oxidation-reduction

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Notes

 

Redox

=

Reduction

and

Oxidation

 

 

(gain of e-)

 

(loss of e-)

 

 

(oxidizing agent)

 

(reducing agent)

 

 

(decrease in oxidation number)

 

(increase in oxidation number)

Example

 

Mn+7 in MnO4− gains 5e− to become Mn2+, therefore MnO4− is the oxidizing agent

 

Fe2+ loses one e- to become Fe3+, therefore Fe2+ is the reducing agent

 

Reduction half-Reaction                     8H+ + MnO4− + 5e− ---> Mn2+ + 4H2O

Oxidation half-Reaction                                               5Fe2+ ---> 5Fe3+ + 5e−

                ----------------------------------------------------------------------------------------------------------

Redox                          8 H+(aq) + MnO4−(aq) + 5 Fe2+(aq) ---> Mn2+(aq) + 4 H2O(l) + 5 Fe3+(aq)

 

                Note that 5 e- are lost and 5 e- are gained. The redox equation is balanced for mass and charge.

 

Cell Potential (Ecell) or Electromotive Force (emf)

In a voltaic or galvanic cell a redox reaction produces a current that can be used to do work. The pull of e− from a reducing agent to an oxidizing agent is called the Cell Potential (Ecell) or Electromotive Force (emf) of the cell. Cell potential values are given in the reference table.

 

E°cell = E°oxidation + E°reduction 

 

The units are the volts (V) and “°” represents standard state (1M and 1 atm for gases)

Example

        Given the reaction 2H+(aq) + Zn(s) ---> Zn2+(aq) + H2(g) what is its cell potential?

    ½ Reduction         2H+ + 2e− ---> H2                             E°red =  0.00 V        see reference table

    ½ Oxidation           Zn ---> Zn2+ + 2e−                          E°oxi = + 0.76 V      see reference table

    E°cell = 0.00 V + 0.76 V = 0.76 V

Example

        Given the reaction Zn(s) + Cu2+(aq) ---> Zn2+(aq) + Cu(s) what is its cell potential?

    ½ Reduction         Cu2+ + 2e− ---> Cu                           E°red = + 0.34 V

    ½ Oxidation           Zn ---> Zn2+ + 2e−                          E°oxi = + 0.76 V

    E°cell = 0.34 V + 0.76 V = 1.10 V

 

Example

        Given the reaction Fe3+(aq) + Cu(s) ---> Cu2+(aq) + Fe2+(aq) what is its cell potential and the balance redox reaction?

 

½ Reduction         Cu ---> Cu2+ + 2e  −                      E°red = - 0.34 V

½ Oxidation           Fe3+ + e− ---> Fe2+                       E°oxi = + 0.77 V

E°cell = - 0.34 V + 0.77 V = 0.43 V

               

The balance redox reaction is the sum of two half-reaction, where number of e− lost is equals the number of e− gained

                        Cu ---> Cu2+ + 2e−

          2(Fe3+ + e− ---> Fe2+)                  Note the ½ oxidation reaction is multiplied by 2 to balance the 2 e- lost

------------------------------------------------------

2Fe3+(aq) + Cu(s) ---> 2Fe2+(aq) + Cu2+(aq)

 

Line Notation

A redox reaction in a voltaic electrochemical cell can be expressed as line notation.

Example

When no metals are present electrodes made of Pt(s) are used.

Example

 

A galvanic cell runs spontaneously in the direction that gives a positive value for E°cell

Example

Given the following information for a galvanic cell made of

Fe2+ (aq)+ 2e− → Fe(s)                                                                     E°red = - 0.44 V

MnO4− (aq) + 5e− + 8H+ (aq)→ Mn2+ (aq) + 4H2O (l)                        E°red = + 1.51 V

1. Calculate E°cell

2. Write the balance redox equation

3. Draw the set up for the galvanic cell

4. Write the line notation

 

First, find which setups give a positive E°cell

                                E°cell = E°oxidation + E°reduction 

 

                                E°cell = (- 0.44 V) + (- 1.51 V) = - 1.07 V               not spontaneous

                                E°cell = (+ 0.44 V) + (+ 1.51 V) = 1.95 V               spontaneous

 

Second, the balanced redox reaction for the cell is

                ½ oxidation                                                           5( Fe --->Fe2+ + 2e− )

                ½ reduction                             2(MnO4− + 5e− + 8H+ ---> Mn2+ + 4H2O)

                -----------------------------------------------------------------------------------------------------------

                Redox balanced    5Fe(s) + 2MnO4−(aq) +16H+(aq) ---> 5Fe2+(aq) + 2Mn2+(aq) + 8H2O(l)

               

                Third, the setup for the galvanic cell is

 

Fourth, the line notation is

 

