SOLUTIONS

Chapter 16

Test Monday 8/09/04

What is a solution?

Any substance – solid, gas, or liquid – that is evenly dispersed throughout another substance – solid, gas, or liquid – is a solution.

Solutions are homogeneous mixtures.

Examples of solutions: Air, 14K gold, sea water

SOLVENT: The substance that is the dissolving medium

SOLUTE: The substance dissolved in the solvent

These are relative terms. The substance you have more of in a solution is the solvent. Ex. CH3CH2OH

Structure of water

The process of Solvation

How can water molecules break the ionic bonds in a crystal of potassium permanganate (KMnO4) to dissolve it?

HH

O

KPermanganate

ion

HH

O

At the surface of the crystal the water molecules becomes associated with the ions in the crystal.

A group of water molecules will surround each ion and isolate it from its neighbors.

These ions are said to be HYDRATED

This process of dissolving a solute in a solvent is called SOLVATION.

Solvation of a chloride ion

Solvation of the sodium ion

Dissociation

The process of decomposition of a crystal into hydrated ions is called DISSOCIATION.

Example

NaCl(s) Na+(aq) + Cl– (aq)

The symbol (aq) means that the ions are hydrated.

Molecular Solvation

Solvation can also occur between polar solute molecules and polar solvent molecules.

Example: methanol CH3OH.

Is methanol capable or forming a hydrogen bond?

Solvated molecule of methanol in water

Solvation of Glucose

The polar ends of the solute molecules are attracted to the oppositely charged polar regions of the solvent molecules.

In some cases the geometry and attraction between polar molecules cause that when two volumes are added the total volume is less than the one expected.

Ex. H2O and CH3CH2OH

Miscibility

Two substances are said to be miscible when there is no limit to the solubility of one substance into the other

When polar and non-polar substances are mixed, they show very little attraction for each other. Solvation does not occur. Ex: oil and water.

Two liquids that do not mix are called immiscible

Solutions

Solutions are transparent. You can see through them.

The mixture remains stable and does not separate after standing for any period of time.

The particles are so small they cannot be separated by normal filtration.

Colloids

Colloids are mixtures with particle sizes that consist of clumps of molecules. The particles have dimensions between 2 to 1000 nanometers.

The colloid looks homogeneous to the naked eye. Fog and milk are examples of colloids.

Colloids frequently appear "murky" or "opaque". The particles are large enough to scatter light.

Colloids generally do not separate on standing. They are not separated by filtration.

Suspensions

Suspensions are mixtures with particles that have diameters greater than 1000 nm. They are are visible to the naked eye. Ex: Blood

Suspensions are "murky" or "opaque". They do not transmit light. Suspensions separate on standing. The mixture of particles can be separated by filtration.

Examples of matter in solution

gas in gas: air ( N2, O2 , Ar, CO2 , etc)

gas in liquid: soda pop (CO2 in water)

liquid in liquid: gasoline (hydrocarbons)

solid in liquid: sea water (dissolved salts)

gas in solid: H2 in palladium

liquid in solid: dental amalgams ( Hg in Ag)

solid in solid: alloys ( brass, (Cu/Zn))

GAS-SOLID Solution: Hydrogen dissolved in Palladium

SOLID-SOLID Solution: Alloy

Like dissolves like

Generally two polar molecules are soluble in each other. Ex. Water and alcohol.

Many non-polar molecules will dissolve in non-polar solvents. Ex. Iodine will dissolve in mineral oil.

The rule of thumb is: like dissolves like

Solubility

SATURATED: A solution that cannot dissolve any more solute at a given T.UNSATURATED: A solution that can dissolve more soluteSOLUBILITY: Is the amount of solute needed to make a saturated solution at a given T.SUPERSATURATED: A solution that contains more solute that it would normally holds at a given T.

Solubility is the amount of substance needed to make a saturated solution at a given temperature.

It is usually given in grams of the solute per 100 g of water.

It depends on the solute and solvent.

The word soluble is not a precise term because solubilities vary greatly. It is commonly used for solubilities greater than 0.1 mol per liter of solution.

A saturated solution has the same rate of precipitation and dissolution

Factors affecting solubility

TEMPERATURE

For many substances solubility increases with temperature.

The solubility of a gas decreases as the temperature increases.

Gases are less soluble at high temperatures

Factors affecting solubility

PRESSURE

The solubility of a gas depends on the pressure.

The higher the pressure the higher the solubility.

                     

Double the pressure……… Double the concentration

Henry’s Law

How does oxygen partial pressure affect dissolved oxygen levels?

Oxygen in water obeys Henry's law; the solubility is roughly proportional to the partial pressure of oxygen in the air:

pO2 = KO2 xO2

PO2 = partial pressured of the gas

KO2 = Henry constant for water

XO2 = Mole fraction of O2 in a saturated solution

Ionic Equations

Ionic Equation

Net Ionic Equation

Spectator Ions

Precipitate (ppt)

Which pair forms a precipitate?

KI(aq) + Pb(NO3)2(aq) ----> KNO3 + PbI2

1. KNO3

2. PbI2

3. Neither pair forms a ppt.

Identify the spectator ion(s)

Fe(s) + Cu2+(aq) + SO42-+(aq) Cu(s) + Fe2+(aq) + SO4

2-

(aq)

1. Cu(s)

2. Cu2+

3. Fe2+

4. SO42-

5. None are spectator ions.

Acid – Base reaction

When an acid and a base react, a salt and water are formed

Reaction of sodium hydroxide and hydrochloric acid

NaOH(aq) + HCl(aq) NaCl(aq) + H2O(l)

Base Acid Salt Water

Net Ionic: OH–(aq) + H+(aq) H2O(l)

Reaction of a metal and H+ ion

Metals more active than hydrogen react with H+ ions in solution to form metallic ions and hydrogen gas.

Reaction of zinc and hydrochloric acid

Zn(s) + 2 HCl(aq) ZnCl2(s) + H2(g)

Net ionic:

Zn(s) + 2H+(aq) Zn2+(aq) + H2(g)

Rxn of an acid and a carbonate

When carbonic acid is form in a reaction it decomposes into CO2 and H2O.

This is an important reaction in living organisms

Reaction of Na2CO3 and HCl

Na2CO3(aq) + 2HCl(aq) 2NaCl(aq) + H2CO3(aq)

===========

H2CO3(aq) CO2(g) + H2O(l)