Transcript Solutions - Livonia Public Schools

Solutions
C-16 Properties of solutions
2015
Solutions…
 Mixture (but special)
 Solute
+ solvent
 Homogeneous
(molecular level)
 Do not disperse light
 Do not settle out
Colloids do disperse light
Colloids
 Have
larger particles
 Do not settle out
 Disperse light (Tyndall effect)
 Ex. Milk (homogenized)
Suspensions
 Have
large particles
 Will settle out on standing
 Can be filtered out
 Particles are not dissolved, just held in the
mixture until they “fall out”
 Ex. Clay in water
Making solutions
 What
the solute and the solvent are
determines

Whether a substance will dissolve.
 How much will dissolve.
SOLVATION - solvent particles attract
solute particles and cause dissolving
Here an ionic crystal is being pulled apart
by water molecules.
Vocab. check
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Soluble – the solute will dissolve
Insoluble – the solute will not dissolve
Electrolyte – a solute that conducts electricity
when dissolved
Nonelectrolyte – a solute that does not
conduct electricity when dissolved
Miscible – two liquids which dissolve
Immiscible – two liquids which do not dissolve
Rate of dissolving
A
substance dissolves faster if-

It is stirred or shaken.
 The particles are made smaller. (surface
area)
 The temperature is increased.
 Know
why
Solutes dissolve until
equilibrium is reached.
How Much?
 Solubility-
The maximum amount of
substance that will dissolve at that
temperature (usually g/L).
 Saturated solution- Contains the
maximum amount of solute dissolved.
 Unsaturated solution- Has less than the
maximum dissolved. (Can dissolve more
solute.)
 Supersaturated-
A solution that is
temporarily holding more than it can, a
seed crystal will make excess come out
Heat of Solution

Another ∆H value that shows the
energy change during the dissolving
process.
If ∆H is negative, energy is released
 If ∆H is positive, energy is absorbed

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Three steps
1) Separate solute 2) Separate solvent
3) Combine
Heat of Solution diagram

Solubility can be shown in
graphs
The line in a graph represents the
maximum amount of solute that can
dissolve at a particular temperature.
Solubility
g/ 100ml
20
40
60
Temp.
80
100
Liquids
 Miscible
means that two liquids can
dissolve in each other.
Ex. Water and Alcohol
 Immiscible means they can’t. Forms
layers

Ex. Water and oil
(Depends on the bonding)
What affects solubility?
 For
solids in liquids as the temperature
goes up the solubility goes up.
 For gases in a liquid as the temperature
goes up the solubility goes down.
 For gases in a liquid- as the pressure goes
up the solubility goes up.

(Henry’s Law)
Pressure increase forces
gas into solution
Temperature and Solutions
 Higher
temperature makes the molecules
of the solvent move around faster and
contact the solute harder and more often.
 Speeds up dissolving.
 Usually increases the amount that will
dissolve.
Measuring Solutions
Concentration
A
measure of the amount of solute
dissolved in a certain amount of solvent.
 Concentrated solution has a large amount
of solute.
 Dilute solution has a small amount of
solute.
 Sometimes g/l or g/mL or g/100 mL.
 But chemical reactions don’t happen in
grams
Molarity
 The
number of moles of solute in 1 Liter of
the solution.
M
= moles/Liter
 What
is the molarity of a solution with 2.0
moles of NaCl in 4.0 Liters of solution?
 What is the molarity of a solution with 3.0
moles dissolved in 250 mL of solution?
Making solutions
 Pour
in a small amount of solvent.
 Then add the solute and dissolve it
 Fill to final volume.
 M x V(L) = moles
Dilution
Adding water to a solution
Dilution
 The
number of moles of solute doesn’t
change if you add more solvent.

The moles before = the moles after
 M1 x V1 = M2 x V2
 M1 and V1 are the starting concentration and
volume.
 M2 and V2 are the final concentration and
volume.
 Stock
solutions are pre-made to known M
Colligative Properties
Depends only on the number of
dissolved particles
Not the type of particle
Vapor Pressure
 The
IMF between molecules keep
molecules from escaping.
 In a solution, some of the solvent is busy
keeping the solute dissolved.
 Lowers the vapor pressure.
 Electrolytes form ions when dissolved →
more pieces.
+
 NaCl  Na + Cl 2 pieces
 More pieces →bigger effect.
Boiling Point Elevation
 The
vapor pressure determines the boiling
point.
 Lowered vapor pressure →causes raised
boiling point.
 Salt water boils above 100ºC
Freezing Point Depression
 Solids
form when molecules make an
orderly pattern.
 The solute molecules break up the orderly
pattern.
 Makes the freezing point lower.
 Salt water freezes below 0ºC
Molality
a
new unit for concentration
 m = Moles of solute
kilogram of solvent
 What is the molality of a solution with 9.3
mole of NaCl in 450 g of water?
 molality = 9.3 mol / 0.45 kg = 20.7m
Why molality?
 The
size of the change in boiling point is
determined by the molality.
 DTb = Kb x m x n
 DTb is the change in the boiling point
 Kb is a constant determined by the solvent.
 m is the molality of the solution.
 n is the number of pieces it falls into when
it dissolves.
What about Freezing?
The size of the change in freezing point is
determined by the molality.
 DTf = Kf x m x n
 DTf is the change in the freezing point
 Kf is a constant determined by the solvent.
 m is the molality of the solution.
 n is the number of pieces it falls into when it
dissolves.
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