6.0 Electrode Potential

When a metal is placed in the solution of its ions, the metal acquires either positive or negative charge according with respect to the solution and due to which a potential difference gets developed between the both i.e., metal and its solution. This potential difference is called electrode potential.

For example. when a zinc is placed in a salt zinc sulphate which contains $Z{n^{2 + }}$ions, it becomes negatively charged with respect to the solution and a potential difference is developed in between metal and its ions(or solution), which is called electrode potential of zinc.

Similarly when a copper is placed in a salt copper sulphate which contains $C{u^{2 + }}$ions, it becomes positively charged with respect to the solution and a potential difference is developed in between metal and its ions (or solution), which is called electrode potential of copper.

These will result into two reactions, termed as oxidation and reduction.

Magnitude of electrode potential depends on following factors:

i) Concentration of its ions

ii) Temperature

iii) Nature of electrode

Consider a redox reaction:

$$Zn(s) + C{u^{2 + }}(aq) \to Z{n^{2 + }}(aq) + Cu(s)$$

This reaction is the combination of two half cell reactions occurring in two separate cell or compartments. So lets write the separate reactions

Oxidation half reaction: $Zn(s) \to Z{n^{2 + }}(aq) + 2{e^ - }$

Reduction half reaction: $C{u^{2 + }}(aq) + 2{e^ - } \to Cu(s)$

These reactions are categorized as oxidation half reaction and reduction half reaction on the basis of the the oxidation number (whether it is increasing or decreasing), If oxidation number of species is increasing or species loses electron then that it is considered to be oxidation half reaction. The electrode at which oxidation occurs is anode.

Reduction half reaction in which species gain electrons and the electrode at which reduction takes place is cathode.

These definitions of anode and cathode hold good for all cells including electrolytic cell. Once we have decided the anode and cathode then it is easy to determine the direction of flow of electron in the external portion of the circuit.

So anode in voltaic cell has negative sign and cathode has positive sign. In left compartment, oxidation occurs and in right compartment, reduction occurs.

FOR MEMORY: LOAN

L-Left

O-oxidation

A-Anode

N-Negative

Reduction half reaction occurs at copper electrode and oxidation half reaction occurs at zinc electrode. These two portions are called redox couples.

So we can construct number of galvanic cells by taking combination of different half cells. These two cells must be connected by a metallic wire through voltmeter and switch externally. And electrolytes are connected by salt bridge.It is also possible that both electrodes are dipped in the same electrolyte and in that case we don't need salt bridge.

At an interface of electrode-electrolyte, metal ions from the solution have the tendency to deposit on the metal electrode to make it positively charged and at the same time, metal atoms of the electrode have a tendacy to go into solution as ions, leaving behind the electrons at the electrode to make them negatively charged. At equilibrium there will be some separation of charges and depending on the tendencies of two opposing reactions, electrode will be positively charged or negatively charged with respect to the solution.

Potential difference which developed between the electrode and the electrolyte is called electrode potential. When the concentrations of all the species are unity then this potential is called standard electrode potential.