Electrochemical Cells

If we place a strip of zinc metal in a beaker containing a solution of CuSO₄ Zn is oxidized to Zn²⁺ ions, while Cu²⁺ ions are reduced to metallic copper

Zn(s)  +  Cu²⁺(aq)  ==>  Zn²⁺(aq)  +  Cu(s)

The electrons are transferred directly from the reducing agent, Zn, to the oxidizing agent, Cu²⁺ in solution.  We have already seen, when we balanced redox equations, that we can separate a redox reaction like the one above into two half- reactions.  One of the half- reactions being oxidation and the other being a reduction half- reaction.

Zn(s) ==> Zn²⁺(aq)  + 2 e^-           oxidation half- reaction

Cu²⁺(aq)   + 2 e^-  ==> Cu(s)      reduction half- reaction

It turns out that we can physically separate the two half-reactions and thereby force the electron transfer to occur by way of an external conductor.  We can do this by using an electrochemical cell.  An electrochemical cell is an apparatus consisting of electrodes that dip into an electrolyte and in which a chemical reaction either produces electricity or is caused by an electric current.  A voltaic, or galvanic, cell is an electrochemical cell in which a spontaneous chemical reaction generates an electric current.  An electrolytic cell is an electrochemical cell in which a nonspontaneous chemical reaction results from the flow of an electric current through the cell.   Below we show a drawing of a voltaic cell in which the redox reaction above takes place.   Notice that the voltaic cell is made up of two compartments.  Each of these compartments is called a half-cell, and the two half-cells are connected by a salt bridge containing an electrolyte such as KCl.  The zinc and copper strips are called electrodes.  By definition, the anode in an electrochemical cell, voltaic or electrolytic, is the electrode at which oxidation occurs and the cathode is the electrode at which reduction occurs.  The anode in a voltaic cell has a negative sign whereas the cathode has a positive sign.  The electrons to be transferred, flow or move from the anode to the cathode through a wire or other metallic conductor (electrons do not move through the solution).  There is a transfer through the solution in the salt bridge of anions from the cathode half-cell to the anode half-cell, with cations moving in the opposite direction.  Without the salt bridge connecting the two solutions, the buildup of positive charge in the anode compartment, from the formation of Zn²⁺ ions, and the negative charge in the cathode compartment, caused by the Cu²⁺ ions being reduced to Cu, would quickly prevent the cell from operating.  The oxidation half-reaction is shown occurring in the anode compartment and the reduction half-reaction in the cathode compartment.  If we sum the two half-reactions we obtain the overall or cell reaction.  We must make sure that the electrons cancel when we add the two half-reactions, we can do this by multiplying each half-reaction by the appropriate small integer.  The voltmeter shown in the diagram would measure the cell potential or cell emf, E_cell.  And finally notice immediately under the cell a shorthand way of designating the cell.  The Zn|Zn²⁺(aq) represents the anode half-cell, the || represents the salt bridge and Cu²⁺(aq)|Cu represents the cathode half-cell.

Once we know which electrode is the anode or cathode, all of the other information shown in the diagram above falls into place by rules. To answer the question, which electrode is the cathode, you must look at the next topic, electrode potentials.