Problem # 36 Galvanic Cell [Solved]
Question: In problem # 35, the notion of Electrode Potential has been explained. From that, do explain what's an Electrochemical Cell/Galvanic Cell
Answer
In Problem # 35, we have seen that to measure or determine the Standard Electrode Potential of a given Electrode, the Electrode must be connected to a Standard Hydrogen Electrode. By doing so, we have created an Electrochemical Cell, type Galvanic Cell or Voltaic Cell. In this case, the Electrode chambers are in Standard conditions.
Therefore, a Galvanic Cell is a set up where two Electrodes are connected: at one Electrode, called Cathode, a Reduction reaction takes place; at the other Electrode called Anode. an Oxidation reaction takes place. The reactions at both electrodes are called Half-reactions or Redox couples
Examples of Galvanic or Voltaic Cells
(1)
Cathode: Cu2+ + 2e → Cu Eo = 0.34 V Reduction half-reaction
Anode: H2 → 2H+ + 2e Eo = 0 V Oxidation half-reaction
Overall reaction: Cu2+ + H2 → Cu + 2H+ Redox reaction: EMF = 0.34 V + 0V = 0.34 V
(2)
Cathode: Cu2+ + 2e → Cu Eo = 0.34 V Reduction half-reaction
Anode: 2Li → 2Li+ + 2e Eo = +3.04 V Oxidation half-reaction
Overall reaction: Cu2+ 2Li → Cu + 2Li+ Redox reaction: EMF = 3.38 V
(3)
Cathode: Cu2+ + 2e → Cu Eo = 0.34 V Reduction half-reaction
Anode: Zn → Zn2+ + 2e Eo = 0.76 V Oxidation half-reaction
Overall reaction: Cu2+ + Zn → Cu + Zn2+ Redox reaction: EMF = 1.10 V
You have noticed that at the Anode, Eo used is the Standard Electrode Oxidation Potential, because the half-reaction at the anode is an oxidation reaction; in this case Eo bears the opposite sign of the Standard Electrode Reduction Potential.
In the overall reaction, the electrons produced at the Anode are consumed or captured at the Cathode.
EMF stands for Electromotive Force of the Galvanic/Voltaic cell. A positive EMF indicates that the Galvanic cell can deliver electricity as shown in the examples above.
Representation of a Galvanic Cell
By convention,
The anode is represented by the chemical symbol of the metal followed by a slash, followed by the metal ion: ex: Zn/Zn2+
The cathode is represented by the chemical symbol of the metal cation followed by a slash, followed by the metal symbol: ex: Cu2+/Cu
The Cell is represented by the two Electrodes connected; the connection is represented by a double slash (//), representing a porous disk or a salt bridge that allows ions to pass from one chamber to another. For a Galvanic cell to operate and deliver electricity, the two electrodes are connected by an external electrical conductor as shown below. Keeping in mind that the anode is at the left and the cathode at the right:Zn/Zn2+//Cu2+/Cu.
The figure below shows this Galvanic Cell that bears also the name of Daniell Cell.
(en.wikipedia.org)
A Galvanic cell can produce electrical energy, only if its EMF is positive. As the battery discharges, that potential difference decreases. When the potential difference reaches zero, the battery or the system doesn’t deliver energy anymore, it has reached the equilibrium; it is dead and its EMF = 0 V. As you can see, the driving force in a working battery is the potential difference between the two electrodes.
Conditions for an Electrochemical or Galvanic cell to generate electricity:
(1) The two electrodes, cathode and anode, must be different; because when they are different, their Electrode Potentials, i.e. tendencies to gain or lose electron are different. The difference can be in the nature of the two electrodes or in the concentrations of the two electrolytes. It is this difference in the Electrode Potentials that causes the chemical reaction in the Voltaic Cell to occur naturally (spontaneously) and to produce energy in form of electrical energy.
This can be compared to what happens with a Waterfall on a river. Waterfall results from the difference in altitudes of two points of a river. Those two points have different Potential Energies due to their positions and that results in the Waterfall from the higher altitude to the lower altitude. And the kinetic energy of the Waterfall can be converted into electrical or mechanical energy.
(2) If the two Electrodes were identical, the same as if a river is flowing on almost the same altitude, there wouldn’t be any difference of Potential Energies, and hence no chemical reaction would happen and no electrical energy would be produced.
However, generally an electrode is made of a metal in contact with a solution of a cation of that metal: this constitutes an electrode chamber. If the metal in the two electrode chambers is the same, but the concentration of the metal ion different, there is a difference in their Electrode Potentials and the two electrode chambers will form an electrochemical cell (voltaic cell), this kind of cell is called “concentration cell”.