Transcript Document
Redox Reactions &
Electrochemical Cells
I. Balancing Redox Reactions
I. Balancing Redox Reactions
STEP 1. Split Reaction into 2 Half-Reactions
STEP 2. Balance Elements Other than H & O
STEP 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
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)
Example
Complete and Balance Following Reaction:
CuS (s) + NO3 - (aq)
Cu2+(aq) + SO42- (aq)
+ NO (g)
STEP1. Split into 2 Half-Reactions
a.1
CuS
b.1
NO3 -
Cu2+ + SO42NO
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
b.3 NO3-
Cu2+ + SO42-
NO + 2H2O
I. Balancing Redox Reactions
(continued)
STEP 4. ACIDIC, so Balance H by inserting
H+ as needed
a4. CuS + 4H2O
b4. NO3- + 4H+
Cu2+ + SO42- + 8H+
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+ + 3SO42- + 24H+
+ 24eb6.Multiply by 8x
8NO3- + 32H+ + 24e8NO + 16H+
+ 24e-
I. Balancing Redox Reactions
(continued)
STEP 7. Add Eqns. and Cancel Out
Duplicated Terms
(a7 + b7)
8H+
3CuS + 12H2O + 8NO3- + 32H+ + 24 e3Cu2+ + 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 SO42-(aq) + 8NO(g) +
+ 4H2 O(l)
Checking mass balance and
charge balance in Equation
L.H.S
R.H.S.
3 x Cu
3 x S
8 x N
24 x O
8 x H
3 x Cu
3 x S
8 x N
24 x O
8 x H
(8 x 1-) + (8 x H+) = 0
(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
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
Al2O3 dissolved in molten cryolite (Na3AlF6)
at 950 0C (vs. 2050 0C for pure Al2O3)
Steel case
Graphite Anodes (+)
Al
C lining
(Cathode)
(-)
Al2O3 in molten Na3AlF6
Molten Al
Al
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.