chapter 21. two types: ◦ voltaic cell: electrons flow spontaneously ◦ electrolytic cell:...
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Chapter 21
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Two types:◦ Voltaic cell: electrons flow spontaneously ◦ Electrolytic cell: electrons are forced to flow
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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.
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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).
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The electrode at which oxidation occurs is called the anode.
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The electrode at which reduction occurs is called the cathode.
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The salt bridge allows ions to flow into each half-cell to keep them electrically neutral.
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◦ Dry Cells A dry cell is a voltaic cell in which the electrolyte is
a paste. The half reactions for a typical dry cell:
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Both dry cells and alkaline batteries are single electrochemical cells that produce about 1.5 V.
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◦ Lead Storage Batteries A battery is a group of cells connected together.
The half-reactions for a lead storage battery:
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A 12-V car battery consists of six voltaic cells connected together.
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◦ Fuel Cells Fuel cells are voltaic cells in which a fuel substance
undergoes oxidation.
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The hydrogen-oxygen fuel cell is a clean source of power. Such cells are often used in spacecraft.
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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.
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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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21.3
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Uses:◦ Electrolysis of brine◦ Obtaining oxygen and hydrogen from water◦ Electroplating
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To produce chlorine and sodium hydroxide in electrolytic cells, electricity is passed through brine, a sodium chloride solution.
21.3
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21.3
Battery
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Oxygen and hydrogen gas are obtained.
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21.3
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Copper, silver and gold are used to plate jewelry and flatware, etc.
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The cathode is the object to be plated. The anode is the plating metal.
21.3