Solutions

What Is A Solution?

Recall that a solution is a homogeneous mixture. (the same throughout)Solvent – major component of a solution.

Dissolves the solute(s). (Usually Water)

Solute(s) – minor component(s) of a solution.Active ingredients of a solution. (The salt in salt

water)

Solution = solvent + solute(s)

Some Common Solutions

The Solution Process

3 things happen when a solute dissolves:1. Solute particles separate.

This requires energy to overcome the intermolecular forces holding them together.

2. Solvent molecules separate.This also requires energy.

3. Solvent particles surround solute particles.Known as “solvation”.Releases energy.

The Solution Process

1. Particles separate from solute.

2. Solvent molecules move apart to accommodate solute.

3. Solvation

Ions in Solution

Solvated cation

Solvated anion

The Solution Process

Energy Changes in Solutions

The formation of a solution always involves a temperature change. Can be a large or small temperature change. Can get warmer or cooler as solute dissolves.

H = enthalpy (heat) of solution Energy absorbed or released as a solute dissolves in

a solvent. +H = endothermic sol’n process.

More heat absorbed than released. Temperature decreases as sol’n is formed.

-H = exothermic sol’n process. More heat released than absorbed. Temperature increases as sol’n is formed.

Energy Changes in Solutions

Solubility

Not all substances dissolve in water. Remember, “like dissolves like”.

Polar solutes tend to dissolve more in polar solvents. Non-polar solutes tend to dissolve more in non-polar

solvents.

Some (but not all) ionic compounds will dissolve in water. Of those that do, temperature affects how much

they dissolve. Solids usually dissolve more in warm water. Gases usually dissolve more in cool water.

Solubility

Insoluble – less than 0.1 grams of solute will dissolve in 100 grams of solvent. BaSO4 is insoluble in water. Only 0.0002448 grams of BaSO4 will dissolve in 100 grams of

water. Slightly soluble – 0.1 to 1 gram of solute will dissolve in

100 grams of solvent. Ag2SO4 is slightly soluble in water. About 0.8 grams of Ag2SO4 will dissolve in 100 grams of water

at 20ºC. Soluble – more than 1 gram of solute will dissolve in

100 grams of solvent. NaCl is soluble in water. 35.9 grams of NaCl will dissolve in 100 grams of water at

20ºC.

Solubility Rules

Use your list of solubility rules to determine whether each of the following salts is soluble or insoluble in water.NaClK2SO4

CaCO3

NH4NO3

Cu3(PO4)2

AgBr

NaCl is soluble.K2SO4 is soluble.CaCO3 is insoluble.NH4NO3 is soluble.Cu3(PO4)2 is insoluble.AgBr is insoluble.

Saturation

Saturated solution A sol’n containing the max. amt. of solute for a given amt. of

solvent at constant temp. and pressure. If additional solute is added it will not dissolve.

Unsaturated solution A sol’n that does not contain the max. amt. of solute for a

given amt. of solvent. Additional solute can dissolve in an unsaturated solution.

Supersaturated solution A sol’n that contains more solute than it can theoretically hold

at a given temperature. Addition of more solute causes the excess solute to precipitate.

Solubility Curves

Solubility curve chart – compares the solubilities of several compounds in water as a function of temperature.Usually expresses solubility in terms of

grams of solute per 100 grams of water.

Solubility Curves

How many grams of KNO3 can dissolve in 100 grams of water at 30ºC? Answer: About

48 grams.

Solubility Curves

How many grams of KClO3 can dissolve in 100 grams of water at 55ºC? Answer: About

25 grams.

Solubility Curves

How many grams of NH4Cl can be dissolved in 200 grams of water at 40ºC? Answer: About

96 grams.

Solubility Curves

A saturated solution of NaNO3 is prepared at 70ºC in 100 grams of water. If the temperature is lowered to 40ºC, how many grams of NaNO3 will precipitate? Answer: About

30 grams will precipitate out.

Solubility Curves

If 35 grams of NH4Cl are dissolved in 100 grams of water at 30ºC, is the solution unsaturated, saturated, or supersaturated? Answer: The

solution is unsaturated.

Solubilities of Gases

Henry’s Law – The solubility of a gas in a liquid increases as the pressure above the liquid increases. Explains decompression

sickness (the “bends”). Nitrogen is forced to

dissolve in scuba divers’ blood by high pressure.

If the diver ascends too quickly, the nitrogen bubbles come out of solution and form painful and possibly fatal gas embolisms.