Pg:- 78,79,80,81

Ch:- 4

Constructing an equation in chemistry involves writing a balanced chemical equation that represents a chemical reaction. This equation describes the transformation of reactants into products, showing their chemical formulas and the relative amounts involved.

1. Identify the Reactants and Products

• Reactants: The starting substances that undergo change.

• Products: The substances formed as a result of the reaction.

• Example: When hydrogen reacts with oxygen, water is formed.

  • Reactants: H2​

  • Product: H2O

2. Write the Unbalanced Equation

Write the chemical formulas of the reactants on the left-hand side (LHS) and the products on the right-hand side (RHS), separated by an arrow (→) indicating the direction of the reaction.

Example:

H2 + O2 → H2O

3. Balance the Equation

Ensure the number of atoms of each element is the same on both sides of the equation (law of conservation of mass). Use coefficients (numbers before formulas) to balance the atoms.

• For H2 + O2 → H2O:

  • There are 2 hydrogen atoms on the LHS but 2 hydrogen atoms in 1 water molecule (on RHS), so the hydrogen is already balanced.

  • For oxygen: O2​ on the LHS has 2 oxygen atoms, but each H2O molecule has 1 oxygen atom. To balance oxygen, we need 2 H2Omolecules.

  • Adjust coefficients: 2H2 + O2 → 2H2O.

4. Indicate States of Matter

Specify the physical states of reactants and products using symbols:

• (s): Solid

• (l): Liquid

• (g): Gas

• (aq): Aqueous (dissolved in water)

Example:

2H2(g) + O2(g) → 2H2O(l)