An electrochemical cell is a device in which chemical energy of a redox reaction is converted into electrical energy. The redox reaction is carried out indirectly and the reaction being spontaneous produces decrease in free energy. This free energy gives rise to electrical energy i.e., an e.m.f.
The simplest electrochemical cell is Daniel cell or Galvanic cell in which a zinc rod is placed in a solution of ions (say ) in the left container and a bar of copper metal is immersed in a solution of ions (say ) in the right container. The two metals which act as electrodes are connected by a metallic wire through a voltmeter. The two solutions are joined by an inverted U-tube containing semi-solid paste of either KCl, or in gelatin or agar-agar jelly. This arrangement of u-tube is called salt bridge. A diagram of this cell has been shown in figure.
The overall cell reaction :
can be split into two half cells. There is a deflection in the voltameter which indicates the flow of current through the external circuit. The conventional current flows through the outer circuit from copper metal to zinc metal, which implies flow of electrons from zinc to copper bar.
(a) At zinc electrode the metal undergoes oxidation and releases two electrons (de-electronation).
Since oxidation is taking place, the electrode behaves as anode (–ve polarity).
These electrons travel through wire and reach the copper metal.
The above reaction occurs at the copper end. Electronation takes place which is a reduction process and that is why it acts as cathode (+ve polarity).
As a result of the two half cell reactions, zinc metal dissolves in anode solution to form ions, while the ions are discharged at the cathode by accepting two electrons and are deposited at cathode. The electrical neutrality is maintained in two half cells using a salt bridge. The anions of the inert electrolyte in the salt bridge migrate to the anodic chamber and cations to the cathodic chamber. As a result, as the reaction progresses, copper bar gains weight whereas zinc rod loses weight. As a consequence the cell continues to function till either zinc metal or copper ions in solution are consumed fully.
Since electrons are released at anode, it acquires negative polarity and cathode becomes positive because it receives electrons. This observation is against the usual electrolytic cell where anode is + ive and cathode is – ive.
(i) Salt Bridge and ts Functions
The electrolytes that are often used in salt bridge are called inert electrolytes which are supposed
(i) not to interact chemically with either of the solutions present in anodic or cathodic chamber.
(ii) not to interfere with overall cell reaction.
(iii) only those electrolytes can be used as a salt bridge in which mobility of ions is almost the same. Example, KCl, etc.
A salt bridge carries out two important functions :
(a) it allows only flow of ions through it. Thus, the circuit is completed.
(b) it also maintains the electrical neutrality.
As the electrons flow from anode to cathode, a net positive charge develops around the anode due to increase in the concentration of cations and a net negative charge around the cathode due to deposition of cations at the cathode.
(ii) Cell Diagram or representation of an electrochemical cell
The following conventions or notations are applied for writing the cell diagram in accordance with IUPAC recommendations. The Daniel cell is represented as follows :
(a) Anode half cell is written on the left hand side while cathode half cell on right hand side.
(b) A single vertical line separates the metal from aqueous solution of its own ions.
(c) A double vertical line represents salt bridge which allows the passage of ions through it but prevents the mixing of two solutions.
(d) The molar concentration (C) or activity (a) are placed in brackets after the formula of the corresponding ion.
(e) The value of e.m.f. of the cell is written on the extreme right of the cell. For example,
(f) If an inert electrode like platinum is involved in the construction of the cell, it may be written along with the working electrode in bracket, say for example, when a zinc anode is connected to a hydrogen electrode.
(iii) Reversibility of Daniel Cell
1. When external voltage is less than 1·10 V, electrons flow from Zn to Cu but current flows from Cu to Zn i.e., in opposite direction. Zinc dissolves at anode and copper deposits at cathode [see Fig (a)]
2. When external voltage applied is less than 1·10 V and is increased slowly, it is observed that the reaction continues to take place till the external voltage attains the value 1·10 V. When this is so, reaction stops altogether and no current flows [see Fig. (b)].
3. When the value of external voltage exceeds the voltage of Daniel cell (1·10 V), the reaction takes place in opposite direction, i.e., the cell functions like an electrolytic cell. [See Fig. (c)].
Functioning of Daniel cell when external voltage opposing the cell potential is applied
(iv) Electrode Potential
It may be defined as the tendency of an element, when it is placed in contact with its own ions to either lose or gain of electrons and in turn becomes positively or negatively charged.
The electrode potential will be named as oxidation or reduction potential depending upon whether oxidation or reduction has taken place.
(i) Both oxidation and reduction potentials are equal in magnitude but opposite in sign.
(ii) The reduction potential shows an increase with increasing concentration and decrease with decreasing concentration of ions in a solution.
(iii) It is not a thermodynamic property, so values of E are not additive.
(v) Standard Hydrogen Electrode (SHE)
It is a reference electrode. Standard Hydrogen electrode consists of a platinum wire coated with finely divided platinum black containing pure hydrogen gas at 1 bar pressure 298 K and solution of HCl(C = 1M) so as to maintain the equilibrium between ions and (g). Platinum black acts as a catalyst.
SHE is shown in above figure.
The hydrogen electrode can act both ways, i.e., as an anode or as a cathode.
Representation of SHE
(vi) Standard Oxidation Potential
It is the potential difference when a given electrode in contact with its own ions of 1M concentration undergoes oxidation when coupled with SHE.
Here SHE acts as cathode (reduction electrode) and the metal electrode as anode (oxidation electrode).
(vii) Standard Reduction Electrode Potential
It is the potential difference developed in a cell in volts, when a given metal electrode in contact with its ions (c = 1M) undergoes reduction when it is coupled with SHE.
Here SHE acts as anode (oxidation electrode) and the metal electrode as cathode (reduction electrode)
(viii) Standard Electrode Potential
According to IUPAC convention, the mode of reaction is always that of reduction, then,
The electrode potential in this mode is called standard electrode potential.
(ix) E.M.F. of the Cell
An electrochemical cell consists of two half cells. The e.m.f. of a single electrode cannot be determined without combining this electrode with any reference electrode. As the circuit is completed the oxidation occurs at the electrode which has lower oxidation potential and acts as anode and the reduction occurs at the electrode with higher reduction potential. The difference between the reduction electrode potentials of the two cells is called the e.m.f. of the cell. Thus,
Since anode is put on left and cathode on right, it follows therefore,
For a Daniel cell, therefore
Difference between the EMF (Electromotive force) of a cell and the cell potential can be summarized by the following table.
It measure the potential difference of the two half cells when electric current flows through the cell.
EMF is the potential difference between two electrodes, when no current is flowing in the circuit.
Cell potential is always less than the maximum voltage obtained from the cell.
EMF is the maximum voltage obtained from the cell.
It does not correspond to maximum useful work obtained from the galvanic cell.
It corresponds to the maximum useful work.
Cell potential is measured using a voltmeter.
EMF is measured by a potentiometer.
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