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New Jersey Center for Teaching and Learning
Progressive Science Initiative
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Solutions:
Formation and Properties
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Heats of Solution
Solution Formation
Solute formation requires the weakening of the Coulombic
attractions within the solvent and solute so new Coulombic
attractions may form between the solvent and solute.
Example: Formation of an aqueous glucose solution.
C6H12O6(s) --> C6H12O6(aq)
Step
What happens?
Enthalpy
Change
1
Intermolecular forces between glucose
(solute) molecules must weaken.
+
2
3
Intermolecular forces between solvent
(water) molecules must weaken.
New coulombic attractions will form
between the solute and solvent
+
-
The net change in enthalpy for the solution formation
process is called the heat of solution and is specific to
a particular solute-solvent combination.
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Heats of Solution
Ideal solutions are solutions in which when the solutes are
mixed the heat of solution would be equal to zero.
Solutions behave most ideally when the solute and solvent
are extremely similar in molecular structure and polarity.
Examples of nearly ideal solutions
The heat of solution will vary depending on the affinity of the
solute for the solvent.
Ethanol (CH3CH2OH) has very different heats of solution when
dissolved in water (H2O) and hexane (C6H14).
Ethanol dissolved in water (
H = -10.7 kJ/mol)
The hydrogen bonds between the solute and solvent
release large amounts of energy when formed.
Ethanol dissolved in hexane (
H = +23 kJ/mol)
Since ethanol is polar and hexane is non-polar, very few
coulombic attractions form to offset the energy required to
weaken the solute-solute and solvent-solvent attractions.
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Heats of Solution
The heat of solution can be calculated by monitoring the
temperature change when the solute and solvent are mixed.
Example: When 5.3 grams of NH4Cl are dissolved in 100 grams
of water @22.0 C, the temperature of the solution drops to 18.7
C. Assuming the specific heat of the solution is 4.2 J/gC, what is
the heat of solution?
CH3OH and CH3CH2OH
C6H14 and C7H14
C6H6 and C7H8
The heat of solution for these solutions is near zero because the
the Coulombic attractions between the solute molecules and
solvent molecules are almost identical to those that would form
between solute and solvent.
Energy lost by solution = 105.3 g x 3.3 C x 4.2 J = 1460 J
gC
Expressed in kJ/mol = 1.460 kJ /0.1 mol NH4Cl = 14.6 kJ/mol
This process is endothermic and therefore will likely
become more favorable as the temperature increases.
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Solution Formation
Electrolytes
Soluble ionic compounds and strong acids make excellent
electrolytes. Covalent molecular materials make poor
electrolytes as they do not dissociate into ions.
Ionic solutes dissociate into ions in aqueous solvent while
covalent molecular solutes do not.
Dissolution of NaCl in H2O
The more ions that are produced in solution, the stronger
the electrolyte.
NaCl(s) --> Na+(aq) + Cl-(aq)
Each ion becomes solvated by water molecules
Comparing equimolar NaCl(aq) and MgCl2(aq)
NaCl(s) --> Na+(aq) + Cl-(aq)
Dissolution of glucose (C6H12O6) in H2O
C6H12O6(s) --> C6H12O6(aq)
Which compound is the stronger electrolyte?
The entire glucose molecule becomes solvated
by water molecules
Ionic solutes are called electrolytes. Why?
MgCl2 producesMove
1 iontoofsee
Mg2+
and 2 ions of Clanswer
when it dissociates, so it is the stronger electrolyte.
Since ionic solutes produce ions in solution resulting in increased
electrical conductivity, there are referred to as electrolytes
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Which of the following is NOT true regarding the
formation of an aqueous glucose solution?
1
Soluble ionic compounds and strong acids make excellent
electrolytes. Covalent molecular materials make poor
electrolytes as they do not dissociate into ions.
The more ions that are produced in solution, the stronger
the electrolyte.
A
Covalent bonds within the glucose molecule must
be broken
B
Intermolecular coulombic forces will form between
glucose molecules and water molecules
C
The hydrogen bonding network between water
molecules must be disrupted
Comparing equimolar HF(aq) and HBr(aq)
HBr(aq) --> H+(aq) + Br-(aq)
Which compound is a stronger electrolyte?
Answer
Electrolytes
D The glucose molecule remains un-ionized
Since HBr is a strong acid, it produces many more ions
to see
answer
compared to HF, a Move
weak acid
in which
very few of the HF
molecules have ionized.
