valdosta state university experiment 11 oxidation-reduction chemistry valdosta state university

Valdosta State University

Experiment 11Oxidation-Reduction

Chemistry



Purpose

To observe and predict the EMF of a voltaic cell and a concentration cell. Also, to balance redox equations in acidic and alkaline solutions.



Background


Voltaic Cells• An electrochemical cell that uses spontaneous chemical reactions to

produce a voltage.

• The voltage is produced by potential difference between two substances.

• The transfer of electrons from one ion or molecule to another occurs:• Oxidation – when one substance loses electrons.• Reduction – when one substance gains electrons.• Remember – LEO the lion goes GER (Loss of Electrons is

Oxidation, Gain of Electrons is Reduction)

Zn(s) + Cu2+(aq) Zn2+(aq) + Cu(s)

• The interesting part is converting this equation to a workable cell.


Background


Voltaic Cells


Background


Voltaic CellsCurrent is carried by:• Electrons (through wire)• Ions (solution, salt bridge)

Both pathways are required to form a completed circuit.


Background


Voltaic CellsNotice that each beaker

(half-cell) contains the complete half-reaction.

• Anode (Oxidation)Zn(s) Zn2+(aq) + 2 e-

• Cathode (Reduction)Cu2+(aq) + 2 e- Cu(s)

• Remember the phrase Red Cat (Reduction occurs at the Cathode) to help you remember this.

V


Background


Voltaic Cells – Cell EMF

Cell electromotive force (Ecell) – the voltage a voltaic cell generates.

The voltage generated by a voltaic cell depends on a number of factors, a few of which are:

• the half-cells used• the concentrations of the reagents• the temperature of the cell.


Background


Voltaic Cells – Standard Cell Potential


If the cell is operated at standard state (298K, 1M solution concentrations and 1 atmosphere pressure) the voltage generated is referred to as a standard cell potential ( )

ocellE


Background


Voltaic Cells – Standard Cell Potential


If the cell is operated at standard state (298K, 1M solution concentrations and 1 atmosphere pressure) the voltage generated is referred to as a standard cell potential ( )

ocellE

Zn(s) + Cu2+(aq) Zu2+(aq) + Cu(s) VE ocell 10.1


Background


Voltaic Cells – Standard Reduction Potential

• Reduction potentials are summarized in a table.• These potentials can be used to determine the cell potential.

)()( anodeEcathodeEE ored

ored

ocell

• If you do the calculation for Eocell and get a negative

number, then you chose the wrong anode and wrong cathode and need to switch both of them.


Background


Voltaic Cells – Concentration Cell

• Solution concentration has an impact on cell emf.• The Nernst equation can be used to predict the emf.

QnF

RTEE o ln

• Simplifying the equation yields (at T = 298.15K):

Qn

VEE o log

0592.0

*Ions move towards diluted solutionProducts/ReactantsDiluted/Concentrated


Background


Balancing Oxidation-Reduction Reactions1. Break up the overall reaction into two half reactions omitting H+, OH-, and H2O. 2. For each half reaction, balance all of the elements except O and H.3. Balance for oxygen. For each half reaction, balance the O's by adding H2O to the

side of the reaction that is deficient in oxygen. 4. Balance for hydrogen. For each half reaction, add H+ to the side of the reaction that is

deficient in hydrogen. If you are balancing a reaction in acid solution, skip to step 5BASE ONLYa) For each half reaction, add OH to both sides of the equation. Add enough OH-

to neutralize the H+ ions added in step 4.b) For each half reaction, take the H+ and OH and combine them to form water.

Cancel out water molecules that appear on both sides of the reaction.5. For each half reaction, balance the charge by adding electrons. The electrons are

added to the more positive side, and the difference in charge gives the number of electrons to add.

6. Multiply one (or both) half reactions by a whole number so that both half reactions have the same number of electrons.

7. Add the half reactions. The electrons will cancel, as will some of the H+, OH, and H2O.


Background


Balancing Oxidation-Reduction Reactions (Acid)

1. Break up the overall reaction into two half reactions omitting H+, OH-, and H2O.

H2S(aq) + NO3-(aq) NO(g) + S(s)

NO3-(aq) NO(g)H2S(aq) S(s)

Oxidation half reaction Reduction half reaction


Background



2. For each half reaction, balance all of the elements except O and H.




Background



3. Balance for oxygen. For each half reaction, balance the O's by adding H2O to the side of the reaction that is deficient in oxygen.




Background



4. Balance for hydrogen. For each half reaction, add H+ to the side of the reaction that is deficient in hydrogen.




Background



5. For each half reaction, balance the charge by adding electrons. The electrons are added to the more positive side, and the difference in charge gives the number of electrons to add.




Background







Background




3H2S(aq) 3S(s) + 6H+(aq) + 6e-

6e- + 8H+(aq) + 2NO3-(aq) 2NO(g) + 4H2O(l)


Background




3H2S(aq) 3S(s) + 6H+(aq) + 6e-

6e- + 8H+(aq) + 2NO3-(aq) 2NO(g) + 4H2O(l)

2H+(aq) + 2NO3

-(aq) + 3H2S(aq) 2NO(g) + 3S(s) + 4H2O(l)


Background


Balancing Oxidation-Reduction Reactions (Alkaline)

1. Break up the overall reaction into two half reactions omitting H+, OH-, and H2O.

Cr3+(aq) + MnO2(s) Mn2+(aq) + CrO42-(aq)

Cr3+(aq) CrO42-(aq) MnO2(s) Mn2+(aq)

Oxidation half-reaction Reduction half-reaction


Background



2. For each half reaction, balance all of the elements except O and H.




Background



3. Balance for oxygen. For each half reaction, balance the O's by adding H2O to the side of the reaction that is deficient in oxygen.




