electro chemistry - wikispaces · electro chemistry prepared by odyssa ... chemist who established...
TRANSCRIPT
Electro chemistry
Prepared by
Odyssa Natividad RM. Molo
Effect of Concentration on Cell emf
• As a voltaic cell is discharged, the Reactant/s
of the rxn are consumed & the Product/s are
generated so the conc of these substances
change. The emf progressively drops until
E=0, at which point, we say the cell is “dead”.
At that point the conc of the R & P cease to
change; they are at eqlbm.
• The emf generated under nonstandard
conditions can be calculated using an eqn
first derived by Walther Nernst, a German
chemist who established many of the
theoretical foundations of electrochemistry.
Nernst Equation
Practice Exercise
• Calculate the emf at 298K generated
by the cell below:
1) Zn(s) + Cu2+(aq) Zn2+(aq) + Cu(s)
where [Cu2+] = 5.0M & [Zn2+] = 0.050M
2) Al(s) + I2(s) Al3+(aq) + I-(aq) where [Al3+] = 4.0 x 10-3M & [I-]=0.010M
3) Zn(s) + Cd2+(aq) Zn2+(aq) + Cd(s)
where [Cd2+] = 0.0750M & [Zn2+] =
0.950M
Cell emf & Chemical Equilibrium
• From Nernst eqn: As R are converted to
P, the value of Q inc, so the value of E
dec. The cell emf eventually reaches E
= 0, where G = 0; system is at eqlbm;
Q = K.
• Rearranging Nernst eqn (@ T = 298K):
Concentration Cells
• Cell based solely on the emf generated because of a difference in a concentration.
• more dilute soln = anode; more conc = cathode.
• Applications: pH meter; regulation of heartbeat in mammals.
Electrolysis
• In contrast to spontaneous
redox reactions, which result in
the conversion of chemical
energy into electrical energy,
electrolysis is the process in
which electrical energy is used
to cause a nonspontaneous
chemical reaction to occur.
Electrolytic Cells
• Is an apparatus for carrying out
electrolysis
• Same principle with galvanic
cells
• Example: Electrolysis of molten NaCl
Electrolysis of water
Electrolysis of an aqueous NaCl soln
Electrolysis of molten NaCl
Electrolysis of molten NaCl
• Molten NaCl is electrolyzed
to form Na metal & Cl gas.
• Cation: Na+ Anion: Cl-
• Electrode: anode (C); cathode (Fe)
• Battery serves as electron pump
• Redox rxn:
• Major source of pure Na metal & Cl gas.
• Theoretical E° value is -4V which means a
minimum of 4V must be supplied by battery to
carry out the rxn.
Electrolysis of Water
Electrolysis of Water
• Water is electrolyzed
to form H & O gas.
• Electrodes: Pt immersed in H2O
• When connected to battery, nothing happens
since not enough ions (1 x10-7 M @ 25°C) in
pure water to carry much of an electric current,
hence 0.1M H2SO4 soln is used.
• Redox rxn:
• No net H2SO4 is consumed. Why?
• What is the minimum voltage needed?
Electrolysis of aqueous NaCl soln
Electrolysis of aqueous NaCl solution
Comparison
• Electrolysis of molten NaCl
• Electrolysis of water
• Electrolysis of an aqueous NaCl
soln
Electrolysis of Mixture of Ions
More Example & Practice
Analysis of Electrolysis
• Cations are likely to be reduced at the
cathode and anions are likely to be oxidized
at the anode.
• In aqueous solutions, water itself may be
oxidized and/or reduced.
• The outcome depends on the nature of other
species present.
• Electrolysis has many applications
in industry, mainly in the extraction
and purification of metals.
Electrochemical vs Electrolytic Cell Electrochemical electrolytic
Electrodes Cathode Site of
reduction
(+ charge)
Site of
reduction
(- charge)
Anode Site of
oxidation
(- charge)
Site of
oxidation
(+ charge)
Ions Cations Migrate to
cathode
Migrate to
cathode
Anions Migrate to
anode
Migrate to
anode
Electron flow anode to
cathode
anode to
cathode
Voltage + voltage
Produces a
voltage
- voltage
Requires a
voltage
source
Spontaneity Spontaneous Non
spontaneous
Quantitative Aspects of Electrolysis
END PART 2