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.