This preview shows page 1. Sign up to view the full content.
Unformatted text preview: Zumdahl Chapter 11: Electrochemistry
Techniques analyze for trace pollutants that may signal development of a specific disease.
E l e c t r oc h e m is t r y : the study of the interchange of chemical and electrical energy.
Generation of an electric current from a chemical reaction
Use of a current to produce chemical change
11.1 Galvanic Cells
Electrons are transferred directly as the reactants collide
Releases heat, but no useful work is done
Sustained electron flow cannot occur under conditions where a wire is stuck in 2 solutions
Current spots flowing because of charge buildups in the 2 compartments
S a l t b r i d ge: a U-tube filled w/ an electrolyte
Po r o u s d i s k
Allows ion flow w/o extensive mixing of solutions
Electrons flow through the wire from reducing agent to oxidizing agent, and ions flow
between the compartments to keep the net charge zero in each
G a l v a n i c c e l l : a device in which chemical energy is changed to electrical energy.
The opposite process is called electrolysis
A nod e : the electrode at which oxida t ion occurs
C a t h od e : the electrode at which r educ t i on occurs
C e l l Po t e n t i a l
The pull, or driving force on the electrons. Also called the e l e c t r o mot i v e f o r c e (emf).
V ol t : the unit of electrical potential defined as 1 joule of work per coulomb of charge
V ol t m e t e r : draws current through a known resistance, allowing us to measure the cell
Typically measure a lower potential as frictional heating wastes some energy.
To determine maximum potential, perform under conditions of zero current
Insert variable-voltage device in opposi t i on to the cell potential
P e n t e t io m e t e r adjusted until no current flows in the cell circuit
Cell potential is equal in magnitude and opposite in sign to the voltage setting
of the potentiometer. This is maximum cell potential.
11.2: Standard Reduction Potentials
Reaction in galvanic cell can always be broken down into ½ reactions
S t a n d a r d h y d r oge n e l e c t r od e : platinum electrode in contact w/ 1M H+ and bathed by
hydrogen gas at 1 atm.
The standard hydrogen potential is the reference potential against which all ½ rxn.
Potentials are assigned
Potentials of ½ reactions given as r educ t i on processes
S t a n d a r d R e d u c t i o n Po t e n t i a l s
Combining 2 ½ reactions to get a balanced redox rxn. Requires 2 things 1. One of the reduction ½ reactions must be reversed. Half reaction with the largest
positive potential will run as written and the other will be reversed (will be the
oxidation reaction). The net potential of the cell will be the di f f e r enc e between the
two. E cell = E cathode E anode
2. Number of electrons lost must equal the number gained, the half=reactions must be
multiplied by integers as necessary to achieve electron balance. The va lu e of E is not
changed when a half-reaction is multiplied by an integer. A standard reduction
potential is an int ensive prope r ty
occurs) the potential is not multiplied by the integer required to balance the cell
A ga lvani c c e l l runs spont aneously in the di r e c t ion tha t gives a posi t ive va lue for E c e l l
S u m m a r y : I t e ms N e e d e d f o r a D es c r i p t i o n o f a G a l v a n i c C e l l
The cell potential and the balanced cell reaction
The direction of electron flow, obtained b inspection the half-reactions and using the
directions that give a positive E cell.
Designation of the anode and the cathode.
The nature of each electrode and the ions present in each compartment. A chemically
inert conductor is required if none of the substance participating in the half-reaction is a
11.3: Cell Potential, Electrical Work, and Free Energy
emf = potential differene (V) = work (J)/charge (C)
wmax = -qEmax
The work is never the maximum possible if any current is flowing
In any r e a l , spont a neous proc ess some ene rgy is a lw ays wast ed the a c tua l wor k
r e a l ized is a lw ays l ess than the c a l cul a t ed maximum
Only maximized in hypothetical reversible process
W = - qE
E= actual potential difference (V or J/C) q = heat transferred in C
F a r a d a y ( F ) : 96,485 coulombs of charge per mole of electrons
G = nFE T he maximum c e l l pot ent i a l is di r e c t l y r e l a t ed to the f r e e ene rgy di f f e r enc e
be twe en the r e a c t ants and the produc ts i n the e l l
11.4: Dependence of the Cell Potential on Concentration
T h e N e r nst E q u a t ion
E = E (RT/nF)(lnQ)
Relationship between cell potential and the concentrations of the cell components
E = E (.0591/n)(logQ) valid at 25 C
The c e l l wi l l spont aneously discha rge unt i l i t r e a ches equi l ibr i um whe r e E c e l l = 0
A t equi l ibr i um the components i n the two c e l l compa r tments have the same f r e e ene rgy
Cell no longer has the ability to do work (dead battery)
I on -Se l e c t i v e E l e c t r od es
Standard electrode of known potential
Glass electrode that changes potential depending on concentration of H+
Potentiometer I on -se l e c t i v e e l e c t r od es : electrodes that are sensitive to the concentration of a particular ion.
C a l c u l a t ion of E q u i l i b r i u m C onst a n ts fo r R e do x R e a c t ions
For a cell at equilibrium Ecell = 0 and Q = K
Large equilibrium constant common for a redox reaction
C on c e n t r a t ion C e l ls
A cell in which both compartments have the same components but at difference
B a t t e r y : a group of galvanic cells connected in series where the potentials of the individual
cells add to give the total battery potential.