E All of these are true
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Answer
2 What is the heat of solution in (kJ/mol) of KCl if when
14.8 grams of KCl was dissolved in 400 grams of
water, the temperature dropped 4.3 C? Assume a
specific heat of solution of 4.2 J/gC.
3 How much would the temperature of a solution
prepared by dissolving 10.6 grams of LiNO2 in 300
grams of water increase? Assume a specific heat of
solution of 4.2 J/gC and a heat of solution of LiNO2 of
-11.0 kJ/mol.
Answer
HF(aq) --> H+(aq) + F-(aq)
MgCl2(s) --> Mg2+(aq) + 2Cl-(aq)
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5 Which of the following correctly ranks the solutions
from highest to lowest conductivity?
A 0.1 M NaF > 0.1 M CH3OH > pure water
B CH3OH
B 0.2 M AlCl3 > 0.2 M NaF > pure water
C C6H12O6
C pure water > 0.1 M NaF > 0.1 M CH3OH
Answer
A HCN
D H2SO4
E HC2H3O2
D 0.1 M CH3OH > 0.1 M AlCl3 > 0.1 M NaF
E None of these
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6 Which of the following pairs of liquids would form the
most IDEAL solution?
A C6H14(l) and H2O(l)
Colligative properties of solutions depend exclusively on
the
number of solute particles in the solution, not on their kind.
Vapor pressure lowering
C CH3OH(l) and CH3COCH3(l)
Boiling point elevation
Answer
E None of these
Colligative Properties
Examples of colligative properties
B CH3OH(l) and C6H14(l)
D C5H12(l) and C6H14(l)
Answer
4 Which of the following would be the strongest
electrolyte when dissolved in water?
Freezing point depression
Osmotic pressure elevation
In essence, the addition of solute to any solvent will decrease
the vapor pressure and hence raise the boiling point. The
solution will freeze at a lower temperature and require more
pressure to prevent osmosis into the solution.
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Vapor Pressure
When a liquid or solid evaporates, the vapor above the liquid
exerts pressure on the surface of the liquid.
When condensation and evaporation occur at equal rates,
the vapor is in equilibrium with its liquid.
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Vapor Pressure
The vapor pressure is influenced by the strength of the solvent's
particle interactions.
The stronger the particle interactions, the lower the
vapor pressure of a pure liquid at a given temperature.
H 2O
CH3COCH3
Vapor
Liquid
VP = 55.3 mm Hg @ 40 C
H-Bonds
less evaporation
VP = 400 mm Hg @ 40 C
no H-bonds
more evaporation
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Vapor Pressure
Vapor Pressure
The vapor pressure is directly proportional to the temperature.
VP of Various Substances vs. Temp.
As heat is added, more evaporation results, leading to a higher
vapor pressure.
Note the larger
hydrocarbons
have a lower
vapor pressure
at a given
temperature
due to higher
LDF's.
VP H2O vs Temp
When a liquid's vapor pressure equals the atmospheric/
external pressure it will boil. The stronger the particle
interactions, the more energy must be added to raise the vapor
pressure hence the higher boiling points.
Note the relationship is not linear.
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Vapor Pressure
To reach the boiling point, either the vapor pressure must be
increased, the atmospheric pressure must be lowered, or both.
The BP can be reached by raising the vapor pressure by
heating
H2O @25 C
23.88 mm Hg
H2O @100
C
760 mm Hg
Status @ 1 atm external
pressure
The freezing point of the solution will be lower than the pure solvent
B
The boiling point of the solution will be higher than
the pure solvent
C
The osmotic pressure of the solution will be higher
than the pure solvent
D
The vapor pressure of the solution will be lower
than the pure solvent
not boiling!
boiling!
The BP can also be reached by lowering the atmospheric
pressure
The atmospheric pressure at the peak of Mt. Everest is just
253 mm Hg so water must be heated only to roughly 72 C to
obtain a vapor pressure of 253 mm Hg and thus boil.
E All of these would occur
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8 Which of the following would have the lowest vapor
pressure at a given temperature?
9 Which of the following would be TRUE?
A Water will boil at a lower temperature at high
altitude because its Coulombic attractions are
stronger at these pressures
A C6H14(l)
C H2O(l)
D CH3COCH3(l)
Answer
B C4H10(l)
B Water will boil at a lower temperature at high
altitude because its vapor pressure is higher at
these altitudes.
