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Electrochemical Cells: The Voltaic Cell

Mr. Shields Regents Chemistry U14 L03

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Half cell NomenclatureHalf cell NomenclatureWe’re now going to discuss some practical applications ofRedox reactions

- Recall that Redox reactions involve electron transfers

- Participating atoms either provide or accept these e-

Example: Zn0 Zn+2 + 2e- Oxidation half-cellCu+2 + 2e- Cu0 Reduction half-cell

These reactions can also be written in “Shorthand” nomenclatureAs Follows:

Zn0 |Zn+2

Cu+2 |Cu0oxidationReduction

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Electrochemical cellsElectrochemical cellsThe electrical nature of a Redox reaction allows one the abilityto construct several types of electrical cells

An apparatus that uses Redox reactions to produce electrical Energy OR uses electrical energy to cause a chemical reaction isKnown as an ELECTROCHEMICAL CELL

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Electrochemical cellsElectrochemical cellsThere are two types of electrochemical cells:

- VOLTAIC (also known as GALVANIC) CELLS- ELECTROLYTIC CELLS

Devices that convert CHEMICAL ENERGY into ELECTRICAL ENERGY through a Spontaneous REDOX reaction are called VOLTAIC CELLSThey represent EXOTHERMIC reactions.

Note Well:Note Well:NYS Regents Refers to only the NYS Regents Refers to only the VOLTAIC CELLSVOLTAIC CELLS as an as an ELECTROCHEMICALELECTROCHEMICAL cell cell

(even though this is not quite accurate, (even though this is not quite accurate, remember itremember it!)!)

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Electrochemical cellsElectrochemical cells

In an ELECTROLYTIC CELL electrical energy is provided toForce a NON-SPONTANEOUS Redox reaction to happen. TheseCells are ENDOTHERMIC reactions.

Electrolytic cells are typically used to

- plate metals on other metals- obtain pure metal from it’s compounds- to recharge batteries

We’ll talk about GALVANIC CELLS first and later we’ll discussELECTROLYTIC CELLS

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Remember … We used Table J to determine if a Rxn will occur spontaneously.

A Rxn will be spontaneous if the substance to be oxidized is above the substance to be reduced.

Example 1: Will the reaction Cu+2 + K Cu + K+ be spontaneous?

Cu+2 Cu (reduction) and K K+ (oxidation)

Since K is oxidized & above Cu on the table the Rxn is spontaneous.

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Example 2: Will the reaction Li+ + Al Li + Al+3 be spontaneous?

Li+ Li (reduction) Al Al+3 (oxidation)

Since Al is oxidized but below Li on the table, the Rxn is non-spontaneous.

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Electrochemical CellsElectrochemical CellsMost Voltaic cells share certain common features.They all have:

- Two physically separated half cells- A Cathode (+) in one cell (Reduction occurs here)- An Anode (-) in the other cell (Ox. Occurs here)- Two different metal electrodes- A solution in each cell containing a dissolved salt

made from the metal in the electrode- An electrical connection between Anode and Cathode

- A physical connection between two separated cells containing ions that can move freely into each cell

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For Redox reactions you’ve remembered the phrase “OIL RIG”

For Voltaic cells there is another phrase to help you rememberhow Voltaic cells are constructed.

Anode Cathode

Oxidation Reduction

Negative Positive

A- oxidation C+ ReductionFe Fe+2 + 2e- Cu+2 +2e- Cu

“AN Ox ate a Red Pussy Cat”

Memory Jogger for Galvanic cells

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The basics of a Voltaic electrochemical cell

Anode Cathode

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Electrochemical CellsElectrochemical CellsVoltaic (galvanic) Cells generate usable electricity

Let’s see how this happens. Consider the followingreaction:

Fe + CuCl2 FeCl2 +Cu

This is a spontaneous Redox reaction. Fe is being oxidized &since it’s higher in Table J than Cu, Cu will be reduced.

What are the 2 half cell reactions?

