1.Chemical Equations

Chapter 1

Chemical reactions and Equations

Chemical Reaction: The transformation of chemical substance into another chemical substance is known as Chemical Reaction. For example: Rusting of iron, the setting of milk into curd, digestion of food, respiration, etc.
In a chemical reaction, a new substance is formed which is completely different in properties from the original substance, so in a chemical reaction, a chemical change takes place. Only a rearrangement of atoms takes place in a chemical reaction.
The substances which take part in a chemical reaction are called reactants.
The new substances produced as a result of a chemical reaction are called products.
Characteristics of Chemical Reactions: 

(i) Evolution of gas: The chemical reaction between zinc and dilute sulphuric acid is characterised by the evolution of hydrogen gas.
Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g) ↑
(ii) Change in Colour: The chemical reaction between citric acid and purple coloured potassium permanganate solution is characterised by a change in colour from purple to colourless.
The chemical reaction between sulphur dioxide gas and acidified potassium dichromate solution is characterized by a change in colour from orange to green.
(iii) Change in state of substance: The combustion reaction of candle wax is characterised by a change in state from solid to liquid and gas (because the wax is a solid, water formed by the combustion of wax is a liquid at room temperature whereas, carbon dioxide produced by the combustion of wax is a gas). There are some chemical reactions which can show more than one characteristic.
(iv) Change in temperature: The chemical reaction between quick lime water to form slaked lime is characterized by a change in temperature (which is a rise in temperature).
The chemical reaction between zinc granules and dilute sulphuric acid is also characterised by a change in temperature (which is a rise in temperature).
(v) Formation of precipitate: The chemical reaction between sulphuric acid and barium chloride solution is characterised by the formation of a white precipitate of barium sulphate.
BaCl₂(aq) + H₂SO₄(aq) → BaSO₄(s) (ppt) + 2HCl(aq)
Chemical Equation: 

Representation of chemical reaction using symbols and formulae of the substances is called Chemical Equation.
Example: A + B → C + D
In this equation, A and B are called reactants and C and D are called the products. The arrow shows the direction of the chemical reaction. Condition, if any, is written generally above the arrow.
When hydrogen reacts with oxygen, it gives water. This reaction can be represented by the following chemical equation:
Hydrogen + Oxygen → Water  H₂ + O₂ → H₂O
In the first equation, words are used and in second, symbols of substances are used to write the chemical equation. For convenience, the symbol of substance is used to represent chemical equations.
A chemical equation is a way to represent the chemical reaction in a concise and informative way.
A chemical equation can be divided into two types: Balanced Chemical Equation and Unbalanced Chemical Equation.

(a) Balanced Chemical Equation: A balanced chemical equation has the number of atoms of each element equal on both sides.
Example: Zn + H₂SO₄ → ZnSO₄ + H₂
In this equation, numbers of zinc, hydrogen and sulphate are equal on both sides, so it is a Balanced Chemical Equation.
According to the Law of Conservation of Mass, mass can neither be created nor destroyed in a chemical reaction. To obey this law, the total mass of elements present in reactants must be equal to the total mass of elements present in products.

(b) Unbalanced Chemical Equation: If the number of atoms of each element in reactants is not equal to the number of atoms of each element present in the product, then the chemical equation is called Unbalanced Chemical Equation.
Example: Fe + H₂O → Fe₃O₄ + H₂
In this example, a number of atoms of elements are not equal on two sides of the reaction. For example; on the left-hand side only one iron atom is present, while three iron atoms are present on the right-hand side. Therefore, it is an unbalanced chemical equation.
Steps to form balanced chemical equation
To show how to balance the equation, the following equation is used-
Fe + H₂O → Fe₃O₄ + H₂
Step 1: First of all, draw the boxes around each formula as shown below-

Step 2: Find out the number of atoms of each element. For Example, on reactant side, 1 for Fe, 2 H, and 1 O and on product side we have, 3 for Fe, 4 for O and 2 for H.
Step 3: Start to balance the equation with the compound having maximum number of atoms. While balancing does not alter the formula of the compound.
Step 4: One by one balance each element on reactant and product side.

Step 5: After balancing number of atoms on both the side of the equation, finally check the correctness of the balanced equation.

Step 6: then write the symbols of the physical state of reactants and products as shown below-

3Fe(s) + 4H₂O (g) → Fe₃O₄ (s) + 4H₂ (g)

This above equation represents the balanced equation.