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Chemical Reactions and Equations | Class 10 CBSE | Web Notes | Part 1 | Chemical Equations

  • chemical reaction is a process in which one or more reactants are converted to one or more products.
  • During a chemical reaction, a chemical change occurs.
  • The substances that undergo chemical change are called reactants, and the new substances formed are called products.

Examples of Chemical Reactions

  • Burning of a clean magnesium ribbon with a dazzling white flame to form a white powder, magnesium oxide. This is due to the reaction of magnesium with oxygen in the air.
  • Take lead nitrate solution in a test tube and add potassium iodide solution. A yellow precipitate of lead iodide appears at the bottom.
  • Take a few zinc granules in a conical flask or test tube. Add dilute HCl or H2SO4. Bubbles are observed around the zinc granules due to the release of hydrogen, and the conical flask becomes hot.

The following observations helps to determine whether a chemical reaction has taken place.

  • Change in state.
  • Change in color.
  • Evolution of a gas.
  • Change in temperature.

Chemical Equations

  • The description of a chemical reaction can be written in a shorter form using a word-equation. For example:
    • Magnesium + Oxygen → Magnesium oxide
  • Reactants are written on the left-hand side (LHS) with a plus sign (+) between them. Products are written on the right-hand side (RHS) with a plus sign (+) between them. The arrowhead points towards the products, indicating the direction of the reaction.

Writing a Chemical Equation

  • Chemical equations can be simplified using chemical formulae. A chemical equation represents a chemical reaction. For example:
    • Mg + O2 → MgO (skeletal chemical equation)
  • If the number of atoms of each element is the same on the LHS and RHS, the equation is balanced. If not, it is unbalanced and called a skeletal chemical equation.

Balanced Chemical Equations

  • According to the law of conservation of mass, mass can neither be created nor destroyed in a chemical reaction.
  • The total mass of the elements present in the products is equal to the total mass of the elements in the reactants.
  • The number of atoms of each element remains the same before and after a chemical reaction, so a skeletal chemical equation must be balanced.
  • For example, the word-equation:
    • Zinc + Sulphuric acid → Zinc sulphate + Hydrogen

    It is represented by the chemical equation:

    Zn + H2SO4 → ZnSO4 + H2
    Element Number of atoms in reactants (LHS) Number of atoms in products (RHS)
    Zn 1 1
    H 2 2
    S 1 1
    O 4 4

    Thus, it is a balanced chemical equation.

Steps of Balancing a Chemical Equation

Balancing a chemical equation using the least whole number coefficients is called the hit-and-trial method. The steps are as follows:

Fe + H2O → Fe3O4 + H2

Step I:

Draw boxes around each formula. Do not change anything inside the boxes.

Fe + H2O → Fe3O4 + H2

Step II:

List the number of atoms of different elements

Element Number of atoms in reactants (LHS) Number of atoms in products (RHS)
Fe 1 3
H 2 2
O 1 4

Step III:

Select the compound (reactant or product) having maximum number of atoms (Fe3O4). In that, select the element having maximum number of atoms (oxygen).

Atoms of oxygen In reactants In products
(i) Initial
(ii) To balance		 1 (in H2O)
1 × 4		 4 (in Fe3O4)
4

Fe + 4 H2O → Fe3O4 + H2

Step IV:

Balance the number of hydrogen atoms

Atoms of hydrogen In reactants In products
(i) Initial
(ii) To balance		 8 (in 4 H2O)
8		 2 (in H2)
2 × 4

Fe + 4 H2O → Fe3O4 + 4 H2 (partly balanced)


Step V:

Balance the number of iron atoms

Atoms of iron In reactants In products
(i) Initial
(ii) To balance		 1 (in Fe)
1 × 3		 3 (in Fe3O4)
3

3 Fe + 4 H2O → Fe3O4 + 4 H2

Step VI:

Count atoms of each element on both sides of the equation to check the correctness.

3Fe + 4H2O → Fe3O4 + 4H2 (balanced equation)

Step VII:

If necessary, physical states such as gaseous (g), liquid (l), aqueous (aq), and solid (s) are included in a chemical equation. Aqueous (aq) means the reactant or product is present as a solution in water.

3Fe(s) + 4H2O(g) → Fe3O4(s) + 4H2(g)

H2O(g) indicates that water is used in the form of steam.

Sometimes, reaction conditions (temperature, pressure, catalyst, etc.) are indicated above and/or below the arrow in the equation. For example:

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1. Introduction, Chemical Equations 2. Types of Chemical Reactions
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