Redox Rxns:Part II: Common Redox Rxns

Chapter 6 Sec 3, 4 & 5of Jespersen 6TH ed)

Note: We skip Sec 6.2 this semester. It is covered in Gen Chem II.

Dr. C. Yau

Spring 20131

Redox Rxns

Review: Redox Rxns involve a transfer of electrons.

Oxidation is INCREASE in oxidation number. e.g. 2KCl Cl2

-1 0Reduction is DECREASE in oxidation numbere.g. MnO2 MnSO4 +4 +2

Cl has lost one e- each.

Mn has gained 2 e- each. 2

SummaryOxidation: Increase in oxid #

Loss of electron(s)

Reduction: Decrease in oxid #

Gain of electron(s)

3

LEO the lionSays GER.Loss of Electron OxidationGain of Electron Reduction

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 MetalsMg + 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 Metals3Mg + 2AlCl3 3MgCl2 + 2Al

2Al + 3MgCl2 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

Table6.3p. 232

Mg is more active than Al

Nickel Ni2+

Least Active

Most Active

Slide 6

Slide 9

Slide 107

The Activity Series tells us that Mg is more "active" than Al.

This means Mg can replace Al, and not the reverse:

3Mg + 2AlCl3 3MgCl2 + 2Al

2Al + 3MgCl2 2AlCl3 + 3Mg

YES

NO

8

Example 6.7 p.233What will happen if an iron nail is dipped into

a solution containing copper(II) sulfate? What is the molecular equation?

Example 6.8 p. 235What 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. 235Activity Series 9

Redox Rxns of Metals with WaterNote that in the Activity Series, H is included

even though it is not a metal. It is best to view water as HOH.

The most active metals will react with HOH as liquid or steam,

And least active will not react with water.

Al + H2O ?

Pb + H2O ?

Modified Activity Series

Na + HOH NaOH + H2

10

Slide 10

Slide 13

Slide 14

Nickel Ni2+

Slide 1511

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 AcidsThe 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 ?

Activity Series

You will not be asked to memorize the activity series, but you are expected to know how to use it.

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

Nonoxidizing acids are acids where H+ is being reduced. Examples we have just gone through were of nonoxidizing acids.

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 + 2H2OGive the oxidation numbers of all the

elements. 15

Oxidizing Acids Can React with Most MetalsHNO3: (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 Fe2O3

(s) (g) (s)

Mg (s) + O2 (g) ?

Do Pract Exer 22,& 23 p.195 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 nuclear power plants in Japan?

21

Schematics of a Nuclear Power PlantControl 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 the above reactions will go. 24

Working with net ionic eqns in redox rxns:

Note: Net charges must balance also!

Give the possible products & decide whether the reaction will go:

Al + Zn2+

Pb2+ + Fe

25

Activity Series

gold Ag+

copper Cu2+

hydrogen H+

lead Pb2+

tin Sn2+

zinc Zn2+

manganese Mn2+

aluminum Al3+

magnesium Mg2+

Stoichiometry of Redox RxnsIn 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?

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

2+4

2+4

8

0.0500 mol MnO 5 mol Sn 1 mol Sn 118.7 g Sn#g Sn=0.00808 L

1 L 1 mol Sn1 mol Sn2 mol MnO

0.119 g = Sn

-

8% Sn = x 100 =

0.3000 g o

0.119

re sam39

g.9

ple

Sn % Sn

Do example on p.240, Ques on p.249 #6.79 & 6.81 28