Chapter 14
Solutions
I have no doubt that in reality the future will be vastly more surprising than anything I can imagine. Now my own
suspicion is that the universe is not only queerer than we suppose, but queerer than we can suppose.
John Burdon Sanderson Haldane
Outline
I. Solutions
II. Units of Concentration III. Properties of Solutions IV. Solubility
V. Reactions
2
Solute and Solvent
Solutions
• are homogeneous
mixtures of two or more substances
• contain a solute,
usually in smaller quantity
• contain a solvent,
Solution: a homogeneous mixture of one or more solutes and a solvent.
Solvent: dissolving agent that is usually the most abundant substance in the mixture.
Note: a solution does not always just refer to liquids.
Example: Air is a solution composed of N2, O2, Ar and CO2
N2 is the solvent as it composes 78% of air. Solute: substance being dissolved.
General Properties of Solutions
Solubility: the amount of a substance that will dissolve in a specific amount of solvent at a given temperature.
Example 27 g KBr/100g H2O at 23 ºC
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
5
Formation of a Solution
Na+ and Cl ions
• on the surface of a
NaCl crystal are attracted to polar water molecules
• in solution are
hydrated as several H2O molecules
Attractions in Solutions
Interactions between solutes and solvents that
• are similar in polarity will form a solution • are not similar will not form a solution
7
When NaCl(s) dissolves in water, the reaction can be written as
H2O
NaCl(s) Na+(aq) + Cl(aq)
solid separation of ions
8
Two substances form a solution
• when there is an attraction between the
particles of the solute and solvent
• when a polar solvent such as water dissolves
polar solutes such as sugar and ionic solutes such as NaCl
• when a nonpolar solvent such as hexane
(C6H14) dissolves nonpolar solutes such as oil or grease
9
Like Dissolves Like
Solvents Solutes
Water (polar) Ni(NO3)2
CH2Cl2(nonpolar) (polar)
10
Will the following solutes dissolve in water? Why? 1) Na2SO4
2) gasoline (nonpolar) 3) I2
4) HCl
11
In water, strong electrolytes
• completely dissociate in water • produce ions
• conduct an electric current
Examples:
H2O 100% ions
NaCl(s) Na+(aq) + Cl− (aq)
H2O
CaBr2(s) Ca2+(aq) + 2Br−(aq)
Solutes: Strong Electrolytes
12
In water, weak electrolytes
• dissolve mostly as molecules • produce a few ions
• conduct a weak current
Examples:
HF(g) + H2O(l)
H3O+(aq) + F −(aq)
NH3(g) + H2O(l)
NH4+(aq) + OH− (aq)
Solutes: Weak Electrolytes
The weak electrolyte in an
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In water, nonelectrolytes
• dissolve as molecules • do not produce ions
• do not conduct a current
Example:
C12H22O11(s) C12H22O11(aq)
Solutes: Nonelectrolytes
Solutes in Aqueous Solutions
Na+, K+, NH 4+
Cl-, Br-, I- Ag+, Hg
22+, Pb2+
Soluble Insoluble
Nitrates (NO3-)
Acetates, (C2H3O2-)
Sulfates (SO42-),
Ag+, Ca2+ (slightly)
NH4+, Group I
Except
Except
Except
Ba2+, Sr2+, Pb2+
Carbonates (CO32-)
Phosphates (PO43-)
OH-, Sulfides (S2-)
Solubility Rules
Predict the solubility of barium sulfate.
a. soluble b. insoluble
Predict the solubility of NaCl.
a. soluble b. insoluble
Most sulfates are soluble, except Ba2+.
All Na+ salts are soluble.
Solutions Practice
a. soluble b. insoluble
Predict the solubility of silver nitrate.
a. soluble b. insoluble
Predict the solubility of silver hydroxide.
a. soluble b. insoluble
Predict the solubility of ammonium carbonate. All NO3- salts are soluble.
Most hydroxides are insoluble.
All NH4+ salts are soluble.
© 2014 John Wiley & Sons, Inc. All rights reserved.
18
Equations for Forming Solids
A chemical equation shows the complete formulas of all the compounds in a reaction.
Pb(NO3)2(aq) + 2NaCl(aq) PbCl2(s) + 2NaNO3(aq)
An ionic equation shows the ionic compounds as separate ions.
Pb2+(aq) + 2NO
3−(aq) + 2Na+(aq) + 2Cl−(aq)
PbCl2(s) + 2Na+(aq) + 2NO
3−(aq)
A net ionic equation shows only the ions that react to form a solid.
Pb2+(aq) + 2Cl−(aq) PbCl
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Example of Writing Equations for an
Insoluble Salt
Write the net ionic equation for the reaction of Ba(NO3)2(aq) + Na2CO3(aq)
STEP 1 Predict the products of the reaction and determine their phase (s), (l), (g), (aq)
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Equations for an Insoluble Salt
(continued)
STEP 3 Write the ionic equation including any solid.
Ba2+(aq) + 2NO
3−(aq) + 2Na+(aq) + CO32 −(aq)
BaCO3(s) + 2Na+(aq) + 2NO
3−(aq))
STEP 4 Write the net ionic equation. Cross out the spectator ions in the ionic equation.
