Redox Reaction and Electrode Potential

Section 6.1.1 already noted that a displacement reaction is a kind of redox reaction; this section develops that link further using Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s) as the running example, showing it can be carried out in two physically different ways. The simple way is to dip a zinc rod directly into a copper sulfate solution in one container: zinc is oxidised to Zn²⁺ while Cu²⁺ ions are reduced to metallic copper, with the electron transfer happening directly at the point of contact between the zinc metal surface and the solution. The second way keeps the two half reactions physically apart: zinc metal sits in zinc-salt solution in one container, copper metal sits in copper-salt solution in a second container, and the two containers are linked in two different ways at once -- an external electrical wire (running through a switch and a voltmeter) connecting the two metal electrodes directly, and a salt bridge (a U-shaped glass tube filled with a gel of KCl or NH4NO3 in agar-agar) connecting the two solutions. When the switch is closed the circuit is completed and the voltmeter shows a deflection: zinc atoms on the zinc plate spontaneously lose electrons, which travel through the external wire to the copper plate, where Cu²⁺ ions in the second container pick up those electrons and are reduced, depositing as copper metal on the copper plate. Since oxidation occurs at the zinc electrode, it is called the anode (the negative electrode); since reduction occurs at the copper electrode, it is called the cathode (the positive electrode). Meanwhile, within the two solutions, the circuit is completed not by electrons but by the movement of ions through the salt bridge. This whole two-container arrangement, which converts a spontaneous redox reaction directly into a flow of electric current, is called a Daniell cell, and it is a simple example of an electrochemical cell (Figure 6.1). The electrical potential set up at each of a cell's two electrodes is called the electrode potential; i …