Chapter 14: Solutions
FIRST SEMESTER | 2023-2024 |
General Properties of Solutions
Solu%on: a homogeneous mixture of one or
more solutes and a solvent.
Solute: substance being dissolved.
Solvent: dissolving agent that is usually the
most abundant substance in the mixture.
Note: a solu%on does not always just refer to
liquids.
Example: Air is a solu%on composed of N2,
O2, A r and C O2
N2 is the solvent as it composes 78% of air.
Properties of True Solutions
1. A homogeneous mixture of two or
more components whose ra%o can
be varied.
2. The dissolved solute is molecule or
ionic in size (< 1 n m).
3. Can be colored or colorless, though
solu%ons are usually transparent.
4. The solute remains dissolved and
does not sePle (precipitate) out of
solu%on over %me.
5. The solute can be separated from
solvent by physical means (usually
evapora%on).
Solubility
Solubility: the amount of a substance that
will dissolve in a specific amount of solvent
at a given temperature.
Example: 27 g K B r/100 g H2O at 23 degrees
Celsius
Miscible: when two liquids dissolve in each
other.
Immiscible: when two liquids do not
dissolve one another.
A mixture of oil and water is immiscible.
Solubility Rules
Chapter 14: Solutions
FIRST SEMESTER | 2023-2024 |
Factors Affecting Solubility
“Like dissolves like”
Polar compounds dissolve in polar
solvents.
Ethanol (C H3O H) dissolves in water (H O H).
Nonpolar compounds dissolve in nonpolar
solvents.
Carbon tetrachloride (C C l4) dissolves in
hexane (C H3(C H2)4C H3).
Ionic Compound Solubility in Polar
Solvents
Ionic Compound Solubility in Polar
Solvents
Several ionic compounds dissolve in water,
due to strong ion-dipole forces.
The individual ca%ons and anions are
surrounded by H2O molecules (i.e.,
hydrated).
The ca%on is aPracted to the par%ally
nega%ve O atom.
The anion is aPracted to the par%ally
posi%ve H atoms.
Solubility increases with temperature for
most solids (red lines)
Solubility decreases with temperature for all
gases (blue lines).
As a gas increases in temperature, the
kine%c energy increases, which means it
interacts less with the liquid, making it less
easy to solvate.
Pressure and Solubility
Pressure does not affect solubility of liquids
or solids.
Gas solubility in a liquid is propor%onal to
the gas pressure over the liquid.
Example: A boPle of root beer is under high
pressure.
Chapter 14: Solutions
FIRST SEMESTER | 2023-2024 |
As the boPle opens, the pressure decreases,
and the bubbles formed indicate gas loss
from the liquid.
Saturated and Unsaturated Solutions
There are limits to the solubility of a
compound at a given temperature.
Saturated solu%ons: contain the maximum
amount of dissolved solute in a solvent.
Saturated solu%ons are s%ll dynamic;
dissolved solute is in equilibrium with
undissolved solute.
Unsaturated solu%ons: contain less than
the maximum amount of possible dissolved
solute in a solvent.
Supersaturated Solutions
Supersaturated solu%ons: contain more
solute than needed to saturate a solu%on at
a given temperature.
Hea%ng a solu%on can allow more to
dissolve. Upon cooling to ambient
temperature, the solu%on is supersaturated.
These solu%ons are unstable -- disturbing the
solu%ons can cause precipita%on of solute.
Some hotpacks release heat by
crystalliza%on of a supersaturated solu%on of
sodium acetate.
Rate of Dissolving Solids
Effect of par%cle size
- A solid can only dissolve at a surface
that is in contact with the solvent.
- Since smaller crystals have a higher
surface to volume area, smaller
crystals dissolve faster than larger
ones.
-
-
Effect of Temperature
Increasing the temperature normally
increases the rate of dissolu%on of
most compounds.
Solvent molecules strike the solid
surface more oden, causing the solid
to dissolve more rapidly.
The solute molecules are more easily
separated from the solid due to a
higher kine%c energy.
Effect of Solute Concentration
Rate is highest at higher
concentra%on and decreases at lower
concentra%on.
