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

Redox Reactions battery to start car prevent corrosion 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! redox rxns

Predicting Redox Reactions Table J: used to predict if given redox reaction will occur metalsdonatemetals donate electrons to ion of metals below itself non-metaltakesnon-metal takes electrons from ion of non-metal below itself

Predicting SR Redox Reactions Element + Compound  New Element + New Compound If element above swapable ion, reaction is spontaneous If element below swapable ion, reaction is NOT spontaneous

Predicting Redox Reactions A + BX  B + AX If metal A above metal B (Table J): reaction is spontaneous X + AY  Y + AX If non-metal X above non-metal Y (Table J): reaction is 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

Started with: Zn(NO 3 ) 2 & Cu AgNO 3 & Cu Which beaker had Zn ions & which had Ag ions? A

Overview of Electrochemistry TWO kinds of cells: 1.galvanic or Voltaicelectrochemical 1.galvanic or Voltaic (NYS – electrochemical) –use spontaneous rxn to produce flow of electrons (electricity) –Exothermic 2.electrolytic –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 be the same type of cell

Electrochemical Cells use spontaneous SR redox rxn: –produces flow of electrons electrons flow from oxidized substance to reduced substance names: galvanic cells, Voltaic cells, or electrochemical cells (NYS)

Electrochemical Cells Redox rxn arranged so electrons forced to flow through wire when electrons travel through a wire, can make them do work - light a bulb, ring a buzzer oxidation & reduction reactions must be separated physically

Half-Cell place where each half-reaction takes place –½ cells: 2 needed for complete redox rxn wireconnected by wire so electrons flow from 1 st to 2 nd cell salt bridgeconnected by salt bridge to maintain electrical neutrality

Schematic of Galvanic/Voltaic Cell

Parts of a Galvanic/Voltaic Cell 2 half-cells: –one for oxidation rxn –one for reduction rxn –each consists of: aqueous solution container with aqueous solution & electrode electrode –surface where electron transfer takes place –wire connects electrodes –salt bridge connects 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 is location for oxidation half-rxn Reduction – CathodeReduction – Cathode –cathode is 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 Electron Flow (through wire): Anode → Cathode

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

Negative electrode Negative electrode (anode): where electrons originate: here it ’ s Zn electrode 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): where electrons attracted: here it’s Cu electrode 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

Mercury battery

Corrosion

Corrosion Prevention

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