C
Water will boil at a lower temperature at high
altitude because atmospheric pressure is lower
D Water will boil at a higher temperature at high
altitude because atmospheric pressure is higher
Answer
Vapor Pressure
A
Answer
7 Which of the following would NOT occur when a
solute is added to a solvent?
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Vapor Pressure of Solutions
Adding solute to create a solution lowers the solutions vapor
pressure and raises the boiling point.
When solute is mixed with the solvent, attractions form
between the two, resulting in less solvent molecules
evaporating and thereby lowering the vapor pressure.
Vapor Pressure of Solutions
The amount of solute particles produced per solute molecule is
called the Van't Hoff factor. The higher the Van't Hoff factor, the
greater the impact on the colligative properties of the solution.
Molecular solutes all have Van't Hoff factors of 1
CH3OH, C6H12O6, CH3COCH3
Na+
Lower VP
NaCl(aq) --> Na+(aq) + Cl-(aq) VHF = 2
Cl-
Pure H2O
Al(NO3)3(aq) --> Al3+(aq) + 3NO3-(aq) VHF = 4
0.1 M NaCl(aq)
Attractions between the ions and water reduce
evaporation and the vapor pressure.
A given quantity of Al(NO3)3 will have 4x the impact of a molecule solute
and 2x the impact of NaCl(aq) on the vapor pressure of a solution.
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Vapor Pressure of Solutions
Vapor Pressure of Solutions
The vapor pressure of a solution can be calculated using
Raoult's Law.
VPsolution = (Xsolvent)(Psolvent)
The vapor pressure of a solution can be calculated using
Raoult's Law.
Example: If 2.3 grams of NaCl is added to 108 grams of water,
what is the vapor pressure of the resulting solution assuming the
partial pressure of water vapor at this temperature is 18.9 mm Hg?
where....
Find mole fraction of solvent
Xsolvent = mole fraction of solvent
2.3 g NaCl = 0.05 moles NaCl x VHF(2) = 0.10 mol solute
Psolvent = pressure of pure solvent
108 g H2O = 6 moles H2O
As can be seen, as the mole fraction of solvent decreases - due
to high moles of solute particles - the vapor pressure of the
solution will diminish.
mole fraction = 6/6.1 = 0.98
Find VP solution
VPsolution = (0.98)(18.9 mm Hg) = 18.6 mm Hg
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Vapor Pressure of Solutions
The vapor pressure of solution made from two liquids can be
determined using an expanded version of Raoult's Law
VPsolution = (XliquidA)(Pliquid A) + (XliquidB)(Pliquid B)
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10
When a solute is mixed with a solvent, the resulting
solution will have...
A A higher vapor pressure and boiling point
B A higher vapor pressure and lower boiling point
Both liquids evaporate and therefore contribute to the vapor
pressure of the solution
C A lower vapor pressure and a higher boiling point
Example:
D A lower vapor pressure and a lower boiling point
What is the vapor pressure of a solution made by mixing 1 mole
of acetone (C3H6O) with 4 moles of ethanol (CH3CH2OH);
assuming vapor pressures of each being 28 mm Hg and 17.4 mm
Hg at a given temperature.
VPsolution = (1/5)28 mm Hg + (4/5)17.4 mm Hg = 19.5 mm Hg
Answer
Higher VP
Ionic solutes have Van't Hoff factors that correlate to the
number of ions present
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Which of the following substances/mixtures would
have the highest vapor pressure?
12
Which of the following substances/mixtures would
have the lowest vapor pressure?
A 0.2 M LiC2H3O2
A 0.1 M NaCl
B 0.3 M HNO3
B 0.2 M CH3OH
Answer
C 0.1 M CH3CH2OH
C 0.4 M HC2H3O2
Answer
11
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D 0.1 M Al(NO3)3
E Pure H2O
D 0.1 M MgCl2
E 0.05 M Al(NO3)3
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Equi-molar amounts of liquid methanol (VP = 19 mm
Hg) and water (VP = 17 mm Hg) are mixed, what
would be the vapor pressure of the solution?
Answer
14
What is the vapor pressure of a solution prepared
by adding 1.8 grams of glucose (C6H12O6) to 200 mL
of water (D =1 g/mL). Assume water vapor has a
vapor pressure of 12.3 mm Hg at this temperature.
