Chapter 13: Properties of Solutions

Sam White

Pd. 2

Introduction

A solution is any homogenous mixture, which means the components are uniformly intermingled on a molecular level

The Solvent is the most abundant component. It does the dissolving.

The Solute are any of the other components. They are the ones being dissolved

Formation of Solutions

With the exception of gas solutions, solutions form when the attractive forces between solute and solvent are comparable or greater than the intermolecular forces in either component

Formation of Solutions

Example: Salt Water- Attractive forces between Na+ or Cl- and the polar water molecules overcome the lattice energy of solid NaCl

Once separated, the Na+ and Cl- are surrounded by water. This interaction is known in all solutions as solvation

When the solvent is water, this interaction is known as hydration

Energy Change in Solution Formation

In order to form a solution, the solvent must form space to house the solute and the solute must be dissolved, both of which take energy

Enthalpy of Solution

Overall Enthalpy Change:solution=H1+H2+H3

Example with Salt Water:H1 accounts for the separation of NaCl to Na+

and Cl-

H2 accounts for the separation of solvent molecules to accommodate the solute

H3 accounts for the attractive interactions between solute and solvent

Overall Enthalpy Change

Saturated Solutions

As concentration of a solid solute increases, so does it’s chance of of colliding with the surface of the solid and becoming reattached to the solid

This is called crystallization

Solute + Solvent Solution

Saturated Solutions

When the rates of crystallization and dissolving become equal, no increase of solute in solution will occur

When a solution will not dissolve any more solute, it is a saturated solution

When a solution that can still dissolve solute into it is an unsaturated solution

Supersaturation

Under suitable conditions, it is sometimes possible to form a solution with more solute than that needed for a saturated solution

These solutions are supersaturated

Supersaturation

Supersaturation usually occurs because many solutes are more soluble at one temperature than another

Example: Sodium acetate, NaC2H3O2, will dissolve in water more readily at higher temperatures. When a saturated solution is made at higher temperatures then slowly cooled, all of the solute may remain dissolved even though the solubility decreases

Factors Affecting Solubility

The stronger the intermolecular attractive forces between solute and solvent, the greater the solubility

As a result of favorable dipole-dipole attractions, polar liquids tend to dissolve more readily in polar solvents

Water is not only polar, but has hydrogen bonds, making solutes that have hydrogen bonds able to dissolve in water as well

Factors Affecting Solubility

Pairs of liquids that mix in all proportions are miscible

Liquids that do not dissolve significantly in one another are immiscible

Hydrocarbons vs. Alcohols

Many hydrocarbons are immiscible in water because they are nonpolar moleculesAlcohols have an OH group, which are both polar and have hydrogen bonds, making them more readily soluble in waterAs the carbon chain become larger, the effect of the OH group becomes smaller, meaning that larger alcohol chains begin to become less soluble

Pressure Effects

Pressure only affects the solubility of gas in a solvent

As pressure increases, solubility of the gas increases

Henry’s Law

Cg = kPg

Cg is the solubility of the gas in solution (usually expressed in molarity)

Pg is the partial pressure of the gas over solutionk is the Henry’s Law Constant, which is unique for all solute-solvent pairs as well as the temperature

Temperature Effects

As temperature increases, the solubility of solid solutes (such as salts) normally increases

In contrast, as temperature increases, the solubility of gaseous solutes normally decreases

Solubility Charts

Gas Solubility Curve Solids Solubility Curve

Ways of Expressing Concentration

Mass percentage, ppm

Mole Fraction

Molarity

Molality

Mass Percentage and ppm

Mass % of component = (mc / mt) x 100mc = mass of component in solution

mt = total mass of solution

ppm of component = (mc / mt) x 106

mc and mt denote the same things for ppm as they denote for mass % of component

Mole Fraction

Mole Fraction of Component = (molc / molt)molc = moles of component

molt = total moles of all components

Molarity

Molarity = (mols / Ls)mols = moles solute

Ls = liters solution

Molality

Molality = (mols / kgs)mols = moles solute

kgs = kilograms solvent

Colligative Properties

Colligative properties depend on the quanity of solute, not the type of solute

The colligative properties are:Vapor-Pressure ReductionBoiling-Point ElevationFreezing-Point DepressionOsmotic Pressure

Vapor-Pressure Reduction

As the amount of solute increases, the vapor pressure of solution decreasesThis relationship can be expessed through Raoult’s Law:PA = XAPo

A

PA = Partial pressure exerted by solventXA = Mole fraction of solventPo

A = Vapor pressure of pure solvent

Boiling-Point Elevation

As amount of solute increase, boiling point increases

This relationship can be expressed as: Tb = dKbm Tb = total boiling point elevation d = dissociation factor of the solute Kb = molal boiling point elevation constant of the

solvent m = molality of solution

Freezing-Point Depression

As amount of solute increases, freezing point decreases

This relationship can be expressed as: Tf = dKfm Tf = total freezing point depression d = dissociation factor of the solute Kf = molal freezing point depression constant of

the solvent m = molality of solution

Osmotic Pressure

As amount of solute increases, osmotic pressure increasesThis relationship can be expressed as: = (n / V)RT = MRT = osmotic pressure n = number of moles solute V = volume of solution R = ideal gas constant temperature of solution molarity of solution