Experience 6: Properties of Solutions

soluble

the ability for a substance to dissolve in a solvent

insoluble

the inability for a substance to dissolve in a solvent

immiscible

two liquids that mix together but then separate after mixing

miscible

two liquids that are soluble into each other

solvation

the process of surrounding solute particles with solvent particles to form a solution

heat of solution

the overall energy change the occurs to make a solution

solubility

the maximum amount of solute that dissolve into a solvent

saturated solution

contains the max amount of dissolved solute for a given amount of solvent

unsaturated solution

contains less dissolved solute

supersaturated solution

contains more dissolved solute than a saturated solution

Henry's Law

S1 g/L S2 g/L ---------- x ---------- P1 P2

Solution

Homogeneous mixtures of two or more pure substances

Structure of solutions

Solute (minor component) is dissolved uniformly throughout the solvent (major component)

In order for a substance to dissolve—

Intermolecular forces must be greater than

1) solvent-solvent intermolecular forces

2) solute-solute intermolecular forces

Why do salts dissolve in water?

The ion-dipole interactions between the ions and water is stronger than the electrostatic forces between the ions in the salt

How do solutions form?

Solvent particles pull solute particles apart and solvate (surround) them

This process is called solvation

Steps that affect the energetics of solution

Separate the solute particles (DHsolute)

Separate the solvent particles (DHsolvent)

Form solute-solvent interactions (DHmix)

Energy of solution (dissolving the solute)

DHsoln = DHsolute + DHsolvent + DHmix

DHmix

Interaction between solute and solvent particles is expthermic

DHsoln < 0 (overall exothermic)

Most DHsoln are

Exothermic, but some processes are endothermic

what must happen in order for endothermic processes to occur?

overall energy must be lowered

enthalpy is not the only factor

When a process generates -- the energy of the system tends to be --

more disorder, lower

Enthalpy

the change in the relative disorder of a system

Unsaturated Solution

have the capacity to dissolve more solute at a given temp

more solute can be dissolved

Saturated Solution

have both dissolved and undissolved solute particles

hold as much solute as is possible at that temperature (maximum solubility of the solute is reached)

have dissolved and solid particles in equilibrium

Supersaturated Solution

more solute is dissolved than can normally occur at a given temp

unstable; crystallization may occur

A substance is more soluble in a solvent which has

similar intermolecular attractions

A gas's solubility in a liquid is

directly proportional to its pressure

The influence of mass on solubility of gases in liquids

increases with mass due to stronger dispersion forces

Henry's Law

Relates the solubility of a gas to the pressure of the gas above the liquid

Sg = kPg

Sg is the solubility of the gas,

k is the Henry's Law constant for that gas in that solvent, and

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

Molality is...

not dependent on temperature

Colligative Properties

Properties that depend only on the number of solute particles present, not on the identity/type of the solute particles.

Examples of colligative properties

- vapor pressure lowering (of the solvent) - boiling point elevation (of the solvent)

- melting point depression (of the solvent)

- osmotic pressure (of the solution)

Increasing concentration of nonvolatile solute

reduces solvent's vapor pressure due to increased solute-solvent intermolecular attractions

vapor pressure of a solution

is lower than that of pure solvent

Raoult's Law

Partial vapor pressure of a solvent equals the mole fraction of the solvent times the vapor pressure of the pure solvent

Nonvolatile solute-solvent interactions

cause solutions to have higher boiling points and lower freezing points than the pure solvent.

Boiling Point Elevation

The addition of a soluble compound will raise a solvent's boiling point. The change in boiling point (Tb) is proportional to the solution's molality:

Tb = Kb × m

- Kb is the molal boiling point elevation constant; unique for each solvent.

- Tb is added to the normal boiling point of the solvent.

Osmosis

Semipermeable membranes: film which allows smaller particles to pass through it, but not larger particles.

• In biological systems, many semipermeable membranes (e.g. cell walls) allow only water to pass through, but not solutes.

Net migration of solvent from an area of--concentration to an area of--concentration.

higher solvent, lower solvent

Freezing Point Depression

The addition of a soluble compound will lower a solvent's freezing point.

• The change in freezing point (Tf) can be found similarly:

Tf = Kf × m

- Kf is the molal freezing point depression constant of the solvent.

- Tf is subtracted from the normal freezing point of the solvent.

Osmotic Pressure

pressure required to stop osmosis

P = ( Vn ) R T = MRT

n = moles of solute

V = volume of solution R = ideal gas constant M = molarity of solution T = temperature in K

If the osmotic pressure is the same on both sides of a membrane (i.e. same concentrations), the solutions are

isotonic

van't Hoff Factor

The actual amount of dissociated particles

Since colligative properties depend on the number of particles,

electrolyte solutions should show greater changes than nonelectrolyte solutions.

Soluble ionic compounds dissociate into

2 or more ions

When does reassociation occur more?

at higher concentrations

The # of particles is

concentration-dependent

Colloids

Suspensions of particles larger than individual ions or molecules, but too small to settle out by gravity

can aid in the emulsification of fats and oils in aqueous solutions

Tyndall Effect

Scattering of light by colloidal suspensions

Hypertonic

High solute concentration outside

Low solute concentration inside

Hypotonic

Low solute concentration outside

High solute concentration inside