

• Solubility of a gas is directly proportional to the pressure of that gas above the liquid

Increased pressure causes increased solubility of a gas

4. Size

Covalent Compounds

• Increased molecule size (molecular compounds) causes decreased solubility

• Molecules like methanol (CH₃OH) have a non-polar (CH₃) end and a polar (OH) end

• The –OH group predominates and allows the entire molecule to be soluble in water (polar molecule)

• Increasing the size of the non-polar portion decreases solubility

size and ionic compounds

• If the attraction between the ions is very strong, they will be difficult to separate, and won’t be soluble in water

• Solubility usually increases with increased ion size and decreased ion charge (higher, more concentrated charge, stronger attraction between ions, harder to separate, harder to dissolve)

Factors that affect rate of dissolving

How quickly a solute dissolves in a solvent will increase when each of the following is increased:

• Agitation or mixing: increases number of collisions

• Temperature: increased kinetic energy causes more frequent collisions

• Surface area: more solute is in direct contact with the solvent