Zumdahl - Zumdahl Chapter 11: Electrochemistry Techniques...

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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. Redox reactions Generation of an electric current from a chemical reaction Use of a current to produce chemical change 11.1 Galvanic Cells LEO GER 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 transferred. V ol t m e t e r : draws current through a known resistance, allowing us to measure the cell potential. 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 reaction. 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 conducting solid. 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 Effect 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 PH meter 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. Glass electrode 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 concentrations 11.5: Batteries 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 11.6: Corrosion 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. 11.7: Electrolysis 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 solution. 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 direction indicated. 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 metal 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 ...
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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.

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