L e a d S t o r a ge B a t t e r y
Can function for several years under temperatures from -30F to 100F
Anode is lead; cathode is lead dioxide; dipped in an electrolyte solution of sulfuric acid
Automobile lead storage battery
6 cells connected in series, each gives off 2V
Recharged by forcing current thru the battery in the opposite direction to reverse the rxn.
Car battery continually charged by an alternator
Lifetime of 3-5 years in a car
Physical damage from road shock
D r y C e l l B a t t e r i es
Batteries that are in calculators, watches, radios, etc.
A c id ve rsion contains zinc inner (anode) and NH4Cl cathode
A l k a l ine ve rsion contains zinc with KOH or NaOH
Lasts longer because the zinc corrodes in acidic conditions
Si lve r c e l l has a Zn anode with Ag2O cathode
M e r cury c e l ls also have a Zn anode and cathode is HgO
N i c k e l-c admium ba t t e r y can be recharged indefinitely
Products adhere to the electrodes (like a lead storage battery)
F uel C ell
F u e l c e l l : a galvanic cell in which the reactants are continuously supplied.
Energy used to produce an electric current; flows from reducing agent to oxidizing agent
Runs off of H2 and O2
C o r r osion : the process of returning metals to their natural state the ores from which they
were originally obtained.
Oxidation of the metal; metals oxidize easily
Oxidation of most metals by oxygen is spontaneous
Most metals develop a thin oxide coating that protects them from further oxidation
Gold has a more positive standard reduction potential that is larger than that of oxygen and
therefore shows no appreciable corrosion in air
C o r r osion of I r on
Electrochemical reaction Nonuniformities produce areas where the iron is more easily oxidized (anodic regions) than it
is at other (cathodic regions)
Fe2+ ions react with oxygen to form rust
Salt accelerated rusting
Increases conductivity of the aqueous solution and therefore accelerates the process
P r e v e n t ion of C o r r osio n
Important way to conserve natural resources
Application of coating (paint/metal plating) to protect metal from oxygen/moisture
G a l v a n i z i ng :
Oxidation that occurs dissolves the zinc and not the iron
Alloying also used to prevent corrosion
C a t h od i c P r o t e c t ion : used to protect steel in buried fuel tanks/pipelines.
Magnesium is a better reducing agent than iron so electrons are furnished by the
magnesium which must be replaced periodically.
E l e c t r ol y t i c c e l l : uses electrical energy to produce chemical change.
E l e c t r ol ysi s : process of forcing a current through a cell to produce a chemical change for
which the cell potential is negative; electrical work causes an otherwise nonspontaneous
chemical reaction to occur.
Charging a battery, producing aluminum metal, and chrome plating
A m p e r e : 1 coulomb of charge per second.
P l a t ing : depositing the neutral metal on the electrode surface by reducing the metal ions in
E l e c t r ol ysis of W a t e r
Hydrogen and oxygen combine spontaneously to form water
Decrease in free energy can be used to run a fuel cell to produce electricity
Pure water contains so few ions that only a negligible current can flow
E l e c t r o l y s i s o f M i x t u r es o f I o n s
The more positive the E value, the more the reactions has a tendency to proceed in the
The greatest positive E value is the best oxidizing agent
11.8: Commercial Electrolytic Processes
P r od u c t ion of A l u m i n u m
Aluminum is one of the most abundant elements on earth
Very active metal
H a l l- H e roul t proc ess : uses molten cryolite as the solvent for the aluminum oxide
Electrolysis only possible if ions can move to the electrodes
Can be done in water because water is more easily reduced than Al3+
Ion mobility produced by melting a salt
Bauxite is not pure aluminum oxide; also contains oxides of iron, silicon and titanium, etc.
Aluminum produced in electrolytic process is 99.5% pure E l e c t r o r e f i n i n g of M e t a l s
Purification of metals
Noble metal impurities in the anode are not oxidized at the voltage used; they fall to the
bottom of the cell to form a sludge, which is processed to remove the valuable silver,
gold and platinum.
Cu2+ ions are deposited at the cathode producing copper that is 99.95% pure
M e t a l P l a t i ng
Application of a thin coating of a metal that resists corrosion
An object can be plated by making it the cathode in a tank containing ions of the plating
E l e c t r o l y s i s o f So d i u m C h l o r i d e
Sodium metal is produced by the electrolysis of molten sodium chloride.
D ow n s c e l l : designed so that sodium and chlorine produced cannot come in contact with
each other to re-form NaCl
M e r c u r y c e l l : eliminates the contamination of the NaOH by NaCl.
Large overvoltage required to produce hydrogen at a mercury electrode means that Na+
ions are reduced rather than water
C h l o r -a l k a l i p r oc ess : pure solid NaOH is recovered from the aqueous solution and the
regenerated mercury is then pumped back to the electrolysis cell.
Caused mercury contamination to the environment; now treated to remove mercury
Diaphragm cell used because of environmental problems w/ mercury ...
View Full Document
This note was uploaded on 01/25/2012 for the course CHEM CHEM 152 taught by Professor Daruwala during the Spring '09 term at University of Washington.
- Spring '09