Background



4. Balance for hydrogen. For each half reaction, add H+ to the side of the reaction that is deficient in hydrogen.




Background



4. Balance for hydrogen. For each half reaction, add H+ to the side of the reaction that is deficient in hydrogen. a) For each half reaction, add OH to both sides of the equation. Add enough OH-

to neutralize the H+ ions added in step 4.




Background



4. Balance for hydrogen. For each half reaction, add H+ to the side of the reaction that is deficient in hydrogen. b) For each half reaction, take the H+ and OH and combine them to form water.

Cancel out water molecules that appear on both sides of the reaction.




Background



5. For each half reaction, balance the charge by adding electrons. The electrons are added to the more positive side, and the difference in charge gives the number of electrons to add.


Cr3+(aq) + 8OH-(aq) CrO42-(aq) + 4H2O(l) MnO2(s) + 2H2O(l)

Mn2+(aq) + 4OH-(aq)


Background





Cr3+(aq) + 8OH-(aq) CrO42-(aq) + 4H2O(l) MnO2(s) + 2H2O(l)

Mn2+(aq) + 4OH-(aq)


Background




2Cr3+(aq) + 16OH-(aq) 2CrO42-(aq) + 8H2O(l) + 6e-

6e- + 3MnO2(s) + 6H2O(l) 3Mn2+(aq) + 12OH-(aq)


Background





6e- + 3MnO2(s) + 6H2O(l) 3Mn2+(aq) + 12OH-(aq)6e- + 2Cr3+(aq) + 3MnO2(s) + 16OH-(aq) + 6H2O(l) 2CrO4

2-(aq) + 3Mn2+(aq) + 12OH-(aq) + 8H2O(l) + 6e-


Background





6e- + 3MnO2(s) + 6H2O(l) 3Mn2+(aq) + 12OH-(aq)2Cr3+(aq) + 3MnO2(s) + 4OH-(aq) 2CrO4

2-(aq) + 3Mn2+(aq) + 2 H2O(l)


Procedure – Experiment 11

- For this experiment, work in pairs.


Procedure – Experiment 11Voltaic CellsComputer Set-Up1. Hook the rectangular end of the gray cord into the back of the laptop

computer and the round end into Science Workshop black box interface.2. Plug in and turn on the Pasco Science Workshop black box interface. You

should see a green LED on the front face of the computer.3. Plug the DIN plug of the voltage sensor into Channel A on the Scientific

Workshop black box interface. 4.Turn on the computer. When the computer has booted-up, double click on

the “Data Studio” icon. The program should start.5. On the opening screen, select “Create Experiment”.6. Click and drag the “Voltage” sensor to Channel A of the interface box as

shown in the experimental setup window.7. Click and drag the "digits" icon labeled “Voltage Ch A (V)”.8. To have the computer measure the voltage, press “Start”.


Procedure – Experiment 11Voltaic CellsCell Set-Up (Cu / Zn)1. Get Zn, Cu, and Mg electrodes. Polish each electrode with sandpaper.2. Pour approximately 50 mL of 0.1 M CuSO4 into a 150 mL beaker. Pour 0.1 M

Zn(NO3)2 into the porous cup until the cup is about 2/3 full. Put the porous cup into the CuSO4 solution.

3. Place a Zn electrode into the Zn(NO3)2 solution and a Cu electrode into the CuSO4 solution.

4. Hook one of the alligator clips to the Zn electrode, and the other alligator clip to the Cu electrode.

5. Record the voltage on your report sheet. If the voltage is negative, reverse the alligator clips.


Procedure – Experiment 11Voltaic Cells


Procedure – Experiment 11Voltaic CellsCell Set-Up (Cu / Mg)6. Remove the alligator clips and empty the contents of the porous cup into the

waste container. Rinse the inside of the cup with distilled water7. Place 0.1 M Mg(NO3)2 into the porous cup until the cup is about 2/3 full. Put

the Mg electrode into the Mg(NO3)2 solution. Place the porous cup into the CuSO4 solution.

8. Hook one of the alligator clips to the Cu electrode, and the other alligator clip to the Mg electrode.



Procedure – Experiment 11Voltaic CellsCell Set-Up (Zn / Mg)10. Remove the alligator clips. DO NOT EMPTY ANY CONTAINERS!!11. Get another 150 mL beaker and put 50 mL of 0.1 M Zn(NO3)2 in the beaker.

Put the porous cup containing the Mg(NO3)2 solution into the beaker containing the Zn(NO3)2.

12. Place the Zn electrode into the Zn(NO3)2 solution and make sure the Mg electrode is in the Mg(NO3)2 solution.

13. Hook one of the alligator clips to the Zn electrode, and the other alligator clip to the Mg electrode.



Procedure – Experiment 11Concentration CellsCell Set-Up (Concentration Cell)15. Remove the alligator clips and put both the Mg(NO3)2 solution and the

Zn(NO3)2 solution into the waste container. SAVE THE 0.1 M CuSO4 SOLUTION. YOU WILL USE IT NEXT.

16. Wash the porous cup with distilled water and fill it approximately 2/3 full with 0.001 M CuSO4 solution.

17. Hook each of the alligator clips to the Cu electrodes in each solution. 18. Record the voltage on your report sheet. If the voltage is negative, reverse

the electrodes.19. To the porous cup containing the 0.001 M CuSO4 solution, add 10 drops 6 M

NH3 and mix well. Record the voltage on your report sheet.


Safety

• The 6 M NH3 gives off irritating fumes.



Waste Disposal

• All solutions used in this experiment must be placed in the container marked "Recovered Metals and Metal Ions".

• Electrode materials should be returned to the “Used Electrodes” container.


valdosta state university experiment 11 oxidation-reduction chemistry valdosta state university

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