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).

21.3

 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. 21.3

Battery

 Oxygen and hydrogen gas are obtained.

21.3

 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. 21.3