As the solu%on approaches the
satura%on point, the rate of solute
dissolving decreases.
Chapter 14: Solutions
FIRST SEMESTER | 2023-2024 |
By dissolving both compounds in water, the
ions can collide with one another and react
to form an insoluble compound.
KCl(aq) + AgNO! (aq) → AgCl(s) + KNO! (aq)
K " (aq) + Cl# (aq) + Ag " (aq) + NO! # (aq) → AgCl(s) + K " (aq) + NO! # (aq)
-
Effect of Agitation/Stirring
S%rring a solu%on briskly breaks up a
solid into smaller pieces, increasing
surface area, thereby increasing the
rate of dissolu%on.
Concentration of Solutions
Qualita%ve expressions of concentra%ons
- Dilute: a solu%on that contains a
rela%vely small amount of dissolved
solute.
Surface Area
Example:
A 0.1 M H C l solu%on is dilute acid.
-
Concentrated: a solu%on that
contains a rela%vely large amount of
dissolved solute.
Example:
A 12 M H C l solu%on is concentrated
acid.
Solutions: A Reaction Medium
The purpose of dissolving reactants in a
solu%on is oden to allow them to come in
close contact to react.
Example:
Solid-solid reac%ons are generally very slow
at ambient temperature
KCl(𝑠) + AgNO! (s) → No Reaction
Quantitative Expressions of
Concentrations
Chapter 14: Solutions
FIRST SEMESTER | 2023-2024 |
Colligative Properties of Solutions
Colliga%ve property: A solu%on property
that depends only on the number of solute
par%cles not the nature of the par%cles.
Common colliga%ve proper%es:
1. Vapor pressure lowering
- Solu%ons have lower vapor pressures
than pure solvent.
2. Boiling point eleva%on
- Solu%ons have higher boiling points
than pure solvent
3. Freezing point depression
- Solu%ons have lower freezing points
than pure solvent.
4. Osmosis and osmo%c pressure
Vapor Pressure Lowering
Dissolving a solute in a solvent lowers the
vapor pressure of the solvent.
As a result, the solvent’s boiling point is
increased (a) while the freezing point of the
solvent is lowered (b).
the iden%ty, a new concentra%on unit is
used when discussing colliga%ve proper%es.
𝑚=
0.10 mol
= 0.20 𝑚
0.50 kg H$ O
Example:
What is the molality of a solu%on prepared
by dissolving 0.10 mol of starch in 0.50 kg of
water?
molality =
mol solute
kg solvent
Molality Prac%ce
What is the molality (m) of a solu%on
prepared by dissolving 2.70 g of methanol (C
H3O H) in 25.0 g of water?
2.70 g CH! OH ×
𝑚=
1 mol CH! OH
= 0.0843 mol CH! OH
32.04g CH! OH
0.0843 mol
= 3.37 𝑚
0.0250 kg H$ O
Answer: 3.37 m
Colligative Properties
Calcula%ng the change in boiling/freezing
point of a solu%on:
Molality
Since colliga%ve proper%es depend on the
number of par%cles in the solvent and not
Freezing/boiling points and the related K
constants
Chapter 14: Solutions
FIRST SEMESTER | 2023-2024 |
Chapter 14: Solutions
FIRST SEMESTER | 2023-2024 |
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Osmosis
diffusion of water from a dilute
solu%on or pure water, through a
semipermeable membrane into a
solu%on of higher solute
concentra%on.
-
Blood and Osmosis
Isotonic: same concentra%on of
dissolved solute as a cell. (0.9% saline)
Hypertonic: higher concentra%on of
dissolved par%cles rela%ve to cellular
levels. (1.6% saline)
Hypotonic: lower concentra%on of
dissolved par%cles rela%ve to cellular
levels. (0.2% saline)
Effect of different concentra%ons on red
blood cells
Isotonic:
-
Osmotic Pressure
Osmo%c pressure: difference in the
amount of pressure necessary to
apply to a solu%on to stop the flow of
water due to osmosis and the
atmospheric pressure.
Demonstra%ng osmo%c pressure
Hypertonic
Hypotonic