REVERSIBLE (EQUILIBRIUM ELECTRODE POTENTIALS): The Origin of Reversible Electrode potentials

REVERSIBLE (EQUILIBRIUM ELECTRODE POTENTIALS)

The Origin of Reversible Electrode potentials

In electrochemistry we often talk of a potential difference between two electrodes. It is important to consider the origin of such a potential difference.

Consider a case where a clean piece of zinc metal is placed in a solution containing the ions of zinc metal, e.g. zinc sulphate solution. Zinc being a more electropositive metal, there will be a tendency of the zinc atoms to go into solution as Zn²⁺ ions leaving behind electrons on the metal. The metal has excess electrons and is therefore negatively charged. The solution side, next to the electrode, has excess positive ions due Zn²⁺ ions going into solution.

The excess negative charge that accumulates on the metal prevents or stops the tendency of more Zn atoms going into solution as Zn²⁺ ions. The situation can be pictured as a series of positive charges due to Zn²⁺ ions on the solution side and a series of negative charges due to excess electrons on the metal side. 

The nature of the metal and the concentration of the solution into which the metal is dipping will determine or govern the tendency whether the metal ions will go into solution as ions or vice versa. In concentrated solutions, the tendency will be for the metal ions to deposit on the electrode as metal atoms after discharge. In a moderately concentrated solution, the tendency for the metal ions to go into solution and the metal atoms to deposit will take place to similar extents.

Whether the solution is concentrated or not, a time is reached when neither process dominates over the other. In other words a state of dynamic equilibrium is reached. That is for every zinc atom that goes into solution as zinc ion, i.e.

Zn(s) —----> Zn_(aq)²⁺ + 2e                                                                               (1)

another zinc ion is reduced and deposits as zinc atom, i.e.

Zn _(aq) ²⁺ + 2e ----> Zn _(s)                                                                                  (2)

However, an important point to be emphasized here is that when this state of dynamic equilibrium is attained, already there is excess negative charge on the electrode and an excess positive charge on the solution side. At equilibrium, therefore, there is a separation of charge, a negative charge on the metal and a positive charge on the solution side.

Whenever there is charge separation a potential difference (pd) is set-up. Hence there exists a potential difference between the metal and the solution. For a single electrode, this constitutes a single electrode potential.

For a less electropositive metal such as copper, the situation will be the reverse of the one that has been described for zinc. The tendency will be for the copper ions to be reduced and deposit on the copper metal. At equilibrium the metal will have an excess of positive charge and the solution side an excess of negative charge. Still there will be charge separation and hence a potential difference.

It has been stated above that the electrode potential of a single electrode cannot be measured. How is the electrode potential of say zinc electrode designated as Zn/Zn²⁺ determined?

It is common practice to arbitrarily choose one electrode as the standard or reference electrode and the potential differences of the other electrodes are measured against this one normally the electrode chosen as the standard reference electrode is the hydrogen electrode. 

The potential difference of a standard hydrogen electrode arbitrary assigned 0 volts. The standard hydrogen electrode consists of platinum black electrode dipping in a solution of 1M H⁺ ions.

The pressure of the bubbling hydrogen gas is 1 atm and the temperature of the solution is maintained at 25°C. Written as a reduction reaction:

H⁺_(aq)+e1/2H_2(g)                                    E^o =0.00V

The hydrogen electrode is also written as:

Pt, H_2(g) (1 atm)/H⁺_(aq) (1M).