Electrode potential of a cell

Definition of electrical potential of a cell

The electrical potential of a cell is defined as the measure of the cell's ability to generate the electric current. The SI units for electrical potential is voltage, V and it should be understood that it is impossible to measure the electrical potential of a separate half-cell. It is only possible to measure the potential two half-cells when combined. For instance, it is easy to measure the electrical potential of Zn half-cells or a Cu half-cells when combined. The electrical potential of a cell results from competition for electrons between the two half cells. Reduction process will occur at the half-cell that has a greater tendency to acquire electrons. On the other hand, oxidation occurs in the other half-cell where electrons are lost. Reduction potential is the tendency of a given half-reaction to occur as a reduction. Oxidation process will occur at the half-cell having  greater reduction potential. The difference between the reduction potentials of the two half –cells is known as the cell potential.

 

Standard cell potential

 

The standard cell potential denoted by E°cell refers to the measured cell potential when the ion concentrations in the half cells are 1M at RTP.  E°red and E°oxid represent the standard reduction potentials for the reduction and oxidation half-cells respectively. The relationship between these values follows the general relationship for the cell potential as shown below.

E°cell = E°red - E°oxid

 

Now that we are unable to measure the half-cell potential, hydrogen is chosen as a standard electrode to serve as a reference. The standard reduction potentials of hydrogen electrode have been assigned a value of 0.00v. The standard hydrogen electrode is made up of a platinum electrode dipped in a solution with a hydrogen-ion concentration of 1M. At the platinum electrode, hydrogen gas is bubbled at a pressure of 101kPa. The half-cell reaction that takes place at the platinum electrode is shown below.

2H+ + 2e-  H2(g)                       E°H+ =0.00V

 

 

 

Cell Diagram or representation of a cell

A voltaic cell can be made by linking a standard hydrogen half-cell to standard zinc half-cell. We can calculate the overall reaction of such the cell by identifying the half-cell in which reduction occurs. Generally, reduction takes place at the cathode and oxidation takes place at the anode. A voltmeter is always used to show the reading when the zinc electrode is connected to the negative terminal, and the hydrogen electrode is connected to the positive terminal. In this process, zinc is oxidized while the hydrogen ions are reduced.

Zn(s)  Zn2+(aq) + 2e-    Oxidation

2H+(aq) + 2e- H2(g)       Reduction

  Zn(s) + 2H+(aq)   Zn2+(aq) + H2(g)

 

According to the IUPAC the following conventions or notations are applied for writing the cell diagram.

Zn(s)   Zn2+(aq) H+(aq    H2(g)         

 

Important points to note.

  1. Normally, the anode half-cell is represented on the LHS while the cathode half-cell is represented on RHS. The vertical line is used to separate the metal from an aqueous solution of its ions. This can be illustrated as shown below.

  2. Zn(s)   Zn2+(aq)  At the anode

  3. H+(aq    H2(g)    At the cathode    

  4. The salt bridge is represented by the double line

  5. The molar concentration of the overall reaction/process is put in the bracket after writing the corresponding ion.

  6. Always write the e.m.f of the cell at the end of the cell. For instance, Zn(s)    Zn2+(aq) H+(aq    H2(g)     e.m.f = - 0.76V      

  7. In case the cell consists of a platinum electrode, then we write it in brackets along with the working electrode.

 

Questions

  1. What is electrical potential of a cell?
  2. Mention the important points while representing a cell.
  3. How can we make a voltaic cell?



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