Chemistry 161
Lecture 17: Precipitation Reactions, Ion Exchange, Oxidation-Reduction
Reactions, Titrations
Sections 4.6-4.8 in Gilbert, Kirss, Foster, and Davies 2 nd edition
11/13/08
Learning Objectives:
1.
Define precipitation
2.
Define spectator ions
3.
Write net ionic reactions
4.
Appreciate the concept of the solubility window
5.
Recognize the ion exchange that underlies precipitation reactions between salts
6.
Appreciate the fundamental ion exchange reaction that allows water softening
7.
Appreciate the clever rationale employed to “deionize” water
8.
Define oxidation and reduction
9.
Define oxidizing agent and reducing agent
10.
Understand the relationship between oxidation and oxidizing agent, reduction and reducing agent
11.
Know oxidation and reduction occur together
12.
Define oxidation number
13.
Define half-reaction
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14.
Utilize oxidation numbers and half-reactions in order to balance overall redox reactions
15.
Define titration, end point, equivalence point, and indicator
16.
Describe the features of the curve for titrating a strong acid with a strong base
17.
Describe the features of the curve for titrating a strong base with a strong acid
18.
Calculate the amount of strong acid in an unknown sample by titrating with a given volume of a base solution of known concentration
Defining precipitation
19.
Calculate the amount of strong base in an unknown sample by titrating with a given volume of an acid solution of known concentration
Consider the reaction of NaCl with AgNO
3
If we were to quantify this behavior we need to employ the concepts of equilibrium and solubility product, K sp
1
Given the reaction
AgCl s
Ag + aq
+ Cl aq
K sp
= [Ag + ][Cl ] = 1.8 x 10 -10
1 Equilibrium expressions are always written as molar concentrations of reactants over products. For the AgCl reaction above, one might expect AgCl to appear in the denominator of the equilibrium expression, but since AgCl is a solid, its “concentration” is a constant and is incorporated into the equilibrium constant (we will develop chemical equilibrium at length in chapter 15)
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 Notice that a precipitation reaction is the reverse of a solvation reaction
 Also notice that from a quantitative standpoint, there are still Ag + and
Cl ions in solution…just not very many of them 2
A Summary Table of Qualitative Solubility for Common Ions
 Notice that AgCl precipitation is an anomaly (which makes it very useful for testing purposes)
 There is a window of solubility for aqueous media. If a compound interacts too strongly with itself compared to its interaction with water, it will not be appreciably soluble (e.g. CaCO
3
– note the greater electrical interactions compared to NaCl). If the compound interacts weakly with water compared to the interactions of water with other water molecules, it will also be poorly soluble (e.g. motor oil)
2 K sp
for CaCO
3
= 4.8 x 10 -9 , Ksp for MgCO
3
= 6.8 x 10 -6 ; in areas where these solids are common, the few ions that dissolve when water encounters them is enough to make the water ‘hard’
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Spectator Ions Defined
If the ions were floating around in solution before and after the precipitation (or any other reaction for that matter) they are termed spectator ions
Consider the barn dance analogy:
Writing net ionic reactions
Notice that at the end of the show there must be balance of mass (and thus numbers of atoms) and balance of charge. Consider the reaction of Na
2 with CaCl
2 (aq)
CO
3 (aq)
4.7: Ion Exchange
The generation of soap by saponification
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One problem with hard water
The same reaction that makes soaps limited in usefulness in hard water regions allows water to be softened
Question: What might be a health concern of water that has been
“softened”?
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Deionization of water
H + [RC] -
(s)
+ Na +
(aq)
 Na
[RA] + OH -
(s)
+ Cl -
(aq)
+ [RC] -
 [RA] + Cl -
(s)
(s)
+ H +
+ OH -
(aq)
(aq)
H +
(aq)
+ OH -
(aq)
 H
2
O
4.8: Intro to Redox Reactions
3 Important Examples to Help Keep Oxidation and Reduction Clear
 The oxidation of iron (reverse reaction is reduction)
 The oxidation of ethanol (reverse reaction is reduction)
 The 4 electron reduction (with H + chasers) of O
2
(reverse reaction is oxidation)
And 2 Disturbing Ways to Keep Oxidation Clear:
 LEO (the loin) says GER
 OLE likes BEER
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Oxidation and reduction always occur together; that is, if something is being oxidized, something else is being reduced
Oxidizing agents are reduced in a redox reaction, reducing agents are oxidized
There can be no net change in the number of electrons in a redox reaction
 This is to say that viewing a reaction in redox terms is a method for looking at the transfer of electrons, whereas looking at a reaction in acid base terms is typically a method for looking at the transfer of protons
Problem: Write the overall redox reaction for the reaction of aluminum with oxygen to form aluminum(III)oxide
First we must learn a little about oxidation numbers and balancing halfreactions…
On Oxidation Numbers
 The oxidation number of an element is 0
Consider the following elements
Na Ca Cu S O
2
 The oxidation state of a monatomic ion is equal to it’s charge
 Certain elements have the same oxidation number in (almost) all
Cl
2 compounds – in particular, the group 1 or group 2 metals, and H. O
almost always has a -2 charge o Hydrogen actually supersedes O, which is why the oxidation number of O in H
2
O
2
is -1
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 The sum of the oxidation numbers in a neutral species must equal 0; in a polyatomic ion it must equal the charge on that ion o What is the oxidation number of Mn in KMnO
4
? In MnO
4
? o What is the oxidation number of C in CH
4
, CH
3
OH, CO
2
, and CF
4
?
Balancing Half-Reactions
 Balance the atoms of the element being oxidized or reduced
 Balance oxidation numbers by adding electrons
 Balance charge by adding H + in acidic solution, OH - in basic solution
 Balance H by adding H
2
O
 Check: O is balanced?
 In order to balance the redox reaction overall, we simply multiply one of the half-reactions such that the electrons cancel
Returning to our initial problem of writing the overall redox reaction for the reaction of aluminum with oxygen to form aluminum(III)oxide
Question: Write the overall redox reaction for the oxidation of silver by hydrogen peroxide.
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Finally, a couple more complex examples that more fully illustrate the method for balancing redox reactions:
The oxidation of Fe +2 by MnO
4
in acidic media to Fe +3 and Mn +2
Divide into 2 half reactions:
Balance atoms:
Balance charge by adding H + :
Balance H by adding water:
Balance reaction overall by multiplying half reactions so that electrons cancel
The oxidation of Cr(OH)
3
by Cl
2
in basic media to Cl and CrO
4
-2
Divide into 2 half reactions:
Balance atoms:
Balance charge by adding OH :
Balance H by adding water:
Balance reaction overall by multiplying half reactions so that electrons cancel
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4.6: Titration
Definitions
Titration:
Equivalence Point:
Indicator:
End Point:
What do titration curves look like?
Strong Acids Titrated with Strong Bases
Strong Bases Titrated with Strong Acids
Most often, we look for 1:1 reactions when carrying out titrations. This allows us to utilize the familiar M titrations this becomes M
A
 V
 V = M
A
= M
B
 V
B
 V. In the case of acid-base
What is mass of acetic acid in 500 mL of an unknown vinegar solution if 20 mL of 6 M NaOH is required to reach the equivalence point of a 50 mL sample?
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