Transcript Oxidation and Reduction Reactions

Electrolytic Cells &
Electrolysis Reactions
Outside electrical source provides electrons
that force a non-spontaneous redox reaction.
Electricity “splits” a compound into it’s neutral
elements
Ex: H2O(l) + electricity → H2(g) + O2(g)
Electrolysis Set Up
Ex: NaCl(l)
•
Single Cell filled with
electrolyte with +/- ions
•
Attach battery to two electrodes.
•
•
•
Adds e- onto one electrode - Making it NEGATIVE
Pulls e- off one electrode - Making it POSITIVE
Electrodes are made of an inert substance
(like platinum or graphite) that conducts.
Which Way do the Ions Move?
To electrode of
opposite charge
What is Oxidized/Reduced?
At neg. electrode
electrons are gained by ion
(reduction at CATHODE)
At positive electrode
electrons are lost by ion
(oxidation at ANODE)
Remember
AN OX
RED CAT
Anode is where oxidation happens
Cathode is where reduction happens
Half Reactions & Net Equation
Rxn at Anode: (Ox)
ClCl + 1eOr more correctly
2ClCl2 + 2e-
DIATOMIC!!!!!!
Rxn at Cathode: (Red)
Na+ + 1eNa
(Multiply by 2 to balance electrons)
NET:
2Na+ + 2Cl-
2Na + Cl2
Electrolysis of Molten NaCl (l)
Determining Voltage Needed
(Honors)
Use the Voltage Table to determine the total
voltage “needed” to run the Electrolytic cell.
Total voltage should be a NEGATIVE number
Electrolysis of PbBr2(l)
What is oxidized?
What is reduced?
What are the ox/red
half reactions?
Negative
Electrode
Positive
Electrode
What is the net
equation?
Electrolysis of PbCl2(l)
Oxidized: ClHalf Reactions
Ox:
Cl-1
2Cl-1
Reduced: Pb+2
Cl + 1eCl2 + 2e-
Red:
Pb+2 + 2e-
Pb
Net:
Pb+2 + 2Cl-1
Pb + Cl2
Electrolysis of NaCl (aq)
Electrolysis of Water
At Positive Electrode:
Ox: O-2
O + 2ebut there is a diatomic!
2O-2
O2 + 4eAt Negative Electrode
Red: H+1 + 1eH
but there is a diatomic!
2H+1 + 2eH2
Net: 2H2O
2H2 + O2
Electrolysis of Water (Animation)
http://youtu.be/2t13S-KpGeE
Electrolysis of Water (Simple)
http://youtu.be/HQ9Fhd7P_HA
Electroplating
Electrolysis reaction
used to coat a
substance with a thin
layer of metal.
Often coating is a less
reactive metal that is
not easily oxidized or
corroded.
Electroplating

Negative Electrode
Is the OBJECT TO BE
PLATED
 so the positive metal ions
would go towards it and be
REDUCED.
 It is the CATHODE

Red: Ag+ + 1e-
Ag0
Electroplating

Positive Electrode
Made of plating metal
 It dissolves into solution as
metal strip gets OXIDIZED.
 It is the ANODE
 This replenishes the ions for
plating.

Ox: Ag0
Ag+ + 1e-
Electroplating Problems
(Honors)
Coulomb = measure of electrical charge
1 mole e- = 96,500 coulombs
# coulombs = # amps x seconds
Electroplating Problems
(Honors)
Reduction:
Happens on object to be plated
Look at Reduction half reaction
Look at mole relationships
between electrons and metal atoms.
Ex: Ag+ + 1e-
Ag0
Electroplating Problems
(Honors)

You can now answer questions regarding
the amount of a substance in moles or
grams that can be electroplated over a
certain amount of time.
Electroplating Problems
(Honors)
If 10 amps are run through a CuSO4 solution for
5 minutes, calculate the grams of Cu that will plate
onto the spoon.
We Know:
1 mole e- = 96,500 coulombs
# coulombs = # amps x seconds
Red:
Cu+2 + 2eCu0
2 moles electrons make 1 mole of Cu0
1 mole Cu = 63.5 grams
So….Let’s start here
# coulombs
= 10 amps x 300 seconds
= 3000 coulombs
3000 coul. x 1 mole e- x 1 mole Cu x 63.5g Cu = .987 grams
96,500 coul 2 mole e1 mole Cu
Mole ratio from Reduction
half reaction
Cu+2 + 2eCu0
You Try One
How long will it take to deposit 20 grams of
silver from a solution of AgCl onto a copper
tray if a current of 5 amps is used?
Answer = 3, 574 sec
or 59.5 minutes or about 1 hour
You Try One
How many amps are needed to deposit .504g.
of Iron in 40 minutes by passing a current
through a solution of Iron II Sulfate?
Answer: .72 amps