Electrochemistry
Chapter 19
Electrochemical processes are oxidation-reduction
reactions in which:
•
the energy released by a spontaneous reaction is
converted to electricity or
•
electrical energy is used to cause a nonspontaneous
reaction to occur
0
0
2+ 2-
2Mg (s) + O2 (g)
2Mg
O2 + 4e-
2MgO (s)
2Mg2+ + 4e- Oxidation half-reaction (lose e-)
2O2-
Reduction half-reaction (gain e-)
19.1
Amps, Time, Coulombs, Faradays,
and Moles of Electrons
•
•
•
•
•
Three equations relate these quantities:
amperes x time = Coulombs
96,485 coulombs = 1 Faraday
1 Faraday = 1 mole of electrons
The thought process for interconverting between
amperes and moles of electrons is:
amps & time Coulombs  Faradays  moles
of electrons
Calculating the Quantity of
Substance Produced or Consumed
To determine the quantity of substance either
produced or consumed during electrolysis
given the time and a known current flow these
steps:
1. Write the balanced half-reactions involved.
2. Calculate the number of moles of electrons that
were transferred.
3. Calculate the number of moles of substance
that was produced/consumed at the electrode.
4. Convert the moles of substance to desired
units of measure.
Example:
•
A 40.0 amp current flowed through molten iron(III)
chloride for 10.0 hours (36,000 s). Determine the mass
of iron and the volume of chlorine gas (measured at
25oC and 1 atm) that is produced during this time.
• Write the half-reactions that take place at the anode and
at the cathode.
– anode (oxidation): 2 Cl-  Cl2(g) + 2 e– cathode (reduction) Fe3+ + 3 e-  Fe(s)
• Calculate the number of moles of electrons.
• Calculate the moles of iron and of chlorine
produced using the number of moles of
electrons calculated and the
stoichiometries from the balanced halfreactions. According to the equations,
three moles of electrons produce one mole
of iron and 2 moles of electrons produce 1
mole of chlorine gas.
• Calculate the mass of iron using the molar
mass and calculate the volume of chlorine
gas using the ideal gas law (PV = nRT).
Electrolytic cell
Galvanic cell (also called voltaic cell) uses chemical
reaction to produce electrical energy (flow of electrons).
When zinc metal is placed in CuSO4 solution, the following
reaction takes place:
Zn(s) + CuSO4(aq)
ZnSO4(aq) + Cu(s)
Oxidation: Zn(s)
Reduction: Cu+2 + 2e-1
Overall:
Zn(s) + Cu+2
Zn+2 + 2e-1
Cu
Zn+2 + Cu(s)
Electrolytic cell
• Electrons will not flow in the following apparatus:
• Why not?
The circuit is not complete. There must be
a continuous flow of charge for the
electrons to flow.
Galvanic Cell
• But if the reaction is carried out using
a salt bridge to complete the circuit
and maintain charge neutrality,
electrons are transferred from Zn° to
Cu+2 through a wire producing
electrical energy.
CHEMICAL CHANGE --->
ELECTRIC CURRENT
•To obtain a useful current,
we separate the oxidizing
and reducing agents so that
electron transfer occurs thru
an external wire.
This is accomplished in a GALVANIC or
VOLTAIC cell. http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/galvan5.swf
A group of such cells is called a battery.
Galvanic Cell
Anode
Cathode
Oxidation
Reduction occurs
Electrons
produced
Electrons are
consumed
Has negative sign
(-)
Has positive sign
(+)
Anions migrate
toward
Cations migrate
toward
occurs
Galvanic Cells
The difference in electrical
potential between the anode
and cathode is called:
• cell voltage
• electromotive force (emf)
• cell potential
Cell Diagram
Zn (s) + Cu2+ (aq)
Cu (s) + Zn2+ (aq)
[Cu2+] = 1 M & [Zn2+] = 1 M
Zn (s) | Zn2+ (1 M) || Cu2+ (1 M) | Cu (s)
anode
cathode
19.2
Standard Electrode Potentials
Zn (s) | Zn2+ (1 M) || H+ (1 M) | H2 (1 atm) | Pt (s)
Anode (oxidation):
Zn (s)
Cathode (reduction): 2e- + 2H+ (1 M)
Zn (s) + 2H+ (1 M)
Zn2+ (1 M) + 2eH2 (1 atm)
Zn2+ + H2 (1 atm)
19.3
Shorthand Notation for Cells
Shorthand Notation for:
+2
Zn° + Cu
Anode
electrode
+2
Zn
+ Cu°
Cathode
electrode
Shorthand Notation for Cells
• Write shorthand notation for:
Fe(s) + 2Fe+3(aq)
3Fe+2(aq)
Fe
Fe2+ + 2e- = oxidation (anode)
Fe3+ + 1eFe2+ = reduction (cathode)
Anode
Fe° Fe+2
Cathode
Fe+3 Fe+2
Shorthand Notation for Cells
Write shorthand notation for:
2Ag+1(aq) + Ni(s)
Ni° Ni+2
2Ag(s) + Ni+2(aq)
Ag+1 Ag°