14.2a Voltaic Cells

Basic Function

Voltaic Cell Basics

• Electrodes and electrolytes chemically react to form ions that move in or out of solution.

• = electrode where electrons are transferred out of the solution (ox. occurs)

• = electrode where electrons are transferred into the solution (red. occurs)

• can be a liquid or a paste• Each half-cell contains one and one

Voltaic CellsVoltaic Cells

• device that changes chemical energy into electrical energydevice that changes chemical energy into electrical energy• involves a reactioninvolves a reaction• reducing agent transfers electrons to the oxidizing agentreducing agent transfers electrons to the oxidizing agent

oxidation: oxidation:

of electrons of electrons

( ( ox. state) ox. state) reduction: reduction:

of electronsof electrons

( ( ox state) ox state)

Voltaic CellsVoltaic Cells

• if the two half-rxtns are combined in the same if the two half-rxtns are combined in the same container, the electron exchange occurs directly as container, the electron exchange occurs directly as work and released as heat (not ideal)work and released as heat (not ideal)

• to harness the energy, keep each half-rxtn in a to harness the energy, keep each half-rxtn in a separate container so the electron transfer occurs separate container so the electron transfer occurs through a wire ( )through a wire ( )

• when the metal solid of the metal ion is not a when the metal solid of the metal ion is not a possibility then an is usedpossibility then an is used

• usually carbon or platinumusually carbon or platinum

Half-cellsHalf-cells

• A or porous disk is used to allow for unrelated A or porous disk is used to allow for unrelated ions to move between cells, allowing balance of chargeions to move between cells, allowing balance of charge

• is also usedis also used

Particle movementParticle movement• which direction will the electrons flow in?which direction will the electrons flow in?

from reducing agent to oxidizing agentfrom reducing agent to oxidizing agent

• Locations Locations of each half-rxtnof each half-rxtn anode: anode:

oxidation/SRAoxidation/SRA cathode: cathode:

reduction/SOAreduction/SOA

cations ---><--- anions

Cell NotationCell Notation

• short hand for describing cellsshort hand for describing cells• anode is on L and cathode is on Ranode is on L and cathode is on R• separate anode and cathode half-rxtn with separate anode and cathode half-rxtn with ||||• separate phases in one half-rxtn with separate phases in one half-rxtn with ||• electrodes go on far ends of notationelectrodes go on far ends of notation• Mg(s) | MgMg(s) | Mg2+2+(aq) || Al(aq) || Al3+3+(aq) | Al(s)(aq) | Al(s)• Pt(s) | ClOPt(s) | ClO33

--(aq), ClO(aq), ClO44--(aq) || MnO(aq) || MnO44

--(aq), Mn(aq), Mn2+2+(aq) | Pt(s)(aq) | Pt(s)

Zn Zn2+(1.0 M) Cu2+(1.0 M) Cu

Electrode

Salt bridge

Species (withconcentrations) in contact with electrodes

anode (-) anode (-) || electrolyte electrolyte |||| electrolyte electrolyte || cathode (+) cathode (+)|------------------------- e

- flow ----------------------->

cations ---><--- anions

AnodeAnode CathodeCathode

Basic Concepts Basic Concepts of Electrochemical Cellsof Electrochemical Cells

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

CHEMICAL CHANGE --->CHEMICAL CHANGE --->ELECTRIC CURRENTELECTRIC CURRENT

CHEMICAL CHANGE --->CHEMICAL CHANGE --->ELECTRIC CURRENTELECTRIC CURRENT

Zn metal

Cu2+ ions

Zn metal

Cu2+ ions

With time, Cu plates out onto Zn metal strip, and Zn strip “disappears.”

With time, Cu plates out onto Zn metal strip, and Zn strip “disappears.”

•Zn is oxidized Zn is oxidized and is the reducing agent and is the reducing agent Zn(s) ---> ZnZn(s) ---> Zn2+2+(aq) + 2e-(aq) + 2e-•CuCu2+2+ is reduced is reduced and is the oxidizing agentand is the oxidizing agentCuCu2+2+(aq) + 2e- ---> Cu(s)(aq) + 2e- ---> Cu(s)

•To obtain a useful To obtain a useful current, we separate current, we separate the oxidizing and the oxidizing and reducing agents so ereducing agents so e-- transfer occurs thru an transfer occurs thru an external wireexternal wire. .

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

CHEMICAL CHANGE --->CHEMICAL CHANGE --->ELECTRIC CURRENTELECTRIC CURRENT

CHEMICAL CHANGE --->CHEMICAL CHANGE --->ELECTRIC CURRENTELECTRIC CURRENT

This is accomplished in a This is accomplished in a GALVANICGALVANIC or or VOLTAICVOLTAIC cell. cell.

A group of such cells is called a A group of such cells is called a batterybattery..

http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/galvan5.swf

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

••Electrons travel thru external wireElectrons travel thru external wire•Salt bridge Salt bridge allows anions and cations allows anions and cations to move between half cellsto move between half cells

••Electrons travel thru external wireElectrons travel thru external wire•Salt bridge Salt bridge allows anions and cations allows anions and cations to move between half cellsto move between half cells

Zn --> ZnZn --> Zn2+2+ + 2e + 2e-- CuCu2+2+ + 2e + 2e-- --> Cu --> Cu

<--Anions<--AnionsCations-->Cations-->

OxidationOxidationAnodeAnodeNegativeNegative

OxidationOxidationAnodeAnodeNegativeNegative

RedReductionuctionCatCathodehodePositivePositive

RedReductionuctionCatCathodehodePositivePositive

Cell descriptionCell description

• Describe a voltaic Cell:Describe a voltaic Cell: balanced chemical eqtn. balanced chemical eqtn. give the direction of electron flowgive the direction of electron flow assign the anode and cathodeassign the anode and cathode give cell notationgive cell notation

ExampleExample

• Describe the Galvanic cell based on the following Describe the Galvanic cell based on the following half-reactions:half-reactions: AgAg++ + e + e-- Ag Ag FeFe3+3+ + e + e-- Fe Fe2+2+

• Write balanced equationWrite balanced equation Fe half-rxtn is reversed since FeFe half-rxtn is reversed since Fe2+2+ is SRA is SRA AgAg++ + Fe + Fe2+2+ Ag + Fe Ag + Fe3+3+

ExampleExample

• Give the direction of electron flowGive the direction of electron flow oxidation: Feoxidation: Fe2+2+ Fe Fe3+3+ + e + e--

reduction: Agreduction: Ag++ + e + e-- Ag Ag electrons flow from Feelectrons flow from Fe2+2+ half-cell to Ag half-cell to Ag++ half- half-

cellcell

• Assign cathode and anodeAssign cathode and anode anode: oxidation: Feanode: oxidation: Fe2+2+ Fe Fe3+3+ + e + e--

cathode: reduction: Agcathode: reduction: Ag++ + e + e-- Ag Ag

• Cell NotationCell Notation

Pt(s) | FePt(s) | Fe2+2+(aq), Fe(aq), Fe3+3+(aq) || Ag(aq) || Ag++(aq) | Ag(s)(aq) | Ag(s)

Homework

• Read p.622 - 626 in textbook

• Questions p.626 #1-8

• LSM 14.2 C & D