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

Reduction is DECREASE in oxidation number e.g. MnO2  MnSO4 Redox Rxns Review: Redox Rxns involve a transfer of electrons. Oxidation is INCREASE in oxidation number. e.g. 2KCl  Cl2 -1 0 Reduction is DECREASE in oxidation number e.g. MnO2  MnSO4 +4 +2 Cl has lost one e- each. Mn has gained 2 e- each.

Oxidation: Increase in oxid # Loss of electron(s) 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

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?

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?

Redox Rxns of Metals The 2nd eqn is the reverse of the 1st. 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 Table 6.3 p. 232 Least Active Mg is more active than Al Nickel Ni2+ Slide 6 Slide 9 Most Active Slide 10

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

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

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, And the least active metals will not react with water. Na + HOH NaOH + H2 Al + H2O ? Pb + H2O ? Modified Activity Series

Nickel Ni2+ Slide 10 Slide 13 Slide 14 Slide 15

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?

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

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

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.

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.

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.

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

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

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

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.

Schematics of a Nuclear Power Plant Control rods made of elements such as B or Cd absorb neutrons to slow down the nuclear fission.

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)

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.

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 will go: Al + Zn2+  Pb2+ + Fe  Answer is YES for both reactions! Activity Series gold Au3+ copper Cu2+ hydrogen H+ lead Pb2+ tin Sn2+ Iron Fe2+ zinc Zn2+ manganese Mn2+ aluminum Al3+ magnesium Mg2+

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?

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? Chem FAQ: How do I determine the percent of metal in an ore using a redox titration?

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