Part 3: Colligative Properties Notes
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Colligative properties- property that depends on the concentration of solute particles, not their
identity
Freezing point depression (Δtf)- freezing point of a solution is lower than the freezing point of
the pure solvent
Boiling point elevation (Δtb)- boiling point of a solution is higher than the boiling point of the
pure solvent
Calculating Changes in Freezing or Boiling Points
Boiling Point Elevation: A solution will boil at a higher temperature than the pure
solvent. This is the colligative property called boiling point elevation. The more
solute dissolved, the greater the effect. An equation has been developed for this
behavior. It is:
ΔTb = i Kb m
Freezing Point Depression: A solution will solidfy (freeze) at a lower temperature than
the pure solvent. This is the colligative property called freezing point depression.
The more solute dissolved, the greater the effect. An equation has been developed
for this behavior. It is:
ΔT f = i Kf m
ΔTb or ΔTf = change in boiling point and freezing point i= number of ions (particles)
m= molality kb or kf = boiling point elevation constant and freezing point depression constant
Example: If 45.0 g of glucose are dissolved in 255 g of water what will the boiling point and freezing point of the solution? The kb of water is 0.515 oC/m; the kf of water is 1.86 o C/m. (A reminder ΔT is the change in temperature not the new temperature of the freezing point or boiling point) i = The Number of Ions
Covalent molecules do not
Boiling Point ΔTb = i Kb m disassociate in water so
C6H12O6 is covalent i = 1, kb = 0.515 oC/m, m = moles
i=1
Kg
Ionic compounds
Convert 45.0 g of glucose to moles and 255g of water to kg disassociate in water so
ΔTb = i Kb m = (1) (0.515 oC/m) 0.25 moles
i = the number of ions
0.255 kg
(elements) in the compound.
ΔTb = 0.505 oC so new boiling point is
NaCl i = 2
100 oC + 0.505 oC = 100.5 oC
11 MgCl2 i = 3
AlCl3 i = 4
Freezing Point ΔTf = i Kf m C6H12O6 is covalent i = 1, kf = 1.82 oC/m, m = moles
Kg
Convert 45.0 g of glucose to moles and 255g of water to kg
ΔTf = i Kf m = (1) (1.86 oC/m) 0.25 moles
0.255 kg
o
ΔTf = 1.82 C
so new freezing point is
0 oC - 1.82 oC = - 1.8 oC
Electrolytes and Nonelectrolytes.
‐ Electrolyte – a compound that conducts electric current when it is in an aqueous
solution or in the molten state.
o Examples: Ionic Compounds
‐ Nonelectrolyte – a compound that does not conduct electric current in either an
aqueous solution or in the molten state.
o Examples: Covalent Compounds
‐ For conduction to occur, the ions must be mobile.
Human body requires Na+1, K+1 and Ca+2 for bodily functions.
Molarity and Stoichiometry:
Ammonium chloride and calcium hydroxide react according to the following unbalanced
equation:
NH4Cl (aq) + Ca(OH)2 (aq)  CaCl2(aq) +
NH3 (g) +
H2O (l)
a. What mass of ammonium chloride is needed to make 1.5 liter of a 5.0 M
ammonium chloride solution?
Calculate the moles in a 5.0 M solution of NH4Cl (aq)
M = moles = 5.0 M = X
x= 7.5 moles
L
1.5 L
Convert Moles to grams
7.5 mol NH4Cl
53.45 g
1 mol
12 = 400 g NH4Cl
b. What mass of calcium hydroxide is needed to make 2.5 liters of a 5.0 M
calcium hydroxide solution?
NH4Cl(aq) + Ca(OH)2(aq)  CaCl2(aq) +
NH3(g) +
H2O(l)
Calculate the moles in a 5.0 M solution of Ca(OH)2 (aq)
M = moles = 5.0 M = X
x= 12.5 moles
L
2.5 L
Convert Moles to grams
12.5 mol Ca(OH)2
74 g
1 mol
= 925 g Ca(OH)2
After we have converted to grams then we can follow our regular stoichiometry
steps to solve any stoichiometry problem.
13 To make a reaction go faster:
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Increase temperature
- a direct relationship between kinetic energy & temperature exists
- the higher the temp, the faster the molecules will go
- more chaotic motion will lead to more collisions
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Increase surface area of reactants
- the more sites exposed to react, the more collisions can occur
- a large piece of copper will react slower in acid than many small pieces
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Increase concentration of reactants
- more moles (therefore particles) available to react in a 6.0M solution vs. a
0.10M solution
- copper will react faster with 6.0 M acid than 0.10 M.
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Add a catalyst
- will speed up the reaction by lowering activation energy, Ea
pathway with added catalyst......
Exothermic
Endothermic
Why do reactions occur at different rates?
The rate of a reaction is the speed at which a chemical reaction happens. If a reaction
has a low rate, that means the molecules combine at a slower speed than a reaction
with a high rate. Some reactions take hundreds, maybe even thousands, of years while
others can happen in less than one second. If you want to think of a very slow reaction,
think about how long it takes plants and ancient fish to become fossils (carbonization).
Ultimately: Molecules moving too slowly, elements electronegativity, phases (best is
liquid).
What prevents a reaction from occurring immediately? Energy: .Some molecules will
have high energy; some low; many intermediate. Only those with energies greater
than the activation energy will be able to react
Why don’t all products form at the same instant? Energy
 Each reaction is special. Conditions are different for each reaction.
Sometimes it takes longer for molecules/atoms to arrange themselves so they
can react, therefore forming new substances
14