Answer
13
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If equi-molar amounts of liquid methanol (VP = 19
mm Hg) and water (VP = 17 mm Hg) are mixed, what
would be the mole fraction of methanol in the vapor
phase?
(Hint: For each liquid - its vapor pressure is calculated by Xsolvent*Psolvent. Remember
also that the mole fraction can be determined as a pressure fraction (P/Ptot)
Boiling Point Elevation
The increase in boiling point of a solution compared to the pure
solvent is directly proportional to the molal concetration (m) of
solute, the boiling point constant (Kb) and the Van't Hoff factor of
the solute (i).
Tb = Kb*m*i
Answer
15
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Higher BP
Lower BP
Pure H2O
0.1 m NaCl(aq)
The solute impedes evaporation thereby requiring more heat to
be added to get the vapor pressure = atmospheric pressure.
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Boiling Point Elevation
Measuring the boiling point elevation of a solution can be used
to determine the molar mass of a non-volatile molecular solute.
Example: What is the molar mass of a solute that when 3.4
grams of it are mixed with 300 grams of benzene (Tb = 80.1C,
Kb = 2.53 C/m), the boiling point is 80.6 C.
Use
Freezing Point Depression
The addition of solute disrupts the formation of the crystal lattice
to freeze the solvent, thereby requiring a lower temperature to
freeze the solution.
The degree to which the freezing point has been depressed
can be calculated by...
Tb and Kb to find molality
0.5 C x
1 m
Tf = Kf*m*i
= 0.198 m
where Kf = freezing point constant of solvent
2.53 C
Use molality and kg of solvent to get moles solute
The molar mass of solute can be calculated as done with
boiling point elevation.
0.300 kg solvent x 0.198 mol solute = 0.059 mol
1 kg solvent
Divide the grams by moles
3.4 g/0.059 mol = 57.6 g/mol
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Freezing Point Depression
The osmotic pressure of a solution is the pressure required to
prevent the osmosis (flow) of water into it.
Antifreeze is a mixture of water
and ethylene glycol and it
allows the coolant in your
engine to stay liquid even
below the normal freezing point
of either material.
Some frogs release glucose into
their bloodstream in cold
temperatures so their blood
doesn't freeze!
Pure
H 2O
No net movement of water
Osmotic Pressure = 0
Pure
H 2O
The osmotic pressure is directly proportional to the M of the
solution, the Van't Hoff factor of the solute, and the
temperature.
Osmotic Pressure = iM*R*T
M = Molarity, R = 0.0821 L*atm/mol K, T = Kelvin temperature, i = VHF
0.1 M
NaCl
Osmotic Pressure of 2.44
mmHg needed to prevent
net-flow of water
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16
Compared to the pure solvent, a solution will have
a...
A
higher boiling point and lower freezing point
B
higher boiling and freezing point
C lower boiling point and higher freezing point
D lower boiling point and lower freezing point
E
Plants use high
concentrations of solute
within their cells to draw
water in to create the turgor
pressure necessary to keep
the cells rigid.
Pure
H 2O
0.1 M
NaCl
Net movement of water into
solution
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Osmotic Pressure
Pure
H 2O
the same freezing and boiling points
Answer
Depressing the freezing point of a solvent has many
applications.
Osmotic Pressure
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0.02 M CH3OH
A
0.45 M HC2H3O2
B
0.02 M KNO3
B
0.45 M HI
C
0.02 M HC2H3O2
C
0.15 M Al(NO3)3
D
0.02 M CaCl2
D
0.15 M HC2H3O2
E
Pure water
E
0.15 M HI
Answer
A
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What would be the boiling point of a 200 mL
solution of 0.78 M MgCl 2? Assume a Kb of
0.51 C/m for water and a density of the solution of
1.01 g/mL.
20
When a 2.3 gram sample of a non-ionic solute is
added to 250 grams of water, the freezing point is
found to be -3.45 C. Assuming a Kf of -1.86 C/m
for water, what is the molar mass of the solute?
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Answer
Answer
19
Which of the following solutions would have the
lowest boiling point?
18
Answer
Which of the following solutions would have the
lowest freezing point?
17
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21 What is the osmotic pressure of a solution
created by adding 5.8 grams of NaCl to water to
produce a 340 mL solution @10 C?
Answer
Next up....Equilibrium!!!