Ba2+(aq) + 2NO
3−(aq) + 2Na+(aq) + CO32 −(aq)
BaCO3(s) + 2Na+(aq) + 2NO
3−(aq))
Ba2+(aq) + CO
32−(aq) BaCO3(s)
21
Learning Check
Write the formula of any insoluble salt, if any. Write the net ionic equation for any of the
following which form insoluble salts: A. BaCl2(aq) + Na2SO4(aq)
B. AgNO3(aq) + KCl(aq) C. KNO3(aq) + NaCl(aq)
Example – Conventional, Total Ionic and
Net Ionic Equations
•
Zinc metal reacts with aqueous copper(II)
Example – Conventional, Total Ionic and
Net Ionic Equations
•
Aqueous ammonium nitrate reacts with aqueous
sodium hydroxide to produce aqueous sodium
nitrate and ammonia gas and water.
•
What is the total ionic equation?
A. NH
4NO
3 (aq)+ NaOH
(aq) NaNO
3 (aq)+ NH
3 (g)+ H
2O
(l) B. NH4 +(aq) + NO3- (aq) + Na+ (aq) + OH- (aq) Na+ (aq) + NO3- (aq) + NH3 (g) + H2O (l)
C. NH
4 +(aq)
+ OH-
(aq) NH
3 (g)+ H
2O
(l)D. All spectator ions
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Solubility
• is the maximum amount of solute that dissolves
in a specific amount of solvent
• can be expressed as grams of solute in 100
grams of solvent, usually water g of solute
100 g water
25
Comparing Unsaturated and
Saturated Solutions
There are limits to the solubility of a compound at a given temperature.
Saturated solutions: contain the maximum amount of dissolved solute in a solvent.
Saturated solutions are still dynamic; dissolved solute is in equilibrium with undissolved solute.
Unsaturated solutions: contain less than the maximum amount of possible dissolved solute in a solvent.
undissolved solute dissolved solute
Saturated and Unsaturated
Solutions
Supersaturated solutions: contain more solute than needed to saturate a solution at a given temperature.
These solutions are unstable -- disturbing the solutions can cause precipitation of solute.
How is this possible?
Heating a solution can allow more to dissolve. Upon cooling to ambient temperature,
the solution is supersaturated.
Some hotpacks release heat by crystallization of a supersaturated solution of sodium acetate.
Supersaturated Solutions
Example - Solutions
• What type of
solution is sodium acetate at 75 °C and 50g/100 g H2O?
A. Saturated B. Unsaturated C. Supersaturated D. Not enough
What happens to a supersaturated solution if more solute is added?
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Effect of Temperature on Solubility
Solubility
• depends on
temperature
• of most solids
increases as
temperature increases
• of gases decreases as
Effect of Agitation/Stirring
Stirring a solution briskly breaks up a solid into smaller pieces, increasing surface area,
thereby increasing the rate of dissolution.
© 2014 John Wiley & Sons, Inc. All rights reserved.
32
Solubility and Pressure
Henry’s law states
• the solubility of a
gas in a liquid is directly related to the pressure of that gas above the liquid
• at higher pressures,
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A. Why could a bottle of carbonated drink possibly burst (explode) when it is left out in the hot sun?
B. Why do fish die in water that is too warm?
34
The concentration of a solution
• is the amount of solute dissolved in a specific
amount of solution amount of solute amount of solution
• as percent concentration describes the amount
of solute that is dissolved in 100 parts of solution
amount of solute 100 parts solution
35
The mass percent (m/m)
• concentration is the percent by mass of solute in a
solution
Mass percent (m/m) =
mass of solute x 100% mass of solute + mass of solvent
• is the g of solute in exactly100 g of solution
Mass percent = g of solute x 100% 100. g of solution
36
Mass percent (%m/m) is calculated from the
grams of solute (g KCl) and the grams of solution (g KCl solution).
g of KCl = 8.00 g
g of solvent (water) = 42.00 g
g of KCl solution = 50.00 g
8.00 g KCl (solute) x 100 = 16.0% (m/m) 50.00 g KCl solution
37
38
A solution is prepared by mixing 15.0 g of
Na2CO3 and 235 g of H2O. Calculate the mass percent (m/m) of the solution.
Example – Mass Percent
•
A solution consists of 31.7 g of AgNO
3in
Example – Mass Percent
•
How much of a 25.0% glucose solution is
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The volume percent (v/v) is
• percent volume (mL) of solute (liquid) to volume
(mL) of solution
Volume percent(v/v) = mL of solute x 100% mL of solution
• solute (mL) in 100 mL of solution
Volume percent (v/v) = mL of solute x 100% 100 mL of solution
42
A volume percent (v/v)
solution is prepared by
• using a specific volume
(mL) of solute (liquid)
• adding water to the
volume line
43
Percent Conversion Factors
• Two conversion factors can be written
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• How many grams of NaCl are needed to prepare
225 g of a 10.0% (m/m) NaCl solution?
• How many grams of NaOH are needed to prepare
75.0 g of a 14.0% (m/m) NaOH solution?
• How many milliliters of a 5.75% (v/v) ethanol solution
can be prepared from 2.25 mL of ethanol?
Saline is a 0.9 m/v % NaCl solution. What mass of sodium chloride is needed to make 50 mL of saline?
= 0.45 g NaCl
Knowns 50.0 mL solution (solution volume) 0.90 m/v% (mass/volume %)
Calculate
Solve for mass of solute (NaCl)
50.0 x 0.90 100
mass solute =
Formula
g solute
mL solution x 100
m/v % = mass solute = m/v % x mL soln100