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• O2 H2O and CO2

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 metalsingle metal donatesdonates electrons to ions of metals below itself (oxidaizes)

• single non-metalsingle non-metal takestakes 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 + AlCl3

• Cs + CuCl2

• I2 + NaCl

• Cl2 + KBr

• Fe + CaBr2

• Mg + Sr(NO3)2

• F2 + MgCl2

yesyes

yesyes

nono

yesyes

nono

nono

yesyes

placed Cu(s) in beakers: one contains Zn(NO3)2(aq) other contains AgNO3(aq)

Which beaker contains which ions?

contains Zn ions? & contains Ag ions?

A

B

Overview of Electrochemistry

TWO kinds of Electrochemical Cells

1.1. galvanic or Voltaicgalvanic or Voltaic cells: (Regents: electrochemical cellselectrochemical cells)

–use spontaneous rxn to produce flow of electrons (electricity)

–exothermic

2.2. electrolytic electrolytic cells: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:

container with aq soln:aq soln:

•+/- ions

•electrodeelectrode– surface where e- transfer takes place

– wire connects 2 electrodeswire connects 2 electrodes

– salt bridge connects 2 solutionssalt 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 OxAn Ox ate a Red CatRed 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 anode composed of metal HIGHER on Table Jon Table J

Cathode

• Cathode = Reduction = Electron Acceptor

– cathode composed of metal LOWER cathode composed of metal LOWER on Table Jon 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 electrodenegative 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 electrodepositive 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+2 + 2e-

red: Cu+2 + 2e- 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?

ZnZn

ZnZn+2+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: [remember:

negative electrode: where electrons originate, positive electrode: attracts electrons]negative electrode: where electrons originate, positive electrode: attracts electrons]

Electron flow

Al: anode

Pb: Pb: cathodecathode

wire

Salt bridge

Al+3 & NO3-1

Pb+2 & NO3-1

Positive ion flow

(-)

Oxidation:

Al Al+3 + 3e-

Reduction:

Pb+2 + 2e- Pb

Overall Rxn

(Al Al+3 + 3e-)

+ (Pb+2 + 2e- Pb)_____________________________

2Al + 3Pb+2 2Al+3 + 3Pb

2Al + 3Pb+2 + 6e- 2Al+3 + 3Pb + 6e-

2

3

Application: Batteries

Dry Cell Battery

Mercury Battery

Corrosion

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

What’s wrong with this picture?