Class 10 Science

# Chemical Equation

Writing a chemical reaction in sentences can be cumbersome. There is an easier way to write chemical reaction and that is through chemical equation.

Chemical equation can be written as word equation or by using symbols of elements. Following is the word equation of combustion of magnesium.

Magnesium + Oxygen → Magnesium oxide

Word equation can be lengthy and cumbersome. So, the better way to write a chemical equation is by using symbols of elements. Above equation can be written as follows:

Mg + O2 → MgO

Substances written on the LHS (Left Hand Side) of the arrow are called reactants, as they participate in chemical reaction. Substances written on the RHS are called products, as they are made after a chemical reaction.

## Reactant and Product:

Reactant: Substances which take part in a chemical reaction are called reactants.

Product: New substance formed after a chemical reaction is called product.

Example: A + B → C + D

In this equation, A and B are called reactants and C and D are called the products. Arrow shows the direction of chemical reaction. Condition, if any, is written generally above the arrow.

Example: When hydrogen reacts with oxygen, it gives water. This reaction can be represented by following chemical equation.

Hydrogen + Oxygen → Water

H2 + O2 → H2O

In first equation words are used and in second symbols of substances are used to write the chemical equation. For convenience, symbol of substance is used to represent chemical equations. In this case, hydrogen and oxygen are reactants, while water is the product.

Example: When calcium reacts with water, calcium hydroxide and hydrogen are formed. In this reaction, calcium and water are reactants, while calcium hydroxide and hydrogen are products.

Calcium + Water → Calcium Hydroxide + Hydrogen

Ca + 2H2O → Ca(OH)2 + H2

Chemical Equation is a way to represent the chemical reaction in concise and informative way.

## Balanced Chemical Equation

Let us closely observe the number of atoms of elements on LHS and RHS in above equation. The number of magnesium atoms is 1 on both sides of arrow. But the number of oxygen atoms is 2 on RHS and 1 on LHS. Where did the remaining 1 atom of oxygen go?

As per Law of Conservation of Mass, we cannot create or destroy mass. In order to obey the law of conservation of mass, the number of atoms of elements on RHS needs to be equal to that of elements on LHS. When these numbers are equal on both sides of an equation, the equation is said to be a balanced equation.

Let us take another example which is as follows:

Zinc + Sulphuric Acid → Zinc Sulphate + Hydrogen

Zn + H2SO4 → ZnSO4 + H2

In this equation, let us compare the number of atoms of each element on both sides of equation. To make it easier, let us put the numbers in a table.

ElementNumber of atoms on LHSNumber of atoms on RHS
Zn11
H22
S11
O44

You can notice that number of atoms of each element is equal on both sides of equation. This is a balanced chemical equation, as it obeys the law of conservation of mass.

### Calculation:

In this example, mass of zinc is 65 g and mass of sulphuric acid is 98 g.

H2SO4 = 2 × 1 + 32 + 4 × 16 = 2 + 32 + 64 = 98

So, total mass of reactants is 163 g.

Now, mass of zinc sulphate is 161 g and that of hydrogen is 2 g.

So, total mass of products is 163 g. Thus, mass of reactants is equal to mass of products.

So, this equation obeys the Law of Conservation of mass.

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 product, then the chemical equation is called unbalanced chemical equation.

Example: Fe + H2O → Fe3O4 + H2

In this example, 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.

### Calculation:

In this example, mass of Fe is 56 g and that of water is 18 g. So, total mass of reactants is 74 g.

Now, mass of iron oxide is 231 g and that of hydrogen is 2 g.

Mass of Iron Oxide (Fe3O4) = 3 × 56 + 4 × 16 = 168 + 64 = 232 g

Now, total mass of reactants is 232 + 2 = 234 g. So, this equation does not obey the law of conservation of mass.

Note: While balancing an equation, you don't need to calculate atomic masses. These calculations are given to clear your fundamentals. You just need to count the number of atoms of various elements and match them on both sides of an equation.

## Summary

Chemical Reaction: Chemical change is called chemical reaction.

Chemical Equation: Symbolic representation of chemical reaction is called chemical equation.