Example

Given the following information for a galvanic cell made of

Ag+ (aq)+ e− → Ag (s)                       E° = + 0.80 V

Fe3+(aq) + e− → Fe2+ (aq)                  E° = + 0.77 V

1. Calculate E°cell

2. Write the balance redox equation

3. Draw the set up for the galvanic cell

4. Write the line notation

                               

                                E°cell = (+ 0.80 V) + (- 0.77 V) = + 0.03 V             spontaneous

 

                ½ oxidation                           Ag+ + e− ---> Ag     

                ½ reduction                                    Fe2+ ---> Fe3+ + e−            

                ------------------------------------------------------------------------

                Redox balanced    Ag+(aq) + Fe2+ (aq) ---> Ag(s) + Fe3+ (aq)

                               

 

                          

Questions

PART B

 

1.             I               Calcium acts as a reducing agent when it reacts with bromine

                BECAUSE

II             mass is conserved in a chemical reaction.

 

2.             I               The electrolysis of potassium iodide, KI, produces electrical energy

                BECAUSE

II             electrons flow from the anode to the cathode.

               

3.             I               The oxygen atom in a water molecule has a -2 oxidation state

                BECAUSE

II             water molecules exhibit hydrogen bonding.

 

4.             I               Elemental sodium is a strong reducing agent

                BECAUSE

                II             An atom of elemental sodium gives up its valence electron readily.

 

5.             I               In an electrochemical cell, the electrode that is the site of reduction is called the anode

                BECAUSE

                II             oxidation always occurs at the cathode.

 

6.             I               Hydrogen peroxide is a good oxidizing agent

                BECAUSE

                II             the hydrogen in H2O2 has a +1 oxidation number.

 

7.             I               Electrolysis of water requires the input of energy

                BECAUSE

                II             the products formed, H2 and O2, possess more chemical potential energy than H2O.

 

8.             I               Bromine is a stronger oxidizing agent than chlorine

                BECAUSE

                II             it has a large atomic radius

 

9.             I               The oxidation state of Cr in Al2(Cr2O7)3 is +3

                BECAUSE

                II             as a neutral compound, the sum of oxidation numbers of all the atoms must equal zero

 

10.           I               A voltaic cell spontaneously converts chemical energy into electrical energy

                BECAUSE

                II             a voltaic cell needs an externally applied current to work.

 

11.           I               When a Li atom reacts and become an ion, the Li atom can be considered to be a reducing agent

                BECAUSE

                II             the Li atom lost an electron and was oxidized.

12.           I               F2 --> 2F- + 2e is a correctly written half reaction

                BECAUSE

                II             this half reaction must demonstrate proper conservation of mass and charge.

 

13.           I               Al3+ + 3e --> Al is a correctly balanced oxidation reaction

                BECAUSE

                II             Al3+ + 3e --> Al correctly demonstrates conservation of mass and conservation of charge.

 

14.           I               An electrolytic cell makes a nonspontaneous redox reaction occur

                BECAUSE

                II             an electrolytic cell uses an external current to drive a redox reaction.

 

 

PART C

 

1. What is the oxidation state of chromium, Cr, in the compound potassium dichromate, K2Cr2O7?

A             +1

B             +2

C             +3

D             +6

E              +12

 

2. The standard potential difference of an electrochemical cell using the overall reaction below is

2Fe2+ + Cl2 --> 2Fe3+ + 2Cl-

Fe3+ + e --> Fe2+                    Eored = 0.77 volts

Cl2 +2e --> 2Cl-                      Eored = 1.36 volts

A             0.18 volts

B             0.59 volts

C             1.05 volts

D             2.13 volts

E              2.90 volts

 

3. What is the oxidation state of bromine in HBrO3?

A             -3

B             -1

C             +1

D             +3

E              +5

 

4. Which of the following takes place during the reaction below?

Cu(s) + NO3-(aq) + H+(aq) --> Cu2+(aq) + NO2(g) + H2O(l)

A             Cu(s) is oxidized

B             Cu(s) is reduced

C             H+(aq) is oxidized

D             H+(aq) is reduced

E              NO3-(aq) is oxidized

 

5. An oxidation-reduction reaction takes place in a chemical cell, and the flow of electrons is used to provide energy for a light bulb. Which of the following statements is true of the reaction?

A             The reaction is nonspontaneous and has a positive voltage

B             The reaction is nonspontaneous and has a negative voltage

C             The reaction is at equilibrium and has a voltage of zero

D             The reaction is spontaneous and has a positive voltage

E              The reaction is spontaneous and has a negative voltage

 

6. Zn(s) / ZnCl2(aq) // Cl-(aq) / Cl2(g)

In the electrochemical cell describe by the cell diagram above, what reaction occurs at the anode?

