CBSE CLASS X Chemical Reactions and Equations (Science)

Chemical Reactions and Equations (Science)

Oxidation  Reduction Reactions

You must have observed that when butter is kept in the open for a long time, it becomes rancid. Also, its smell and taste undergoes a change.Do you know why? This is because butter undergoes oxidation i.e. it reacts with oxygen and gets oxidized. This process is called rancidity.

What is Oxidation?

Oxidation is defined as a process that involves a gain of oxygen or a loss of hydrogen. When a substance gains oxygen or loses hydrogen during a reaction, it is oxidized.

Let us perform an activity to understand more about these reactions.

Activity:

Take around 1g copper powder (reddish brown in colour) in a china dish and heat it over a burner (as shown in the given figure).

What do you observe?

It will be observed that after sometime, the surface of the powder is covered by the layer of a black substance. When copper powder is heated, it combines with oxygen to form copper oxide.

Actually, in the process, copper powder gains oxygen. Thus, it gets oxidized to form copper oxide when heated. This process is called oxidation.

Now, if hydrogen gas is passed over heated copper (II) oxide, then the black coating on the surface turns brown. This is because a reverse reaction takes place and copper is re-obtained.

Here, copper (II) oxide loses oxygen and gets reduced to copper. This process is called reduction.

What is Reduction?

Reduction is defined as a process that involves a gain of hydrogen or a loss of oxygen. When a substance loses oxygen or gains hydrogen during a reaction, it is reduced.

Oxidation and reduction always take place simultaneously. Therefore, reactions involving oxidation and reduction are known as Redox (‘Red’ for reduction and ‘ox’ for oxidation) reactions. In a redox reaction, one substance is oxidized, while the other is reduced.

The substances that are reduced (provide oxygen or remove hydrogen) in course of the reaction are called oxidizing agents. These substances oxidize other chemicals in the reaction and are reduced in the process. On the other hand, the substances that are oxidized (remove oxygen or provide hydrogen) are called reducing agents.

For example:

In the above reaction, CO₂ gets reduced to CO and here, CO₂ is the oxidizing agent. On the other hand, hydrogen gets oxidized to form water and here, H₂ is the reducing agent.

We come across many examples of redox reactions in our daily life. For example, in the process of corrosion, metal combines with oxygen and gets corroded. Again, food gets spoilt, when it is oxidized and the process is called rancidity. We will discuss them separately.

Corrosion:

It may be defined as a process where materials, usually metals, are deteriorated because of a chemical reaction with air, moisture, chemicals, etc. For example, corrosion causes damage to car bodies, bridges, iron railings, ships, and all objects made of metals (especially those made from iron). Iron, in the presence of moisture, reacts with oxygen to form iron (III) oxide. This reaction is represented as:

This hydrated iron (III) oxide is rust. If not controlled, rusting can corrode the entire iron present in an object. As rust is softer than iron, the strength of the object decreases when rusting takes place. Every year, a large amount of money is spent for the maintenance of structures made of iron such as bridges, rails, ships etc.

Rancidity;

Whenfats and oils are oxidized, they become rancid and their smell and taste also changes.Thus,the oxidation of fats and oils can be easily observed by a change in their taste and smell. Oxidation of food can be prevented in many ways. Two common methods are discussed below.

1. Storing food in air tight containers. By doing so, the oxygen available for oxidation becomes limited. Hence, oxidation can be prevented.

2. Sometimes, antioxidants are added to food to prevent their oxidation. These antioxidants are oxidized first, which slows down the process of rancidity. These are reducing agents. Normally, vitamin C and vitamin E are added as antioxidants.

Questions asked in previous years’ board examinations

Ques. What is an oxidation reaction? Give an example of oxidation reaction. Is oxidation an exothermic or an endothermic reaction? (2 marks) −2009 CBSE Delhi Sol: The reaction in which a substance gains oxygen or loses hydrogen is called an oxidation reaction. For example, when copper (Cu) is heated in the presence of air, it gets oxidised to form copper oxide (CuO). The chemical equation involved in this reaction can be represented as Since heat energy is released in this process, this reaction is exothermic.