Electrochemistry – Important Topics

The study of electricity and how it affects chemical reactions are known as electrochemistry. In electrochemistry, electricity is generated by the passage of electrons from one element to another in a ‘redox’ or oxidation-reduction reaction.

Electrochemistry deals with the relationship between electrical potential and identifiable chemical change, where the electrical potential results from a particular chemical change or vice versa. The movement of electrons drives these reactions.

When a chemical reaction generates an electrical potential or is affected by a potential difference, it is called an ‘electrochemical reaction’. This is because chemical reactions involve the direct transfer of electrons between molecules. In contrast, electrochemical reactions involve the movement of electrons indirectly through an electronically conducting phase towards electrodes segregated by ionic species in a solution.  

We shall explore the fundamentals of electrochemistry as well as important topics in electrochemistry in the next sections of the article. The importance of the topic of electrochemistry in various entrance examinations is considerable and should be read thoroughly. Keep these essential points in mind while studying electrochemistry.

• Certain chemical reactions can create ‘electricity’.
• ‘Electricity’ can make specific reactions happen that would not happen otherwise.
• Electrochemical reactions deal with both these factors.

Electrochemistry – important topics

1. Oxidation and reduction reactions
2. Electrochemical cells
   1. Electrolytic cells
   2. Galvanic cells

Oxidation and reduction reactions

Any electrochemical process that involves changing the oxidation state of a molecule or ion due to the transfer of electrons to or from the molecule or ion is known as ‘reduction-oxidation’ or ‘redox’ reactions. 

The change in the oxidation state of an atom, molecule, or ion, brought about in an electrochemical reaction, is described as oxidation and reduction. 

‘Oxidation’ occurs when an atom, molecule, or ion loses electrons, and the atom’s, molecule’s, or ion’s oxidation state increases. ‘Reduction’ occurs when an atom, molecule, or ion gains electrons and the atom’s oxidation state, molecule or ion, decreases. 

This can be remembered with the help of a mnemonic device: ‘OIL RIG,’ i.e., Oxidation Is Loss of electrons, and Reduction Is Gain of electrons.

In a redox reaction, oxidation and reduction always occur simultaneously. Therefore, an atom, molecule, or ion that undergoes oxidation (or loses electrons) is called the reducing agent. Conversely, an atom, molecule, or ion that undergoes reduction (or gain of electrons) is called the oxidising agent.

For example, in a redox reaction: where a zinc atom loses two electrons on reacting with 2H+, and Zn2+ and H2 are obtained as a result.   

The oxidation number of Zn increases from 0 to 2; therefore, we say that the species is undergoing oxidation. On the other hand, the oxidation number of H+ decreases from +1 to 0; therefore, we say that the species is undergoing a reduction.

In the case of the following reaction:

CO3 2- + 2H+ → CO2 + H2O

Neither of the elements undergoes oxidation or reduction, i.e., each element has the same oxidation number on the reactant side and the product side; therefore, the reaction is not considered a redox reaction.

Electrochemical cells

A device that can utilise chemical reactions to generate electrical energy or utilise ‘electricity’ to make specific reactions happen that would not occur otherwise is known as an electrochemical cell.

The electrochemical cells that utilise chemical reactions to generate electrical energy are known as ‘Galvanic cells’ or ‘Voltaic Cells’. The electrochemical cells that use ‘electricity’ to make specific reactions occur non-spontaneously are known as electrolytic cells.

Electrolytic cells

This device utilises electricity to drive non-spontaneous chemical reactions. For example, in the application of electricity, water can be electrolyzed into its chemical components, hydrogen and oxygen.

An electrolytic cell comprises three components: an electrolyte and two electrodes. The two electrodes, anode and cathode, are separated by ionic species in a solution (the electrolyte). 

When an external voltage is applied, the electrodes gain a cumulative charge, and the ions in the electrolyte with opposite charges are attracted to their corresponding electrodes. i.e., a positively-charged ion will be attracted to a negatively-charged electrode and a negatively-charged ion will be attracted to a positively-charged electrode. 

Charge-transferring reactions take place at the electrodes. In this way, the external voltage applied helps decompose molecules into their chemical components spontaneously, which would not happen otherwise.

Galvanic cells

The electrochemical cells that utilise chemical reactions to generate electrical energy are known as ‘Galvanic cells’ or ‘Voltaic Cells’.

Consider a solution of silver nitrate in which a copper wire is submerged. The copper ions start to pass into the solution of silver nitrate and the silver ions accumulate on the copper wire. 

As a result, the silver nitrate solution slowly turns blue due to the presence of copper ions in the solution. In this case, the copper wire is undergoing oxidation and the silver nitrate solution reduces. 

A Galvanic cell can be constructed to make the following reaction occur non-spontaneously. The redox reaction can be divided into two half cells: the oxidation half-cell and the reduction half-cell.

In Galvanic cells, these half-cells are separated via an external wire. In the case discussed above, the copper wire undergoing oxidation acts as an anode. The anode will be connected to a voltmeter connected to a silver electrode, where reduction takes place. 

The silver electrode therefore will act as the ‘cathode’. Both these electrodes will be submerged into separate 1M solutions of copper nitrate and silver nitrate respectively. 

At the oxidation half-cell, Cu ions lose electrons and pass into the solution as Cu+2          ions. At the reduction half-cell, Ag ions reduce to Ag(s). A salt bridge connecting both the half-cells maintains the charge balance between the half-cells.

Conclusion 

The study of electricity and how it affects chemical reactions are known as electrochemistry. In simple terms, electrochemistry studies the relationship between electricity and chemical reactions. It deals with the relationship between electrical potential and identifiable chemical change, where the electrical potential results from a particular chemical change or vice-versa. The movement of electrons drives these reactions.