Redox Reactions & Electrochemical Cells

I. Balancing Redox Reactions

I. Balancing Redox ReactionsSTEP 1. Split Reaction into 2 Half-ReactionsSTEP 2. Balance Elements Other than H & OSTEP 3. Balance O by Inserting H2O into eqns. as necessary

STEP 4. Balance H with H+ or H2O (see 4a, 4b)STEP 5. Balance Charge by Inserting Electrons as needed

STEP 6. Multiply Each 1/2 Reaction by Factor needed to make no. of Electrons in each 1/2 Reaction Equal

STEP 7. Add Eqns. & Cancel Out Duplicate terms, where possible

I. Balancing Redox Reactions (continued)

STEP 4a. In ACID: Balance H by Inserting H+, as needed

STEP 4b. In BASE: Balance H by (i) inserting 1 H2O for each missing H & (ii) inserting same no. of OH- on OTHER SIDE OF REACTION as H2Os added in (i)

I. Balancing Redox Reactions (continued)

ExampleComplete and Balance Following Reaction:CuS (s) + NO3

- (aq) Cu2+(aq) + SO42- (aq)

+ NO (g) STEP1. Split into 2 Half-Reactions

a.1 CuS Cu2+ + SO42-

b.1 NO3 - NO

I. Balancing Redox Reactions (continued)

STEP 2. Balance Elements Other than H & O

Already O.K. !

I. Balancing Redox Reactions (continued)

STEP 3. Balance O by inserting H2O into equations as necessary

a.3 CuS + 4H2O Cu2+ + SO42-

b.3 NO3- NO + 2H2O

I. Balancing Redox Reactions (continued)

STEP 4. ACIDIC, so Balance H by inserting H+ as needed

a4. CuS + 4H2O Cu2+ + SO42- + 8H+

b4. NO3- + 4H+ NO + 2H2O

I. Balancing Redox Reactions (continued)

STEP 5. Balance Charge by inserting Electrons, where necessary

a5. CuS + 4H2O Cu2+ + SO42- + 8H+ + 8e-

b5. NO3- + 4H+ + 3e- NO + 2H2O

I. Balancing Redox Reactions (continued)

STEP 6. Multiply each Eqn. by factor to make No. of Electrons in Each 1/2 Reaction the Same

a6. Multiply by 3x 3CuS + 12H2O 3Cu2+ + 3SO4

2- + 24H+

+ 24e-

b6.Multiply by 8x 8NO3

- + 32H+ + 24e- 8NO + 16H+

+ 24e-

I. Balancing Redox Reactions (continued)

STEP 7. Add Eqns. and Cancel Out Duplicated Terms

(a7 + b7) 8H+

3CuS + 12H2O + 8NO3- + 32H+ + 24 e-

3Cu2+ + 3SO42- + 24H+ + 8NO +16 H2O

+24e- 4H2O

I. Balancing Redox Reactions (continued)

So, the final, balanced reaction is: 3CuS(s) + 8 NO3

-(aq) + 8H+ (aq) 3Cu2+(aq) + 3 SO4

2-(aq) + 8NO(g) + + 4H2 O(l)

Checking mass balance and charge balance in Equation

L.H.S3 x Cu3 x S8 x N24 x O8 x H

(8 x 1-) + (8 x H+) = 0

R.H.S.3 x Cu3 x S8 x N24 x O8 x H

(3 x 2+ )+(3 x 2- ) = 0

Redox Reactions in Electrochemistry

Two Types of Electrochemical Cells:1. Galvanic 2. Electrolytic

Galvanic Cell - Converts a Chemical Potential Energy into an Electrical Potential to Perform Work

Electrolytic Cell- Uses Electrical Energy to Force a Chemical Reaction to happen that would not otherwise occur

Anode and Cathode in Electrochemistry

ANODE - Where OXIDATION takes place(-e-)

CATHODE - Where REDUCTION takes place (+e-)

Electrochemistry and the Metals Industry

Many Electrochemical Processes are used Commercially for Production of Pure Metals:

e.g. Al Manufacture (by electrolysis of Al2O3)

Mg Manufacture (by electrolysis of MgCl2)

Na Manufacture (by electrolysis of NaCl)

Electrolylitic Production of Al using the HALL CELL

(major plant in ALCOA, TN

C lining(Cathode) (-)

Al2O3 dissolved in molten cryolite (Na3AlF6) at 950 0C (vs. 2050 0C for pure Al2O3)

Graphite Anodes (+)

Al2O3 in molten Na3AlF6

Molten Al

Al

Al

Steel case

Hall Cell for Al Manufacture

Hall Cell Process

Reaction:

2 Al2O3 (sln) + 3C (s) 4 Al (l) + 3CO2

(g)

Location of Hall cell plant in E. Tennessee through

availability of inexpensive Hydroelectric power. Process uses 50,000 –

100,000 A.