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Chemistry 12 Unit III – Solubility Notes

III.1 – A Review of Solubility

• Recall that Solution = Solute + Solvent

• There are only three classes of compounds that form ionic solutions: ACIDS, BASES and SALTS. This unit will deal only with SALTS. Acids and bases will be thoroughly covered in the next unit.

• Acids… inorganic acids begin with H whereas organic acids end with COOH

• Bases… inorganic bases end with OH. Common bases contains GRP I or II metals.

• Salt… combination of metal and non-metal or contains polyatomic ion

Electrolyte: A substance that dissolves to give an electrically conducting solution containing ions. AKA ionic solution. Examples: (NH4)3PO4(s) ! 3 NH4+(aq) + PO4

3-(aq) HCl(s) ! H+(aq) +Cl-(aq)

Non-Electrolyte: A substance that dissolves to give a solution that does not conduct electricity and contains only molecules. AKA molecular solution. Examples: C12H21O11(s) ! C12H21O11(aq)

Br2(l) ! Br2(aq)

• Electrical conductivity requires the presence of ions in solution. The more ions present, the greater the electrical conductivity.

• In summary, we can have either a molecular solution, which contains only neutral molecules, or an ionic solution, which contains ions. How can we recognize ahead of time whether a molecule will form an ionic or molecular solution?

Ionic or Molecular Solution: General Rules

I. A compound made up of a metal and a non-metal is likely to be ionic in solution.

FeCl3(s) ! Fe3+(aq) + 3 Cl-

(aq)

II. A compound made up of a polyatomic ion(s) will be ionic in solution.

NH4NO3(s) ! NH4+

(aq) + NO3-(aq)

III. Covalent compounds (NON-METAL + NON-METAL), especially organic compounds, generally form molecular solutions.

CH3OH(l) ! CH3OH(aq)

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← bigger component of Solin

eg) usually H2O, acetone

eg) vinegar - Ctb COOHH COOH (carboxylic{ OH''

de},metalOHmetal non-metal polyatomic ion eg)N④

Salt crystallization(NH④P⑧cs) E*.ro?NHatcaqtPoEcaqgMgcoH)zcs) S MgYatg,T2OHIaq)base

table sugar→ C,zH21011 Cag)421421011 CS)

Brace) → Brzcaq)

ion= Stronger electrolyte .

metal non-metal⇐ stronger electrolyte

polyatomic compared to

NH4NO3

Non-metal t non-metal nbutthey are not acid nor base

eg ) Which is the strongest electrolyte ? (Data page 5)A. coppe44IiFfpnospGateB.nPau@c.ammlomntiumssuifateo

CH3 OH* Methanol

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• Salts are ionic compounds that differ in their ability to dissolve in water. All salts dissolve in water to produce electrolyte solutions. Some dissolve more than others = stronger electrolyte. How much a salt can dissolve in water is expressed as solubility.

The “old,” Chemistry 11 definition of solubility was:

Solubility: the maximum amount of substance that can dissolve in a given amount of solvent at a given temperature.

• A solution, which has dissolved the “maximum amount” of a substance, is said to be saturated. Once a solution has been saturated with a substance, the addition of more of the substance will simply cause this extra material to accumulate in undissolved form (usually on the bottom of the container).

Saturated Solution: A solution in which there exists a dissolved substance in equilibrium with the undissolved substance

• In contrast, an unsaturated solution contains less than the maximum amount of a substance, which can dissolve. So that there is no undissolved solid present and no equilibrium exists.

• The chemistry 12 definition of solubility is:

Solubility: the equilibrium concentration of the substance in solution at a given temperature. • The common method to determine solubility is to take an accurately measured volume of a saturated

solution and boil it dry. The mass of the dry solute is weighed and concentration can be calculated by moles / litres.

• The solubility is referred to as the MOLAR SOLIBILITY when the concentration of the salt is expressed in “moles/litre” or “M”. Solubility is also commonly expressed in “gram/litre” or “gram/100 ml”. Note that the solubility refers to the salt, not the ions.

• A saturated solution (ionic solution at equilibrium) must meet two conditions:

1. Some undissolved material must be present.

2. Equilibrium must exist between the dissolved and undissolved material.

• We can show that a solution is saturated by writing an equilibrium equation:

Mg(OH)2(s) ⇌ Mg2+(aq) + 2OH-

(aq)

This equation is interpreted as follows:

When solid Mg(OH)2 dissolves and enters the solution we have two opposing reactions:

Dissolving Reaction: Mg(OH)2(s) ! Mg2+(aq) + 2OH-

(aq)

Crystallization Reaction: Mg2+(aq) + 2OH-

(aq) ! Mg(OH)2(s)

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oldsaturated

Undissolved

saturated = at equilibrium

N④,

The* amount of solute needed to produce saturated Soth

(using a given amount of H2O at some T.anumber

molar solubilityMol/L M g l L

91100ML

① Some undissolved solute is visible

② There is an equilibrium

fytaateesareaoeonrstant[pro] t [Rea ]←Salt Cs) → catctaggt AnIag) No macro change

A particular keg isreached

Mg(OH)zcs, → MgZtt 20ft

- Microscopic changesCag) Cag) occur

Mg Tag, t 2OHIag, → Mg COHL Cs)

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Example:

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Solubility depends on

% :*:: : :O:::Temperature