Redox Rxns:
Part II: Common Redox Rxns
Chapter 6 Sec 3, 4 & 5
of Jespersen 6TH ed)
Note: We skip Sec 6.2 this semester. It is
covered in Gen Chem II.
Dr. C. Yau
Spring 2014
1
Redox Rxns
Review: Redox Rxns involve a transfer
of electrons.
Oxidation is INCREASE in oxidation
number.
e.g. 2KCl  Cl2 Cl has lost one e- each.
-1
0
Reduction is DECREASE in
oxidation number
e.g. MnO2  MnSO4
+4
+2
Mn has gained 2 e- each.
2
Summary
Oxidation: Increase in oxid #
Loss of electron(s)
Reduction: Decrease in oxid #
Gain of electron(s)
LEO the lion
Says GER.
Loss of Electron Oxidation
Gain of Electron Reduction
3
Redox Rxns
2AgNO3 (aq) + Cu (s)
Cu(NO3)2 + 2Ag
Ag+
Ag
Cu
Cu2+
What is oxidized?
What is reduced?
Which is the oxidizing agent?
Which is the reducing agent?
4
Redox Rxns of Metals
Mg
+
AlCl3
MgCl2
+ Al
This type of rxns is often referred to as a
"Single Replacement Reaction."
In this reaction, Mg is replacing Al in AlCl3.
It can be easily balanced without
separating it into half-reactions.
What are the oxidation states of Mg in this
rxn? of Al?
Which is undergoing oxidation? reduction?
5
Redox Rxns of Metals
3Mg + 2AlCl3
2Al + 3MgCl2
3MgCl2 + 2Al
2AlCl3 + 3Mg
The 2nd eqn is the reverse of the 1st.
How do we know which is the correct one?
Will Mg replace Al (as in the 1st eqn),
or will Al replace Mg (as in the 2nd eqn)?
The answer lies in the
Activity Series of Metals.
6
Table
6.3
p. 232
Least
Active
Nickel
Mg is
more
active
than Al
Ni2+
Slide 6
Slide 9
Slide 10
Most
Active
7
The Activity Series tells us that Mg is more
"active" than Al.
This means Mg can replace Al, and not the
reverse:
YES
3Mg
+
2AlCl3
3MgCl2
+ 2Al
NO
2Al
+ 3MgCl2
2AlCl3 + 3Mg
8
Example 6.7 p.233
What will happen if an iron nail is dipped into
a solution containing copper(II) sulfate?
What is the molecular equation?
Example 6.8 p. 235
What happens if an iron nail is dipped into a
solution of aluminum sulfate? What is the
molecular equation?
Do Pract Exer 23 & 24 on p. 235
Activity
Series
9
Redox Rxns of Metals with Water
Note that in the Activity Series, H is included even
though it is not a metal. H is used as a reference
& is water is best viewed as HOH.
The most active metals will react with HOH as
liquid or steam,
Na + HOH
NaOH + H2
And the least active metals will not react with water.
Al + H2O
Pb + H2O
Modified
Activity Series
?
?
10
Nickel
Ni2+
Slide 10
Slide 13
Slide 14
Slide 15
11
Do not confuse these redox rxns with the
rxn of metal oxides with water.
Na2O + HOH  2NaOH
CaO + HOH  Ca(OH)2
2Na + 2HOH  2NaOH + H2
Ca + 2HOH  Ca(OH)2 + H2
What is the difference?
One is redox, the other is not.
Which is which?
12
Redox Rxns of Metals with Acids
The more reactive metals reduce H+ in acids
to H2. Again we need to consult the
activity series.
e.g.
Zn + HCl
ZnCl2 + H2
Cu + HCl
?
Mn + HBr
?
You will not be asked to memorize the activity
series, but you are expected to know how to use it.
Activity
Series
13
Redox Rxns of Metals
Write the equation for the reaction of
aluminum with hydrobromic acid. Will it
go?
Is aluminum gaining or losing electrons?
What is happening to the bromine in this
reaction?
Where are the electrons going to?
Write the two balanced half-reactions.
Which is the oxidizing agent in this rxn?
Activity
Series
14
Oxidizing and Nonoxidizing Acids
Examples we have just gone through were of
“nonoxidizing acids,” where H+ is being reduced.
HCl, HBr, HI, dilute H2SO4 are such acids.
Zn + H2SO4
ZnSO4 + H2
The H+ in all acids have the potential of oxidizing
metals that are below H in the activity series.
Oxidizing acids are acids with anions that
also act as an oxidizing agent.
Cu + 4HNO3
Cu(NO3)2 + 2NO2 + 2H2O
Give the oxidation numbers of all the
elements.
15
Oxidizing Acids Can React with Most Metals
HNO3:
(TABLE 6.2 p.229)
(conc) NO3- + 2H+(aq) + e- → NO2(g)+ H2O(l)
(dil) NO3- (aq) + 4H+(aq)+ 3e- → NO(g) + 4 H2O(l)
