Redox Rxns: Part II: Common Redox Rxns Chapter 6 Sec 3, 4 & 5 of Jespersen 6 TH ed) Note: We skip Sec 6.2 this semester. It is covered in Gen Chem II. Dr. C. Yau Spring 2013. Redox Rxns. Review: Redox Rxns involve a transfer of electrons. Oxidation is INCREASE in oxidation number.
Redox Rxns:Part II: Common Redox RxnsChapter 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
Review: Redox Rxns involve a transfer of electrons.
Oxidation is INCREASE in oxidation number.
e.g. 2KCl Cl2
Reduction is DECREASE in oxidation number
e.g. MnO2 MnSO4
Cl has lost one e- each.
Mn has gained 2 e- each.
Oxidation: Increase in oxid #
Loss of electron(s)
Reduction: Decrease in oxid #
Gain of electron(s)
LEO the lion
Loss of Electron Oxidation
Gain of Electron Reduction
2AgNO3 (aq) + Cu (s) Cu(NO3)2 + 2Ag
What is oxidized?
What is reduced?
Which is the oxidizing agent?
Which is the reducing agent?
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?
3Mg + 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.
Mg is more active than Al
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
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
Note 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 activewill not react with water.
Na + HOH NaOH + H2
Al + H2O ?
Pb + H2O ?
Modified Activity Series
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?
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.
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?
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 anionsthat also act as an oxidizing agent.
Cu + 4HNO3 Cu(NO3)2 + 2NO2 + 2H2O
Give the oxidation numbers of all the elements.
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)
(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.
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.
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
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
Control rods made of elements such as B or Cd absorb neutrons to slow down the nuclear fission.
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)
What kind of redox equations are you expected to be able to write and balance?
Single replacement reactions such as:
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
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?
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?
5Sn2+ + 2MnO4- + 16H+5Sn4+ + 2Mn2+ + 8H2
?mol 8.08 mL
0.0500 M KMnO4
Do example on p.240, Ques on p.249 #6.79 & 6.81