Construction of Voltaic Cells

In the half-cell on the left,a zinc metal atom loses two electrons. These flow through the zinc electrode to the external circuit, then to the copper electrode in the half-cell on the right. Negative charge on the copper electrode (from the electrons) attracts a copper ion, which reacts with the electrons on the electrode to form a copper metal atom.The internal connection, discussed later in the text,allows ions to flow between the two half-cells it is required to maintain charge balance. 

A voltaic cell consists of two half-cells that arc electrically connected. Each half-cell is the portion of an electrochemical cell in which a half-reaction takes place. A simple halfcell can be made from a metal strip that dips into a solution of its metal ion. An example is the zinc-zinc ion half-cell (often called simply a zinc electrode), which consists of a zinc metal strip dipping into a solution of a zinc salt. Another simple half-cell consists of a copper metal strip dipping into a solution of a copper salt (coppCT electrode). 

The first half-reaction, in which a species loses electrons, is the oxidation halfreaction. The electrode at which oxidation occurs is called Ihe anode. The second half-reaction, in which a species gains electrons, is the reduction half-reaction. The electrode at which reduction occurs is called the cathode. These definitions of anode and cathode hold for all electrochemical cells, including electrolytic cells. 

A voltaic cell is constructed from a half-cell in which a cadmium rod dips into a solution of cadmium nitrate, Cd(N03)2, and another half-cell in which a silver rod dips into a solution of silver nitrate, AgN03. The two half-cells are connected by a salt bridge. Silver ion is reduced during operation of the voltaic cell. Draw a sketch of the cell. Label the anode and cathode, showing the corresponding half-reactions at these electrodes. Indicate the electron flow in the external circuit, the signs of the electrodes, and the direction of cation migration in the half-cells. 

SOLUTION Because silver ion is reduced at the silver electrode, the silver electrode is the cathode. The half-reaction is 

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