After completing the topic, the student will be able to:
- Define oxidation and reduction reactions.
- Represent a redox reaction in terms of electron tranfer.
- Define oxidizing and reducing agents and give examples.
- Identify the oxidizing or reducing agent in a given reaction.
- Define oxidation number and calculate the change in valency or oxidation number of species in a redox reaction.
- Balance a redox reaction using ion–electron method.
- Discuss everyday examples those involved a redox process.
In chemical reactions, inter molecular bonds between reactants are broken and new bonds are made in the product molecules. This transformation is the simple rearrangement of electrons.
Reactions that involve the transfer of one or more electrons from one reactant to another are called oxidation – reduction reactions.
Simple meaning of oxidation reaction would be an addition of oxygen to a substance. Reduction reaction would mean removal of oxygen form a substance. Since years, we have been burning coal to get heat. We also know that for burning of coal we need supply of fresh air. What happens when coal burns? The oxygen from the air is necessary for the process of burning. We can represent this reaction by the chemical equation as follows:
Carbon dioxide is the product. If sufficient supply of fresh air is not available, the reaction may be as follows:
Carbon monoxide is the product. The products of burning of coal, both CO2 and CO being gases escape in the atmosphere.
Another common experience is burning of magnesium wire, which is popular in the firework
A common example of reduction is the burning of wood. Wood is mainly cellulose, a substance made up of carbon and hydrogen (C6H10O5)n. As wood burns, its carbon – hydrogen bonds break and the carbon and hydrogen atoms form new bonds with oxygen molecules in the air to create carbon dioxide and water. The formation of these products involves the transfer of electrons from one atom to another and so is by definition an oxidation – reduction reaction.
What remains behind is charcoal – which is mostly carbon, C.
Other examples of reduction reactions are
In this reaction, the metal oxide is reduced because oxygen is removed. If you keep a metal oxide in a test tube and heat it, bringing a burning matchstick near the mouth of the test tube, you can test the emission of oxygen the matchstick will glow brighter. This indicates that the gas escaping from the test tube is oxygen.
Certain reactions where in hydrogen is removed, are also grouped as oxidation reactions. Similarly, if hydrogen is added, these reactions are called reduction reactions.
In this reaction H2S is oxidized to S, because hydrogen is removed. Simultaneously, bromine is reduced to HBr because hydrogen is added.
In this reaction, the oxidation – reduction reactions occur simultaneously on different reactant. These are called redox reactions.
Thus, oxidation may be defined as a chemical reaction in which oxygen is added or hydrogen is removed and reduction is the chemical reaction in which hydrogen is added or oxygen is removed.
Let us see a few more examples:
When CuO is heated with hydrogen, copper metal and water are formed.
In this reaction, the hydrogen H2 is changing into water or H2O. That is oxygen is being added to hydrogen. By simple definition of oxidation, addition of oxygen to a substance is known as oxidation. Hence we can say that hydrogen is being oxidized to water.
Further, in this reaction CuO is changing to Cu. This means that oxygen is being removed from the copper oxide. By simple definition of reduction, removal of oxygen from CuO means that CuO is being reduced to copper.
When hydrogen sulphide (H2S) reacts with chlorine (Cl2), sulphur S and HCl are formed.
In this reaction, the hydrogen H2S is changing into sulphur or 'S'. That is hydrogen is being removed from hydrogen sulphide. By simple definition of removal of hydrogen from a substance is known as oxidation. Hence we can say that hydrogen sulphide is being oxidized to 'S'.
Further, in this reaction, chlorine (Cl2) is changing to HCl – addition of hydrogen. This means that hydrogen is being removed from H2S and added to Cl2. By simple definition of reduction, addition of hydrogen from H2S means that Cl2 is being reduced to HCl.
Oxidation – loss of electrons, Reduction – gain of electrons:
Oxidation is the process whereby a reactant loses one or more electrons. Reduction is the opposite process whereby a reactant gains one or more electrons. Oxidation and reduction are complementary and simultaneous processes. They always occur together; you cannot have one without the other. The electrons lost by one chemical in an oxidation reaction don‘t simply disappear; they are gained by another chemical in a reduction reaction. An oxidation – reduction reaction occurs when elemental sodium and chlorine react exothermically to form sodium chloride, as shown in the below figure. The equation for this reaction is
To see how electrons are transferred in this reaction, we can look at each reactant individually. Each electrically neutral sodium atom changes to a positively charged ion – each atom loses an electron and is therefore oxidized:
Each electrically neutral chlorine molecule changes to two negatively charged ions – each atom gains an electron and is therefore reduced:
The net result is that the two electrons lost by the sodium atoms are transferred to the chlorine atoms. Therefore, the two equations shown above actually represent one process, called a half reaction. In other words, an electron won‘t be lost from a sodium atom without there being a chlorine atom available to pick up that electron. Both half reactions are required to represent the whole oxidation – reduction process. Half reactions are useful for showing which reactant loses electrons and which reactant gains them, which is why half reactions are used throughout this chapter. Because the sodium causes reduction of chlorine, the sodium is acting as a reducing agent, which is any reactant that causes another reactant to be reduced. Note that in behaving as a reducing agent, the sodium is oxidized – it loses electrons.
Conversely, the chlorine causes oxidation of the sodium and so it is acting as an oxidizing agent. Because it gains electrons as it causes another reactant to be oxidized, an oxidizing agent is reduced. Just remember that loss of electrons is oxidation, and gain of electrons is reduction. Here is a helpful pneumonic adopted from one popular children‘s story: Leo the lion went "ger".
Different elements have different oxidation and reduction tendencies – some lose electrons more readily, while others gain electrons more readily, as the figure below illustrates. The tendency to do one or the other is a function of how strongly the atoms nucleus holds electrons. Greater the effective nuclear charge, greater the tendency of the atom to gain electrons. Because the atoms of elements at the upper right of the periodic table have the strongest effective nuclear charges (with the noble gases excluded), these atoms have the greatest tendency to gain electrons and hence behave as oxidizing agents. The atoms of elements at the lower left of the periodic table have the weakest effective nuclear charges and therefore the greatest tendency to lose electrons and behave as reducing agents.
• Chlorine get inert gas configuration of Argon (1s22s22p63s23p6) by gaining one electron.