Electrochemical Series

Cell emf and Potential difference

The difference in potential of the two electrodes (or half cells) of a galvanic cell, when measured in the open circuit is called the cell electromotive force or cell emf. When it is measured in a closed circuit with some external load it is called potential difference. Cell emf can be measured by using a potentiometer. It depends on the nature of electrodes, concentration of electrolyte and the temperature.

Standard cell emf

The emf of a cell has a standard value if both its half cells are in their standard states. It is denoted by Eº cell.

Cell emf and electrode potential

The standard cell emf is related to the standard electrode potentials of its anode and cathode.

E⁰ cell = E⁰ cathode – E⁰ anode

= Eº _Right – Eº _Left

Electrochemical Series

Standard potential of a large number of electrodes have been measured and they have been listed in the increasing order of electrode potential in a series called electro chemical
series.

The most active metal lithium is placed at the top and the most active non metal fluorine at the bottom. Lithium is the most powerful reducing agent and flourine is most powerful oxidizing agent.

Applications of Electrochemical Series

It helps to predict a redox reaction

A given ion will oxidize all the metals below it and a given metal will reduce ions of any metal placed above it in the series. For example, Zn has a greater reducing power than Fe or zinc can undergo oxidation more quickly than Fe. Zinc will reduce Fe²⁺ ions and itself undergoes oxidation.

It helps to calculate the emf of a galvanic cell

E⁰ cell = E⁰ cathode – E⁰ anode

E⁰ cell should always be positive. If E⁰ cell comes as –ve it means the cell cannot work and electrodes should be interchanged.

It helps to predict the feasibility of a redox reaction

The feasibility of a redox reaction can be predicted by calculating E⁰ cell for the redox reaction. The redox reaction is broken in two half reactions: oxidation half reaction acts as anode and reduction half acts as cathode. The positive E⁰ cell indicates the redox reaction is possible.

It helps to predict whether a metal can liberate hydrogen from acids

Any metal which is above hydrogen in the electrochemical series can liberate hydrogen from acid since it is a better reducing agent than hydrogen. Thus, metals like, Zinc, Magnesium, Calcium, etc can displace hydrogen from HCl or H₂SO₄ but metals like Copper, silver, etc cannot displace hydrogen from acid.