Chapter 21. Two types: ◦ Voltaic cell: electrons flow spontaneously ◦ Electrolytic cell:...

Chapter 21

Two types:◦ Voltaic cell: electrons flow spontaneously ◦ Electrolytic cell: electrons are forced to flow

Electrical energy is produced in a voltaic cell by a spontaneous redox reaction.

Spontaneous reactions occur between metals and ions below them on Ref. J

Voltaic cells convert chemical energy into electrical energy.

A half-cell is one half of a voltaic cell in which either oxidation or reduction occurs.

The half-cells are connected by a salt bridge, which allows for the flow of ions, and an external circuit, the wire, which allows for the flow of electrons (electricity!).◦ The oxidation half-cell contains the more active

metal electrode (higher up on ref. table J).◦ The reduction half-cell contains the less active

metal electrode (metal lower down on ref. table J).

The electrode at which oxidation occurs is called the anode.

The electrode at which reduction occurs is called the cathode.

The salt bridge allows ions to flow into each half-cell to keep them electrically neutral.

◦ Dry Cells A dry cell is a voltaic cell in which the electrolyte is

a paste. The half reactions for a typical dry cell:

Both dry cells and alkaline batteries are single electrochemical cells that produce about 1.5 V.

◦ Lead Storage Batteries A battery is a group of cells connected together.

The half-reactions for a lead storage battery:

A 12-V car battery consists of six voltaic cells connected together.

◦ Fuel Cells Fuel cells are voltaic cells in which a fuel substance

undergoes oxidation.

The hydrogen-oxygen fuel cell is a clean source of power. Such cells are often used in spacecraft.

An electrolytic cell uses electricity to cause a nonspontaneous redox reaction to occur.◦ Electrons are forced to flow the “wrong way”.

This process is called electrolysis.

There are no half-cells. Both half reactions occur in the same vessel.◦ The anode is positive, and the cathode is negative

(opposite from a voltaic cell).

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Uses:◦ Electrolysis of brine◦ Obtaining oxygen and hydrogen from water◦ Electroplating

To produce chlorine and sodium hydroxide in electrolytic cells, electricity is passed through brine, a sodium chloride solution.

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Battery

Oxygen and hydrogen gas are obtained.

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Copper, silver and gold are used to plate jewelry and flatware, etc.

The cathode is the object to be plated. The anode is the plating metal.

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