Fe Fe+2 +2e- oxidation half cellCu+2 +2e- Cu0 reduction half cell

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Electrochemical CellsElectrochemical CellsIn this redox Reaction the Transfer of e-Between FeAnd Cu is Direct.

In Voltaiccells electron Transfer is indirect.

One way to do this is to separate the Fe and Cu half cells andconnect the metals by a wire.

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Electrochemical cellsElectrochemical cells

+ -

Two separated Half cellsIf we do this, The reaction quicklyComes to a Screeching halt. Why?

The build up of chargeIn both cells stops theReaction from Continuing. But Why doesCharge build up?

Fe+2 goes into sol’n as Feis oxidized releasing 2e-& leaving behind pos. ions

The Cu+2 consumes the2 electrons leaving behind an excess of Cl-. Rxn can no longer proceed.

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Salt BridgeSalt BridgeWe need a way to stop the build up of charge in each half cell.

This is done using what is called a SALT BRIDGE

- A salt bridge allows ions to flow from cell to cell butstill maintains a separationbetween them

-It contains a salt insolution. The Cation (+) isbe chosen such that itwon’t react with the metalElectrodes (Na salts aretypical)

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A membrane on each side of the salt bridge keeps the NaNO3 Inside but allows ions to diffuse into the half cells to keep themElectrically neutral (i.e no build up of ions).

In this case, for each Cu reduced 2 Na+ ions must migrate intoThe reduction half cell and at the same time 2 NO3

- ionsMust migrate into the Oxidation cell (why?)

Pos. IonBuild Up

Neg. IonBuild Up

Salt Bridge Ion diffusion

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Notice that as the Cu+2 is reduced the concentration ofCopper salt solution in that half cell decreases.

On the other hand, as Fe is oxidized the concentration ofIron salt solution in that half cell increases.

WHY?

Changing Concentration of Half cells

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In this voltaic cell e- transfer is from Fe to Cu. Therefore the FeElectrode is negative (excess of e-) and Cu is + (needs electrons).

Therefore electrons flow through the wire from Fe to Cu.

- +

But how do we knowwhich metal is the One to loose e- ?

Table J provides theInformation needed.

Metals higher on the List are more easilyOxidized.

Direction of electron Flow

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Since iron electrodeis negativeit is called the

And since Cu elec. ispositive itis the called the

ANODE (-)

CATHODE (+)

- +

Looses Mass Gains Mass

The electrode at which oxidation occurs loses mass Fe Fe+2 +2e-

The electrode at which reduction occurs gains mass Cu+2 +2e- Cu

Changing Electrode Mass

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A standard Zn/Cu Electrochemical Cell produces about 1.1VA standard Li/Ag Voltaic Cell produces about 3.8V

+

-

Voltage dependsUpon the metalElectrodes used.

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EquilibriumEquilibriumAs an electrochemical cell is used the voltage continuouslyDecreases until the cell reaches equilibrium.

When the oxidized electrode is completely used up the cellvoltage is zero (0) and the cell has reached equilibrium sincethe concentrations are no longer changing.

The cell is now said to be “dead”.

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The following overall reaction occurs in a galvanic cell.

4AgNO3(aq) + Sn(s) Sn(NO3)4(aq) + 4Ag(s)

What’s oxidized?What’s Reduced?What are the balanced REDOX half cell reactions?

4Ag+ +4e- 4Ag Reduction Half cell Sn Sn+4 + 4e- Oxidation Half Cell

What metal electrode gains mass?What metal electrode loses mass?What electrode is negative, which is positive?Which metal is the cathode, Which is the anode?Which Sol’n Conc. Increases?

PROBLEM:

SnAg+

Ag

Sn Ag, SnSn, Ag

Sn+4

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Problem: Draw and Fully label an Al & Zn Electrochemical

cell. (Make sure you Indicate the direction of e- and ion flow; use nitrate solutions ).

Lastly, write a balanced chemical equation that describes this cell NaNO3

2Al + 3Zn(NO3)2 2Al(NO3)3 + 3Zn

Salt Bridge