(v.dil): NO3- (aq) + 10H+ + 8e- → NH4+ (aq) + 3H2O(l)
H2SO4:
(hot, conc.)SO42- + 4H+(aq) + 3 e- → SO2(g) +2H2O(l)
(hot, conc, with strong reducing agent)
SO42- (aq) + 10H+(aq) + 8e- → H2S(g) + 4H2O(l)
You will not be asked to predict these reactions, but
you should be able to recognize these are redox rxns
by the change in oxidation numbers.
16
Molecular Oxygen as a Powerful
Oxidizing Agent
You have previously learned that "burning"
is always a rxn with O2.
These are redox rxns with O2 as the
oxidizing agent.
CH4 + 2O2
CO2 + 2H2O
C6H10O5 + O2
CO2 + H2O
(C6H10O5 =simplified formula for cellulose,
the chief combustible ingredient in wood)
Balance the equation.
17
Molecular Oxygen as a Powerful
Oxidizing Agent
Burning of organic compounds containing sulfur
produces sulfur dioxide.
2C2H5SH + 9O2
4CO2 + 6H2O + 2SO2
SO2 is an air pollutant. Note that this is a
"nonmetal oxide."
In the presence of rain, what does it become?
Do Pract Exer 25, 26, 27p.237
18
O2 as an Oxidizing Agent
Many metals react with oxygen:
Rusting of iron: 2Fe + 3O2
(s)
Mg (s) + O2 (g)
Do Pract Exer 22,& 23 p.195
(g)
Fe2O3
(s)
?
19
Reaction with O2
• O2 is always the oxidizing agent. It
becomes O2-. The products are the oxides
of the each element in the reactant.
• If the reaction is rapid and producing large
amounts of heat, we call it combustion.
• C2H2 + O2
CO2 + H2O
• Mg+ O2
MgO + heat
• If it is slow, it is usually considered
“tarnishing” or “oxidation” of the metal.
• Al + O2
Al2O3
20
What caused the explosion at the
Fukushima nuclear power plant in Japan
after the earthquake hit in 2011?
It was NOT a
nuclear
explosion...luckily.
21
Schematics of a Nuclear Power Plant
Control rods made of elements such as B or Cd
absorb neutrons to slow down the nuclear fission.
22
What caused the explosion at the nuclear
power plants in Japan?
The earthquake did not damage the reactor
containment structure,
but the tsunami heavily damaged the cooling systems,
all 15 backup cooling systems.
Electricity is needed to pump in cooling water.
Batteries lasted a few hours and new ones had to be
brought in.
Special pump used to pump in sea water ran out of
gas.
Sea water was no longer circulating and over heated.
2H2O (g)  2H2 (g) + O2 (g)
23
What kind of redox equations are you
expected to be able to write and balance?
Single replacement reactions such as:
• Mn + AlCl3
• Mn + H2O
• Mn + HCl
• In addition, you should be able to use the
Activity Series to determine whether the
above reactions will go.
24
Working with net ionic eqns in redox rxns:
Note: Net charges must balance also!
Give the possible products, bal. the eqn. &
decide whether the reaction Activity Series
will go:
gold Au3+
Al + Zn2+ 
Pb2+
+ Fe 
Answer is YES for both reactions!
copper Cu2+
hydrogen
lead
Pb2+
tin
Sn2+
Iron
Fe2+
zinc
Zn2+
manganese
aluminum
magnesium
H+
Mn2+
Al3+
Mg2+
25
Stoichiometry of Redox Rxns
In a reaction, 45.0 g of magnesium is to
react with 500.0 mL of 0.300 M HCl. At the
end of the reaction, will there be any
magnesium left?
How much of the excess is left over?
26
Ore Analysis
#6.107 p.250
A 0.3000 g sample of tin ore was dissolved in acid
solution converting all the tin to tin(II). In a titration,
8.08 mL of 0.0500 M KMnO4 was required to
oxidize the tin(II) to tin(IV).
The reaction is as follows:
5Sn2+ + 2MnO4- + 16H+
5Sn4+ + 2Mn2+ + 8H2O
What was the percentage tin in the original sample?
27
# g Sn
% tin in sample =
x 100
# g ore sample
? g Sn
=
x 100
0.3000 g ore sample
5Sn2+ + 2MnO4- + 16H+  5Sn4+ + 2Mn2+ + 8H2
?mol 8.08 mL
0.0500 M KMnO4


0.0500
mol
MnO
4 
x mol MnO4- =0.00808 L 
= 4.040x104 mol MnO 4

1L


x g Sn =4.040x10
4

mol MnO 4 
5 mol Sn 2+   1 mol Sn  118.7 g Sn 

 2 mol MnO    1 mol Sn 2+ 
1
mol
Sn



4 
= 0.1198 g Sn
0.1198 g Sn
% Sn =
x 100 = 39.9 % Sn
0.3000 g ore sample
Do example on p.240, Ques on p.249 #6.79 & 6.81
28