Transcript Chapter 11

Chapter 13
Properties of solutions
Solution Formation
How do solutions form?

Why do some things (solutes) dissolve in water
(solvent) and yet others don’t?
Energetics in solution formation



Sweet’n’Low vs. Equal in water
Factors affecting solubility?
– Assume 3 distinct steps for solution formation:
1. Expanding the solute (H1) - endothermic
2. Expanding the solvent (H2) - endothermic
3. Interaction of solute and solvent (H3) exothermic
Hsoln = H1 + H2 + H3

Oil and water

NaCl and water
Soluble or not?
Spontaneity and disorder


Usually, spontaneous processes are exothermic
(from CHEM 1211)
Exothermic means…..

Take a look at a solution of ammonium nitrate in
water. How does this feel when you touch it?

Not all spontaneous processes are exothermic; some
are….

Why? An increase in disorder, or Entropy of the
system (in this case the solution).
Assessing Entropy changes



Na2SO4 (s) + 10 H2O (g)  Na2SO4.10H2O (s)
(a) Does the system more or less ordered in this process?
(b) Does the entropy of the system increase or decrease?
Assessing Entropy changes

Does the entropy of this system increase or
decrease when the stopcock is opened to allow
mixing of the gases?
Assessing Entropy changes

Silver chloride (AgCl) is essentially insoluble in water.
Would you expect a significant change in the entropy
of the system when 10g of AgCl is added to 500 mL
of water?

Can you draw a picture that would represent AgCl in
water?
Solution Concentrations

Unsaturated

Saturated

Supersaturated
Which solution type is it?

The solubility of Cr(NO3)3.9H2O in water is 208 g per
100 g of water at 15 C. A solution of Cr(NO3)3.9H2O in
water at 35 C is formed by dissolving 324 g in 100 g of
water. When this solution is slowly cooled to 15 C, no
precipitate forms.

Is this solution unsaturated, saturated or
supersaturated?

How might crystallization of this solution be initiated?
Factors affecting solubility



For a high solubility, you want ________ interactions
between solute and solvent.
Like dissolves like (nonpolar vs. polar)
Hydrogen bonds for aqueous solubility
Predicting solubility patterns

Predict whether each of the following is more likely to
dissolve in carbon tetrachloride (CCl4) or in water:
C7H16, Na2SO4, HCl, and I2.

Which of the following 2 vitamins would be more
soluble in water?
Vitamin A
Vitamin C
Pressure Effects







Soda – bottled under high pressure of CO2 (high conc. of
dissolved CO2 in liquid)
Why does soda ‘fizz’ when opened?
Relationship between gas pressure and conc. of gas dissolved
gas: Henry’s Law; C=kP (William Henry – 1801)
C = Concentration
k = Henry’s Law Constant (different for each solute/solvent pair)
P = Partial pressure
Amount of gas dissolved in a solution  pressure of gas above
the solution
Henry’s Law Calculation

Bottle of ‘Jolt’ cola at 25 °C, has CO2 gas present at
7.5 atm above the solution. Assuming that PCO2 in
atmosphere is 4.0 x 10-4 atm, calculate equilibrium
concentrations of CO2 in cola both before and after
bottle is opened.
Temperature Effects


Solids in water –
Gases in water –
Solution composition

Quantitative terms: Concentrated, dilute
Qualitative expressions:

Molarity (M); Units: moles / L

Mass percent (weight percent)
– Mass of solute
x 100
Mass of solution

Mole fraction (); Mixture of A, B and C:
– A = nA / (nA + nB + nC)

Molality (m); Units: moles of solute / kg of solvent

Composition calculations

10.0g of ethanol (C2H5OH), mixed with 100.0g of water, to give
a final volume of 110 mL. Calculate:
(i) Molarity, (ii) mass percent, (iii) mole fraction and (iv) molality
of ethanol in solution.
Solution composition - ppm

An aqueous solution contains 0.00023g of SO2 per
1000g of solution. What is the concentration of SO2
in ppm?
Converting between concentrations

An aqueous solution of hydrochloric acid contains
36% HCl by mass.

(a) Calculate the mole fraction of HCl in the solution.
(b) Calculate the molality of HCl in the solution.

Colligative properties


A property of a solution that depends only on the
quantity of solute particles present, not on their
chemical identity.
Look at several of these:
– Boiling point elevation
– Freezing point depression
– Vapor pressure lowering
– Osmosis and Osmotic Pressure
Freezing Point Depression / BPE

Impurities lower freezing points and raise boiling points of
liquids
T = Kb . msolute
Sugar in water to make candy
T = Kf . msolute
antifreeze in radiator
salt on icy roads
Freezing point depression

What mass of ethylene glycol (C2H6O2), the main
component of antifreeze, must be added to 10.0L of
water to produce a solution for use in a car’s radiator
that freezes at –23.3 °C? Density of water = 1g/mL.
Kf (H2O) = 1.86 °C.kg/mol
Determining molecular weight

FPD (and most colligative properties, in fact) can be
used to determine the molecular weight of a solute:

A sample of a human hormone weighing 0.546g was
dissolved in 15.0g benzene. The fpd was determined
to be 0.240 C. What’s the molecular weight of the
hormone? Kf(benzene) = 5.20 °C.kg/mol
Vapor Pressure Lowering




Pressure of gas (vapor) above solvent/solution (higher for more
volatile solvents)
Liquid solutions – different properties to pure liquids
– Antifreeze (ethylene glycol)
– Salt on icy roads
Solutes change properties of pure solvent (H2O)
Nonvolatile solute reduces vapor pressure of a solvent.
Osmosis

Semipermeable membrane: permits passage of
some components of a solution (cell membranes)
 Osmosis: the movement of a solvent from low
solute concentration to high solute
concentration.
 There is movement in both directions across a
semipermeable membrane.
 As solvent moves across the membrane, the fluid
levels in the arms becomes uneven.
Osmosis calculations



Osmotic pressure ()
 = MRT
The average osmotic pressure of blood is 7.7 atm at
25 C. What concentration of glucose will be isotonic
with blood?
Osmosis and carrots?

Left cylinder contains
concentrated NaCl solution.
 Right cylinder contains pure
water.
 What’s going on in each
case?
 Left carrot has shrunk.
 Right carrot has expanded.
Osmosis and red blood cells

Why is osmosis important to human beings?
 Analogous to situation with carrots we just saw.
 Red blood cells have a certain osmotic pressure;
their surrounding medium (plasma) needs to have
the same osmotic pressure, or nasty things
happen…..


Solutions with equal osmotic pressures known as
isotonic. Thus, contents of red blood cells and
surrounding plasma are isotonic.
When somebody needs intravenous injections, some
solution is needed. This is routinely 0.89% sodium
chloride in water, or saline solution. This is isotonic
with red blood cells, thus safe.
Osmosis and red blood cells
Hypertonic solution
Hypotonic solution
Osmotic pressure calculation


Osmotic pressure ()
 = MRT

The average osmotic pressure of blood is 7.7 atm at
25 C. What concentration of glucose will be isotonic
with blood?

If 0.5 M glucose and 0.25 M MgCl2 were placed into
compartments A and B of an osmosis chamber, in
which compartment would the solution level rise?