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Objectives Identifyparts of an electrochemical cell and their functions. Write electrode half reactions for cathodes and anodes. Describe the operation of voltaic cells, including dry cells, lead-acid batteries, and fuel cells. Identify conditions that lead to corrosion and ways to prevent it. Describe the relationship between voltage and the movement of electrons. Calculate cell voltage/potentials from a table of standard electrode potentials.
Describe the nature of electrolytic cells. Describe the process of electrolysis in the decomposition of water and in production of metals. Explain the process of electroplating. Describe the chemistry of a rechargeable cell.
When a half-reaction is written as an oxidation reaction, the sign of its electrode potential is reversed.
oxidation half-reaction: Zn Zn2+ + 2e− E0= +0.76 V reduction half-reaction: Zn2+ + 2e− Zn E0= −0.76 V
Standard electrode potentials can be used to predict if a redox reaction will occur spontaneously.
A spontaneous reaction will have a positive value for E0cell.
E0cell= E0cathode− E0anode
The half-reaction that has the more negative standard reduction potential will be the anode.
Sample Problem A Write the overall cell reaction, and calculate the cell potential for a voltaic cell consisting of the following half-cells: an iron (Fe) electrode in a solution of Fe(NO3)3 and a silver (Ag) electrode in a solution of AgNO3. Given:A half-cell consists of Fe(s) with Fe(NO3)3(aq) and a second half-cell consists of Ag(s) with AgNO3(aq). Unknown:E0cell Solution: Fe3+(aq) + 3e− Fe(s) E0= −0.04 V Ag+(aq) + e− Ag(s) E0= +0.80 V
Fe in Fe(NO3)3 is the anode because it has a lower reduction potential than Ag. Ag in Ag(NO3) is the cathode.
Multiply the Ag half-reaction by 3 so that the number of electrons lost in that half-reaction equals the number of electrons gained in the oxidation of iron.
Reverse the iron half-reaction to be an oxidation half-reaction.
There are two important differences between the voltaic cell and the electrolytic cell.
The anode and cathode of an electrolytic cell are connected to a battery or other direct-current source, whereas a voltaic cell serves as a source of electrical energy. Electrolytic cells are those in which electrical energy from an external source causes nonspontaneousredox reactions to occur. Voltaic cells are those in which spontaneous redox reactions produce electricity.
In an electrolytic cell, electrical energy is converted to chemical energy.
In a voltaic cell, chemical energy is converted to electrical energy.
An electrolytic process in which a metal ion is reduced and a solid metal is deposited on a surface is calledelectroplating.