Cell reactions:

The cell reactions are the oxidation and reduction products. At copper electrode, copper ions are reduced where electrons are consumed, and this is known as cathode. At zinc electrode, electrons must be supplied which is completed through the oxidation of zinc and this electrode is called the anode.

It is reasonable to write the cell reactions in a logical manner following a convention such that the anode (negative electrode) is always on the left and the cathode (positive electrode) is on the right. Logic in writing in this way is that the chemical reaction taking place in the cell follows the way that occurs in it. In a more general way it is to say that the left hand electrode of the cell, as written or corresponds to the oxidation reaction that is reaction of supply of electrons to the external circuit; and the right hand electrode corresponds to the reduction that is reaction for accepting electrons from the external circuit. Below given cell is illustrated:

Left Zn|Zn²⁺(a_Zn²⁺)||Cu²⁺(a_Cu2+)|Cu Right

where, a single vertical bar indicates a metal solution interface and two vertical parallel bars indicate a liquid-junction and separate the two half cells. The two half cell reactions are:

Zn (s) ↔     Zn²⁺ (aq) + 2e                                                    ... (1.4)

and Cu²⁺ (aq) + 2e ↔    Cu (s)                                             ... (1.5)

The whole cell reaction could be considered as the sum of the two half reactions (1.4 & 1.5) and is,

Zn  +  Cu²⁺   ↔   Zn²⁺ +  Cu                                                ... (1.6)

The emf of a galvanic cell is the deference between the potentials of the two electrodes and the potential difference at the zone of contact of the two electrolyte solutions. The latter is usually termed the liquid junction potential. Thus, when there is no liquid- junction potential, an emf of the cell is the potential difference between the two electrodes.

E_cell = E _right - E_left                                                      ... (1.7)

or       

E_cell  = (E_cathode ) - (E_anode )                             ...(1.8)

where E_right is the potential of the right electrode and E_left that of the left electrode. By the convention, all electrode potentials are reduction potential, therefore the negative sign in Eq. 1.8 is compensating for the electrode reaction occurring in the reverse direction. Using the similar convention, E_cell for the above discussed Daniell cell may be written as

E_cell =E_Cu²⁺_{/ Cu}- E _Zn²⁺_{/ Zn}                                          ... (1.9)

[Note: - The cell emf is always positive. The cell is, thus, written in such a way in which E_cell is positive.]