Electrochemical cells: utilize relationship between chemical potential energy & electrical energy

Everyday Redox Reactions battery to start car prevent corrosion of metals cleaning with bleach (oxidizing agent) Na, Al, Cl prepared or purified by redox reactions breathing O 2  H 2 O and CO 2

Redox Reactions synthesis rxns decomposition rxns SR rxns DR rxns are NOT redox rxns! all are redox rxns

Predicting Redox Reactions Table J: used to predict if given redox reaction will occur single metaldonatessingle metal donates electrons to ions of metals below itself (oxidaizes) single non-metaltakessingle non-metal takes electrons from ion of non-metals below itself (reduces)

Predicting Redox Reactions A + BX  B + AX If metal A above metal B (Table J): rxn is spontaneous If metal A below metal B: rxn is NOT spontaneous

X + AY  Y + AX If non-metal X above non-metal Y (Table J): rxn is spontaneous If non-metal X below non-metal Y: rxn is NOT spontaneous

Spontaneous or not? Li + AlCl 3  Cs + CuCl 2  I 2 + NaCl  Cl 2 + KBr  Fe + CaBr 2  Mg + Sr(NO 3 ) 2  F 2 + MgCl 2 yes yes no yes no no yes

placed Cu(s) in beakers: one contains Zn(NO 3 ) 2 (aq) other contains AgNO 3 (aq) Which beaker contains which ions? contains Zn ions? & contains Ag ions? A B

Overview of Electrochemistry TWO kinds of Electrochemical Cells

1. galvanic or Voltaic 1. galvanic or Voltaic cells: electrochemical cells (Regents: electrochemical cells) –use spontaneous rxn to produce flow of electrons (electricity) –exothermic

2. electrolytic cells: –use flow of electrons (electricity) to force non-spontaneous rxn to occur –endothermic

Vocabulary galvanic cell Voltaic cell electrochemical cell Board of Regents considers all 3 as electrochemical cells - not exactly accurate

galvanic cells/Voltaic cells (NYS: electrochemical cells) use spontaneous SR redox rxn: produce flow of electrons electrons flow from oxidized substance to reduced substance

Cell Set-up components arranged so e - forced to flow through wire when e - travel through a wire, can make them do work: light a bulb, ring a buzzer oxidation & reduction reactions must be separated physically

Parts of a galvanic/Voltaic Cell 2 half-cells: - one for oxidation rxn - one for oxidation rxn - one for reduction rxn - one for reduction rxn each ½ cell consists of: aq soln: container with aq soln: +/- ions electrodeelectrode –surface where e - transfer takes place –wire connects 2 electrodes –salt bridge connects 2 solutions

How much work can you get out of this reaction? can measure voltage by allowing electrons to travel through voltmeter galvanic cell is a battery –not easy battery to transport or use in real-life applications

electrode: surfaces at which oxidation or reduction half-reaction occur anode: oxidation surface decreases in mass cathode: reduction surface increases in mass

An OxRed Cat An Ox ate a Red Cat Anode – OxidationAnode – Oxidation –anode: location for oxidation half-rxn Reduction – CathodeReduction – Cathode –cathode: location for reduction half-rxn

Anode / Cathode how know which electrode is which? Table J:Table J: use to predict which electrode is anode and which electrode is cathode

Anode Anode = Oxidation = Electron Donor –anode composed of metal HIGHER on Table J

Cathode Cathode = Reduction = Electron Acceptor – cathode composed of metal LOWER on Table J

Zn above Cu: Zn is anode; Cu is cathode cathode

Direction of e - Flow (through wire): Anode → Cathode

Direction of (+) Ion Flow (salt bridge): Anode → Cathode

negative electrode negative electrode (anode): e - originate here: Zn electrode in this picture Zn electrode decreases in mass Zn +2 ions increase in concen aq solns contain ions of same element as electrode 

positive electrode positive electrode (cathode): e - attracted here: Cu electrode in this picture Cu electrode increases in mass Cu +2 ions decrease in concen aq solns contain ions of same element as electrode 

Salt Bridge allows migration of ions between half-cells –necessary to maintain electrical neutrality reaction can not proceed without salt bridge

Half-Reactions ox: Zn  Zn e - red: Cu e -  Cu _________________________ Zn + Cu +2  Zn +2 + Cu Which electrode is dissolving? Which electrode is gaining mass? Which species is increasing its concen? Which species is getting more dilute?Zn Zn +2 Cu Cu +2

When the reaction reaches equilibrium voltage is 0! –electrons no longer flow

Construct Galvanic Cell with Al & Pb Use Table J to identify anode & cathode Draw Cell: put in electrodes & solutions Label: –anode –cathode –positive electrode –negative electrode, –direction of electron flow in wire –direction of positive ion flow in salt bridge[remember: negative electrode: where electrons originate, positive electrode: attracts electrons]

Electron flow  Al: anode Pb:cathode wire Salt bridge Al +3 & NO 3 -1 Pb +2 & NO 3 -1 Positive ion flow  (-)  Oxidation: Al  Al e - Reduction: Pb e -  Pb

Overall Rxn (Al  Al e - ) + (Pb e -  Pb) _____________________________ 2Al + 3Pb +2  2Al Pb 2Al + 3Pb e -  2Al Pb + 6e - 2 3

Application: Batteries

Dry Cell Battery

Mercury Battery

Corrosion to resist corrosion: coat one metal with stronger more durable metal

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