The
cell potential that is generated by a
redox reactions is measured or used in a Galvanic (= electrochemical) cell. A Galvanic cell contains two compartments:
- In the oxidation part ("oxidation half cell"), a substance loses one or more electrons: "Loss of Electrons is Oxidation" (OIL)
- In the reduction part ("reduction half cell"), a substance gains one or more electrons: "Reduction Is Gain" (RIG)
The substance needing electrons in the reduction half cell pulls electrons through a wire from the substance in the oxidation half cell:
- The cell in which oxidation occurs is called the anode: oxidation at anode
- The cell in which reduction occurs is called the cathode: reduction at cathode
The picture opposite shows a Galvanic cell made from zinc and tin half cells. The zinc half cell has a strip of zinc metal in a beaker containing Zn2+(aq) ions. The tin half cell has a strip of tin metal in a beaker containing Sn2+ ions. The two half cells are connected together with a wire and a salt bridge.
The cell potential and the overall redox reaction can be worked out using the standard cell potentials. These are E° = -0.76 V for Zn2+ / Zn(s) and -0.14 V for Sn2+ / Sn(s). As the value for the Sn2+ / Sn(s) half cell is less negative, the reduction, oxidation and overall reactions are:
Sn2+(aq) + 2e- → Sn(s)
Zn(s) → Zn2+(aq) + 2e-
Sn2+(aq) + Zn(s) → Sn(s) + Zn2+(aq)
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The standard cell potential is the reading on the volt meter when the cells are first connected:
E°cell = E°reduction half cell - E°oxidation half cell = (-0.14 V) - (-0.76 V) = +0.62 V
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Oxidation involves loss of electrons and always occurs at the anode. Zinc is therefore the anode as oxidation is occuring in the Zn2+(aq) / Zn(s) half cell. Zn atoms on the electrode lose two electrons and enter the solution as Zn2+(aq). The electrons are left behind on the electrode, which becomes negatively charged.
Reduction involves gain of electrons and always occurs at the cathode. Tin is therefore the cathode as reduction is occuring in the Sn2+(aq) / Sn(s) half cell. Sn2+(aq) ions in the solution pick up two electrons from the electrode and become Sn(s). Electrons are taken from the electrode, which becomes positively charged.
In a Galvanic cell, the oxidation process leads to electrons being left on the anode which becomes negatively charged and the reduction process leads to electrons being taken from the cathode which becomes positively charged.
Electrons travel from the negatively charged electrode to the positively charged electrode through the wire. In a Galvanic cell, electrons therefore always flow from the anode to the cathode.
In the example here, electrons therefore flow from the zinc elecrode to the tin electrode.
The salt bridge contains a solution of a salt, commonly NaNO3, KNO3, NaCl or KCl, which plays no part in the reaction. The ions do, however, move in and out of the salt bridge into the half cells when the cell is connected. They do this to ensure that the whole cell remains neutral overall and complete the circuit.
In the anode half cell, Zn2+(aq) ions are entering the solution as oxidation occurs. This would lead to a build up of positively charged cations in the solution without the salt bridge. Anions in the salt bridge such as NO3-(aq) or Cl-(aq), flow from the salt bridge into the anode beaker to compensate for this.
In the cathode half cell, Sn2+(aq) ions are leaving the solution as reduction occurs. This would lead to a lose of positively charged cations in the solution without the salt bridge. Cations in the salt bridge such as Na+(aq) or K+(aq), flow from the salt bridge into the cathode beaker to compensate for this.
