Introduction to Electrochemistry

Introduction

Electrochemistry is the branch of physical chemistry deals with the relationship between electricity, as a measurable and quantitative phenomenon, and identifiable chemical change, with either electricity considered an outcome of a particular chemical change or vice-versa. During the reaction, the electrical charges move between the electrolytes and electrode, therefore, electrochemistry deals with the interaction between electrical energy and the chemical change. In the electrochemistry most widely applicable reactions includes; redox (combination of reduction and oxidation) that involve electron transfer which is a basis for many electrical devices that we use in our day-to-day life such as automobiles, smartphones, electronic tablets, watches, pacemakers and many types of batteries. These devices involves continuous electron transfer resulting in the production of electricity which can be readily converted into useful work. Many metals are purified or electroplated by using redox reactions based on the principles of electrochemistry. All electrochemical systems work by transferring the electrons in the electrode region of the cell. Let us understand some basic concepts of electrochemistry in this articles.

 

 

Concept of Redox reaction

Oxidation refers to the loss of electrons while reduction is the gaining of electrons and both the reactions occur simultaneously without interfering with each other. Redox reactions are divided into two types namely, direct and indirect redox reaction. In a direct redox reaction, both oxidation and reduction reactions occur in the same vessel. Here, the chemical energy is transformed to heat energy.  On the other hand, indirect redox reaction involves reduction and oxidation taking place in different vessels. The chemical energy is converted into electrical energy.  In most cases, an electrochemical cell is used for a redox reaction.

 

 

Electrochemical cells

Electrochemical cell refers to the device used to convert chemical energy into electrical energy during a chemical reaction. The chemical reaction can either be exothermic or endothermic; in which an exothermic process releases heat, causing the temperature of the immediate surroundings to rise while the endothermic process absorbs heat and cools the surroundings . The energy produced by the reactions is found useful in the form of electricity. Electrochemistry as a discipline applied in both industries and  homes. Electrochemistry is defined as the study of the production of electricity from the heat energy released during the spontaneous chemical reaction. The electrochemical energy is mainly generated by flashlights and automobile batteries. Biological systems also use electrochemistry to transport nerve impulses. To understand electrochemistry, the learner should be in a position to explain the relationship between redox reaction and electrochemistry. Electric current is produced by the flow of electrons by dipping some materials in the solutions. For instance, dipping a  zinc strip in a solution containing copper sulphate cause the transfer of electrons from zinc metal to copper ions where electrons flow  due to the generation of electric current. Zinc,  and copper ions are the good examples of the conversion of chemical energy into electrical energy. All electrochemical processes involve redox reactions.

 

 

Types of electrochemical cells

There are two types of electrochemical cells namely, Galvanic cell and the electrolytic cell. Galvanic cells were named after the Italian physicist and physician Luigi Galvanic. He observed that dissected frog`s leg muscles twitched when he passed a small electric shock,  demonstrating the electrical nature of nerve impulses. A galvanic cell is also known as a voltaic cell which makes use of the energy released during a spontaneous redox reaction ( ΔG<0 ) to generate electricity. This type of electrochemical cell is often called a voltaic cell named after the Italian physicist Alessandro Volta. In contrast to the galvanic cell,  an electrolytic cell makes use of an external electrical energy source resulting in a non-spontaneous redox reaction. However, in both the types, 2 solid metallic electrodes are connected to an external circuit that furnishes necessary electrical connection between the two parts of the system as shown in the picture. The reduction half-reaction occurs at the cathode, and the oxidation half-reaction occurs at the anode.  When the circuit is closed, electrons flow from the anode to the cathode. The electrodes are also connected by an electrolyte, an ionic substance or solution that allows ions to transfer between the electrode compartments, thereby maintaining the system’s electrical neutrality.

 

 

1. Galvanic cells

Galvanic cells refer to the cells that convert chemical energy into electric energy.  In case we need to use a redox reaction as a source of electrical energy, then we should separate the two half-reactions i.e.

Zn(s)  Zn2+(aq) + 2e-         Oxidation reaction

                  Cu2+(aq) + 2e-  Cu(s)         Reduction reaction

Zn is used as a reducing agent, and Cu2+ ion is an oxidizing agent. Electrons in a galvanic cell usually move from the positive terminal to the negative terminal. A half-cell is one part of a galvanic cell in which either oxidation or reduction occurs. The cathode is the half-cell in which reduction process takes place while the anode is the half-cell in which oxidation takes place.

 

 

2. Electrolytic cell

An electrolytic cell is formed when electrodes are dipped into an electrolytic solution which contains anions and cations. Once current is supplied to the cell by use of the battery, ions will migrate towards electrode(s) of opposite charge, and redox reaction will occur.

 

 

Differences between Galvanic and Electrolyte cells

Galvanic Cell / Voltaic Cell

Electrolytic Cell

In a Galvanic cell, chemical energy is converted into electrical energy.

In an electrical cell, electrical energy is transformed into chemical energy.

The redox reactions are spontaneous in nature.

External energy input is needed in the electrolytic cells hence it is non-spontaneous.

The anode is negatively charged and the cathode is positively charged in a Galvanic cell.

It is in reverse when compared to Galvanic cells as the anode is positively charged and the cathode is  negatively charged 

The electrons originate as a result of oxidation.

Electrons come from the external source like a battery.

 

 

Questions

  1. What is electrochemistry?

  2. Name the reaction based on which electrodes work.

  3. What is a redox reaction?

  4. What is half-reaction and why is it necessary?

  5. Distinguish between the galvanic cell and electrolytic cell.

 



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