Solutions Ch 15 & 16
Dec 19, 2015
Solutions
Ch 15 & 16
What is a solution? A solution is uniform mixture that may contain
solids, liquids, or gases.
Known as a homogenous mixture
Solution = solute + solvent Solvent – The substance in greater abundance in
the solution
A solvent dissolves the solute. (dissolving medium)
Solute – The substance dissolved in the solvent
Characteristics of Solutions Soluble – The solute’s ability to dissolve in
a solvent
Insoluble – The solute is not able to dissolve in a solvent
Immiscible – Two liquids that can be mixed together, but separate shortly after you stop mixing them
Miscible – Two liquids that are soluble in each other
Solvation in Aqueous Solutions Solvation – The process of surrounding solute
particles with solvent particles to form a solution
Aqueous solution – A solute dissolved in water
Caused by constant, random molecular motion of solute and solvent particles. (Remember KMT)
In an aqueous solution each ion is surrounded by water molecules
Individual solute ions break away from the crystal and the charged ions become surrounded by solvent molecules (H2O) and the ionic crystal dissolves
Solvation Process
Rules for Solvation Like dissolves like Polar molecules dissolve polar molecules Non-polar molecules dissolve non-polar
molecules
Polar Molecule
Non-polar Molecule
Equal distribution of electrons
CO O O
H H
H2OCO2
Factors that Affect Rate of Solvation (How fast something dissolves) Need to increase collisions between solute
and solvent molecules1.Agitating the mixture (Stirring)
New collisions between solute and solvent will occur
2.Increasing the surface area of the solute (Crushing the solute) Greater surface area allows more collisions to
occur
3.Increasing the temperature of the solvent Increases kinetic energy of molecules and
more frequent collisions occur4.Increasing the pressure (For gases)
Increases the rate of collision between particles.
Solubility (How much is dissolved) Solubility – The maximum amount of solute
that will dissolve in a given amount of solvent at a specified temperature and pressure.
Solubility is usually expressed in grams of solute per 100 g of solvent or in grams/liter.
Solubility cont. . . Saturated solution contains the maximum amount of
dissolved solute for a given amount of solvent at a specific pressure and temperature.
In a saturated solution, solvation and crystallization are in equilibrium (They are happening at equal rates at the same time.)
Unsaturated solution contains less dissolved solute for a given temperature and pressure than a saturated solution
Supersaturated solution contains more dissolved solute than a saturated solution at the same temperature
A supersaturated solution is made at high temperatures, cooled slowly, and is unstable
Factors That Affect Solubility (How much is dissolved)
1. Temperature Most substances as temperature increases
solubility increases
Gases are the exception and solubility tends to decrease as temperature increases, because they are moving quickly escaping the solvent
2. Pressure (For Gases Only) The solubility of a gas in any solvent increases
as the pressure above the solution increases, keeping the gas from escaping in the solvent
Henry’s Law (Pressure and Solubility of a Gas) Henry’s Law states that at a given
temperature the solubility (S) of a gas in a liquid is directly proportional to the pressure (P)
2
2
1
1
P
S
P
S
Describing Solutions - Electrolytes
Electrolytes are compounds that dissociate or ionize in water to form a solution that conducts an electric current
To dissociate or ionize means to break apart into individual ions
Ionic compounds are electrolytes because they dissociate into ions
When NaCl is dissolved in water it breaks up into Na+ and Cl- ions.
Electrolytes dissociate into individual ions and conduct an electric current
Strong Electrolyte
Weak Electrolyte Nonelectrolyte
Conducts a strong current.NaCl
Conducts a weak current.HgCl2
Does not conduct a current.Sugar water
Strong and Weak Electrolytes
A strong electrolyte conducts a strong current and the compound has completely dissociated into ions NaCl
A weak electrolyte conducts a poor current because only part of the solute exists as ions. NH3 (Ammonia)
A nonelectrolyte does not dissociate or form ions, thus does not conduct a current. Most molecular compounds are
nonelectrolytes Sucrose (sugar)
Solubility Example 1
If the solubility of NaCl at 25˚C is 36.2 g/100 g H2O, what is the maximum mass of NaCl that can be
dissolved in 200.0 g of H2O at the same temperature?
Solubility Example 2If the solubility of KNO3 at 20˚C is 79 grams in 250
grams of water, what is the solubility in grams per liter?
Henry’s Law Example
2
2
1
1
P
S
P
S
The solubility of a gas is 0.35 g/L at 25.0 kPa of pressure. What is the solubility when the pressure is
increased to 115 kPa?