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Electrolytic Cells

Mr. Shields Regents Chemistry U14 L04

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Earlier we said there were two types of Cells:

- Electrochemical (i.e. voltaic/galvanic cell)- Electrolytic cell

Electrochemical cells produce electricity through aSpontaneous Redox reaction.

In an ELECTROLYTIC CELL, electrons (i.e. electricity) areprovided to drive a non-spontaneous Redox reaction

Electrolytic CellsElectrolytic Cells

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Electrolytic cells are typically used to:

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

Electrolytic Cell Usage

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They need Electricity to force a redox rxn to occur

There is an external power source req’dThey don’t produce electricity

The polarities are reversed (we’ll see why later)

The Anode is positive The Cathode is negative

Electrolytic cell reactions are also different because they usually take place in one solution/one cell

Electrolytic Cells are different from Voltaic Cells because …

Differences between Electrochemical Cells

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Let’s look at the electrical decomposition of water. This isan example Of an electrolytic cell. It’s called ELECTROLYSIS

2H20(l) 2H2(g) + O2(g)

In this exampleA battery suppliesElectrons to driveThe non-spontaneousReaction forward

What If we drew this electrolytic cell,What would it look Like?

Electrolysis

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Notice that oxidation still takes place at the anode andReduction still takes place at the Cathode.

BUT … now theAnode is positiveAnd the cathodeIs negative.

Electrons from anExternal source Flow into theCathode making itNegative.

Electrons flow fromThe anode intoThe external power sourceMaking this electrode positive

O2 H2

Battery feedse- into cell

+ -

2O-2 O2 +4e-

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Remember our phrase for recalling characteristics ofAn electrochemical (Voltaic) cell?

“ An Ox ate a Red Pussy Cat”

Well there’s another one for an electrolytic cell toalso help you remember.

“A POX on Electrolytic cells”

AnodePositive

Oxidation The Cathode is then just the opposite

Cathode / Negative / Reduction

Memory Jogger

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Voltaic CellVoltaic Cell Electrolytic Electrolytic CellCell

AnodeAnode NegativeNegative PositivePositive

CathodeCathode PositivePositive NegativeNegative

AnodeAnode Oxidation Oxidation takes placetakes place

Oxidation Oxidation takes placetakes place

CathodeCathode Reduction Reduction takes placetakes place

Reduction Reduction takes placetakes place

Electrode ComparisonElectrode Comparison

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Other Electrolytic CellsOther Electrolytic CellsRemember we said earlier that Electrolytic cells are typically used to …

- plate metals on other metals- obtain pure elements from compounds- recharge batteries

We’re going to look at two of these processes:

- Production of pure elements - Mainly Group 1 + 2 elements - Downs Cell

- Metal Plating

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ElectroplatingElectroplatingThis is another example of an electrolytic cell. It is usedTo plate one metal over another conducting object.

For example:

To plate Ag overStainless steel

To plate Au over Cu Or Ni

To plate Cr over Fe

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ElectroplatingElectroplatingIn this diagram what is:

The Cathode

The Anode

At what electrodeDoes Ox. Occur

At which electrodeDoes Red. Occur

What are the halfCell reactions?

Chrome electrode

What’s plated+

-

Chrome Electrode

What’s plated

Cr Cr+2 +2e-

Cr+2 +2e- Cr

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+

SimplifiedCell

+

Downs Cell (fused Salt)Downs Cell (fused Salt)

All reactiveGroup 1 +2 metals,F2 & Al Areproduced This way.

If Na is being producedAt the cathode thenNaCl is melted (Fused)(801 deg C)

To produce a differentMetal I need to use afused salt of that metal

+ -

-

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A manufacturer wants to make Gold forks out ofhis more cheaply made stainless steel (Fe) forks. DrawAn electrolytic cell that he could use to accomplish this Task. Assume Au forms Au+3. Label everything fullyand specify the oxidation and reduction half cellreactions that will take place.

PROBLEM