Electrolytes and
Colligative Properties
Mr. Shields
Regents Chemistry
U12 L07
1
Electrolytes
There are a number of different kinds of pure
Substances that can be dissolved in solvents to
Produce solutions.
-Some of these substance involve dissolving covalent
molecules in liquids
examples: O2, C6H14, C6H12O6, CCL4
-Some of them involve dissolving ionic compounds
in liquids
examples: NaCl, NH4Cl, KI
2
There is a difference between how these two
types of solutes dissolve in a polar liquid such as
such as water.
Molecular compounds…
- remain as single entities in solution
Ionic Compounds …
- separate into the ions from which they are
made
The Separation of a compound into it’s IONS is called
DISSOCIATION
3
Electrolytes
Compounds that produce ions in solution by dissociation are
Called…
4
ELECTROLYTES
Notice that when a compound dissociates the resulting
Solution has more particles in it than what we started with:
Dissolving equations:
NH4Cl + H20  NH4+ (aq) + Cl- (aq) K2SO4 + H20  2K+ (aq) + SO4-2 (aq)
1 particle  2 particles
1 mole
 2 moles
1 particle  3 particles
1 mole
 3 moles
How many moles of particles do I have if I dissolve 1 mole of
Al(NO3)3 in water? What is the dissolving Equation?
How many moles of particles do I have if I dissolve 1 mole of
Sucrose in water? (sucrose = C12H22O11)
1
C12H22O11 +H20  C12H22O11 (aq)
5
An important property of ionic compounds in solution is that
They Conduct electricity.
- Molecular compounds in solution DO NOT
In other words, Electrolytes form conductive solutions but
Non-electrolytes do not
When the ends of an electric circuit are placed in a
Solution of an Electrolyte 2 things happen…
6
Conduction of electricity
1) Positive ions will flow towards the negative electrode
2) Negative ions will flow toward the positive electrode
+
This free flow of ions
Allows electricity to
Be conducted through
The solution
Molecules in solution
Can’t do this!
Cl-
+
- ions
-
+ ions
-
Na+
7
Problem:
1) Which of the following are electrolytes and which
are not?
2) How many particles does each produce in
Solution?
1) Ethyl Alcohol
(C2H5OH)
2) NaCl
3) Sugar
4) CuSO4
5) AlCl3
6) NH4NO3
7) Al2(SO4)3
8
Colligative Properties
Now… When substances dissolve in liquids we change some
Of the physical properties of the liquid
These changes are DEPENDENT only on the Number of
Solute particles present
And … are INDEPENDENT of the nature or kind of solute
Particles
Non-polar
Ionic
CH3CH3
NaCl
9
Colligative Properties
For example:
2 moles of sugar (C12H22O11) in
water would have the same
impact on the solvent’s
properties as 1 mole of NaCl.
So, if changes in properties are dependent only on the
number of solute particles in solution why is the above true?
10
Properties that are dependent on the # of particles but
Are Independent of the nature of the particle are known as
COLLIGATIVE PROPERTIES
When comparing the properties of a pure solvent vs. the
Properties of solution made with the same solvent there are
2 colligative properties of special interest that are
Observed to change:
1) The freezing pt. which decreases
2) The boiling pt. which increases
11
Problem:
a) Which sol’n will have the greatest effect on FP & BP?
how much greater? How many moles of ions are in solution
for each?
1) 2M KBr
2) 0.5M NaCl
b) Which sol’n will have the greatest effect on FP & BP?
1)
2)
3)
4)
5)
1M CuSO4
2M NaCl
3M Sugar
2M CaF2
5 M C2H5OH
2M particles
4M “
3M “
6M “
5M “
12
BP Elevation
The Elevation of Boiling Point (BPE) is one of the Colligative
Properties of solutions
So … What’s the definition
of the Normal Boiling Point?
The NBP is the temperature
at which the VP of the liquid
Equals Standard Pressure
(1 atm)
13
By adding solute to solvent the VP of the solvent is reduced.
Therefore, it takes a higher temperature for the Vapor
Pressure of the liquid To reach 1 Atm (i.e. the Boiling Point)
T1
T2
Normal BP of
Solvent w/o solute
Normal BP of
Solvent w/solute
BP T2 > T1
14
Freezing Point Depression
Depression of Freezing Point is the second of the Colligative
Properties of solutions we’ll discuss.
First … what is the definition of the freezing Point?
The FP is the temperature at
which both solid and
Liquid exist in equilibrium.
This means that liquid becomes solid and solid becomes
Liquid and this oscillates back and forth – in equilibrium15
As the temperature is lowered the attractive forces
between molecules become more significant. These forces
organize liquid molecules into regular, repeating patterns.
In other words into solids. This is what we call solidification
(freezing).
Solutes interfere
With this process. T1
In order to organize the
Solvent molecules a lower
Temperature is now
T2
Required.
As the temp dec. molecular
Motion slows down. The
Attractive forces can now
Overcome the disruptive force of the solute.
16
Seawater is about 3.5% salts
(ionic compounds) in water
The freezing point of seawater is about 28.4°F (-2°C),
instead of the 32°F (0°C) freezing point of ordinary water
And…
As antifreeze is added
To water in your cars
Radiator the freezing
Pt. is markedly depressed
A molecule
17
Now you can appreciate why we add antifreeze
(ethylene glycol) to our car’s cooling systems.
By adding solute we Raise the BP of the water in the
Cooling system AND at the same time Lower the FP of
The water.
So we can protecting our engines
in both winter and summer.
18