A             Zn --> Zn2+ + 2e

B             Zn2+ + 2e --> Zn

C             Cl2 + 2e --> 2Cl-

D             2Cl- --> Cl2 + 2e

E              Zn + Cl2 --> ZnCl2

 

7. The following redox reaction occurs in an acidic solution: Ce4+ + Bi --> Ce3+ + BiO+. What is the coefficient before the Ce4+ when the equation is fully balanced?

A             1

B             2

C             3

D             6

E              9

 

8. The oxidation state of nitrogen is most negative in which of the following compounds?

A             N2

B             N2O

C             NH3

D             NO2

E              NO3

 

9. When chromium metal is used to form K2Cr2O7, the oxidation state of chromium changes from

A             0 to 4

B             3 to 6

C             2 to 6

D             0 to 6

E              2 to 4

 

10. When the following reaction is balanced, what is the net ionic charge on the right side of the equation?

…H+ + …MnO4- + …Fe2+ --> …Mn2+ + …Fe3+ + …H2O

A             +5

B             +7

C             +10

D             +17

E              The net ionic charge on either side must be zero

 

11. Which of the following statements correctly characterizes a galvanic cell?

                I               Oxidation occurs at the anode, which is negative

                II             Oxidation occurs at the anode, which is positive

                III            Reduction occurs at the cathode, which is positive.

A             II only

B             III only

C             I and III only

D             I, II, and III

E              II and III

 

12. According to the following experimental data

Mg --> Mg2+ + 2e                 Eo = 2.37V

Mn --> Mn2+ + 2e                 Eo = 1.03V

H2 --> 2H+ + 2e                      Eo = 0.00V

Cu --> Cu2+ + 2e                    Eo = -0.16V

Which of the following reactions is spontaneous?

A             Mn2+ + H2 -->

B             Mg + Mn2+ -->

C             Mg2+ + Mn -->     

D             Cu + 2H+ -->

E              Mg2+ + Cu -->

 

13.  What is the oxidation state of Mn in KMnO4?

A             -7

B             -3

C             0

D             +3

E              +7

 

14. Which of the following is true of an electrolytic cell?

A             An electric current causes an otherwise nonspontaneous chemical reaction to occur

B             Reduction occurs at the anode

C             A spontaneous electrochemical reaction produces an electric current

D             The electrode to which electrons flow is where oxidation occurs.

E              None of the above

 

15.  Which of the following represents a correctly balanced half reaction?

A             Cl2 + 2e --> Cl-1

B             2e + Fe --> Fe+2

C             O2 --> 2e + 2O2-

D             Al3+ --> Al + 3e

E              2H+ + 2e --> H2

 

16. A student prepares for an experiment involving a voltaic cell. Which of the following is needed the least to perform the experiment?

A             Buret

B             Salt bridge

C             Strip of zinc metal

D             Copper wire

E              Solution of zinc sulfate

 

17. How many moles of electrons are transferred in the following reaction: Ce3+ + Pb --> Ce + Pb4+

A             14

B             12

C             7

D             24

E              3

 

18.  A voltaic cell is set up and a chemical reaction proceeds spontaneously. Which of the following will not occur in this reaction?

A             The electrons will migrate through the wire

B             The cations in the salt bridge will migrate to the anode half cell

C             The cathode will gain mass

D             The anode will lose mass

E              Reduction will occur at the anode

 

19. The oxidation state of the elements in the choices below will be 1- except for

A             F in HF

B             Cl in NaCl

C             O in H2O2

D             F in NaF

E              H in Na2HPO4

 

20. Which metal will not generate hydrogen gas when placed in HCl(aq)?

A             Au

B             Mg

C             Ca

D             Sr

E              Zn

 

21. Which substance is the best oxidizing agent?

A             Fe

B             O2

C             Na

D             Li

E              F2

 

22. Which of the following half reactions is correctly balanced?

A             MnO4- --> Mn2+ + 4H2O

B             Cu + 2Ag+ --> 2Ag + Cu2+

C             H2 + OH- --> 2H2O

D             Pb2+ +2e --> Pb

E              2F- +2e --> F2

 

23. Molten KBr is allowed to undergo the process of electrolysis. Which reaction occurs at the anode?

A             K+ + 1e --> K(s)

B             2Br- --> Br2 + 2e

C             K(s) --> K+ + 1e

D             Br2 --> 2Br- + 2e

E              Br2 + 2e --> 2Br-

 

24. Which reaction will not occur spontaneously?

A             Au3+ + 3e --> Au

B             Mg + 2H+ --> Mg2+ + H2

C             F2 + 2e --> 2F-

D             Li+ + 1e --> Li

E              2Na + Cl2 + 2e --> 2NaCl

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