Chemical Reactions and Equations

A substance which undergoes a chemical change is called the reactant and the substance which is formed as a result of a chemical change is called the product.

For example,

Magnesium + Oxygen → Magnesium oxide

In the above reaction. magnesium and oxygen undergo chemical change and they are the reactants. Magnesium oxide is a new substance formed. It is the product.

Chemical Equation

A chemical equation can be made more concise and useful if we use chemical formulae instead of words. You substitute formulae of magnesium, oxygen and magnesium oxide for the words:

Mg + O₂ → MgO

Chemical equations in which the number of atoms is not equal on both sides of the arrow but still represent chemical reactions are called skeletal chemical equations. Skeletal chemical equations can be balanced by using suitable coefficients in the equation.

Balanced Chemical Equations

According to the law of conservation of mass, matter can neither be created nor destroyed. Thus, mass of each element present in the products of a chemical reaction must be equal to its mass present in the reactants.

In a balanced chemical equation number of atoms of a particular element present in the reactants and products must be equal. If not, equation is said to be "not balanced".

Mg + O₂ → MgO

Number of atoms of magnesium in the reactant side is equal to the number of atoms of magnesium in the product side. However, the number of atoms of oxygen on the reactant side is two (in O₂) but only one atom of oxygen is in the product side (in MgO). To make the same number of atoms of oxygen in the product side, you have to write 2MgO. Now the equation becomes

Mg + O₂ → 2MgO

In the above equation there is a shortage of one atom of magnesium on the left hand side. For balancing the number of magnesium atoms, you need to put 2 before Mg and the equation becomes

2Mg + O₂ → 2MgO

Now the number of magnesium and oxygen atoms is equal on both sides of the arrow and the chemical equation is said to be balanced. This method of balancing of a chemical equation is called the Hit and Trial method.

Types of Chemical Reactions

Chemical reactions are of the following types:

Combination reactions
Decomposition reactions
Displacement reactions
Double displacement reactions

Combination Reactions

In combination reactions, two or more substances (elements or compounds) simply combine to form a new substance. For example,

2Mg (s) + O₂ (g) → 2MgO (s)

C (s) + O₂ (g) → CO₂ (g)

Decomposition Reactions

A decomposition reaction is the one in which a compound decomposes into two or more than two substances (elements or compounds).

CaCO₃ (s) + heat → CaO (s) + CO₂ (g)

In the reactions in which decomposition occurs by application of heat, such reactions fall in the category of thermal decomposition.

A reaction in which a compound decomposes due to electrical energy into two or more than two substances (elements or compounds) is called electrolytic decomposition reaction.

Displacement Reaction

The displacement reaction is one in which one element displaces another element from its compounds. For example,

Fe (s) + CuSO₄ (aq) → FeSO₄ (aq) + Cu (s)

Iron has displaced copper from copper sulphate solution.

Double Displacement Reactions

Reactions in which there is an exchange of ions between the reactants, are called double displacement reactions. For example,

Na₂SO₄ (aq) + BaCl₂ (aq) → BaSO₄ (s)↓ + 2NaCl

Oxidation and Reduction

When a substance gains oxygen during a reaction, it is said to be oxidized and when a substance loses oxygen during a reaction, it is said to be reduced. During the reaction process, one reactant gets oxidized while the other gets reduced. Such reactions are called oxidation reduction reaction or Redox Reactions. For example,

CuO (s) + H₂ (g) → Cu (s) + H₂O (l)

CuO provides oxygen and therefore is an oxidizing agent and hydrogen takes this oxygen and therefore is a reducing agent. In a redox reaction, an oxidizing agent is reduced and a reducing agent is oxidized.

There is no oxidation without reduction and there is no reduction without oxidation. This aspect of redox reactions can also be explained in terms of electron gain and electron loss.

Electron Gain and Electron Loss

A reaction in which a species loses electrons is called an oxidation reaction and a reaction in which a species gains electrons is called a reduction reaction. For example,

Cu (s) + I₂ (s) → CuI₂ (s)

Above reaction can be written in two steps:

Step (i): Cu → Cu²⁺ + 2e^–

Step (ii): I₂ + 2e^– → 2I^–

In step (i) one copper atom loses two electrons to become a cupric ion, Cu²⁺ and in step (ii) iodine gains two electrons and gets converted into two iodide ions. Copper is oxidized by losing electrons and iodine is reduced by gaining electrons.

The substance which oxidizes the other substance is known as an oxidizing agent. An oxidizing agent gets reduced during the reaction. Likewise, the substance which reduces the other substance is known as a reducing agent. A reducing agent gets oxidized during the reaction.

Gain of electron is reduction and loss of electron is oxidation.

Effect of Redox Reactions

Corrosion

Corrosion is a destructive chemical process in which metals are oxidized in presence of air and moisture. The rusting of iron, tarnishing of silver, development of green coatings on copper, brass and bronze are a few examples of corrosion. It causes enormous damage to bridges, ships, cars and to all machines which are made of iron or steel.

Rancidity

Fats and oils undergo oxidation and become rancid. This change is called rancidity. Oxidation of fats/oils results into the formation of acids. These acids give unpleasant smell and bad taste.