## Balancing of Chemical Equations :

**Balancing : **

The process of making atoms of various elements equal in an equation on either side is called balancing.

**Balancing an Equation**

The first step in balancing an equation is to count the number of atoms of each element on both sides of the equation. For example, reactants X and Y2 react to form a compound XY. The word equation for this reaction would be

X + Y_{2} ——————————– ► XY

The number of atoms of elements X and Y in the above-mentioned equation is shown below.

Element | Number of atoms in LHS | Number of atoms in RHS |

X | 1 | 1 |

Y | 2 | 1 |

To balance Y on both sides, multiply RHS by 2, i.e.,

X + Y_{2} —————————— ► 2XY

Now, the number of atoms of Y is balanced but not the number of atoms of X. Therefore, multiply X on the LHS by 2. Thus, the equation becomes

2X + Y_{2} —————————— ► 2XY

This is a balanced equation as the number of atoms of X and Y on both sides is equal.

Keeping these steps in mind, let us now write the chemical equation for the formation of magnesium oxide.

Step 1: Magnesium burns in oxygen to give magnesium oxide. Here, the reactants are magnesium and oxygen. The product is magnesium oxide.

Step 2: Thus, the word equation is

Magnesium + Oxygen —► Magnesium oxide

Step 3: Replacing the names with symbols and formulae, we get the chemical equation as

Mg + O_{2} —► MgO

Step 4: The number of atoms of the elements are

Element | Number of atoms in LHS | Number of atoms in RHS |

Magnesium | 1 | 1 |

Oxygen | 2 | 1 |

To balance oxygen on both sides, multiply RHS by 2, i.e.,

Mg + O_{2} ———————- ► 2MgO

Now, the number of oxygen atoms is balanced but the number of magnesium atoms is not. Therefore, multiply magnesium on the LHS by 2. Thus, the equation becomes

2Mg + O_{2} ———————- ► 2MgO

This is the balanced chemical equation.

Observe the following two chemical equations :

Zn + H_{2}SO_{4} → ZnSO_{4} + H_{2} …..(i)

Na + H_{2}O → NaOH + H_{2} …..(ii)

In equation (i), the number of atoms of Zn, H, S and O are equal on both sides, i.e., the equation is balanced

**Balanced Equations : **The equations in which atoms of various elements on the reactants’ and the products side are equal.

Equation (ii) is not balanced because the number of hydrogen atoms in not equal on both sides. It is called a skeleton chemical equation.

**Reason of Balancing Equations : **

The number of atoms of elements on both sides of a chemical equation should be equal in accordance with the law of conservation of mass.

**How to balance chemical equations step by step :**

A number of steps are involved in balancing a chemical equation,

Example 2: Na + H_{2}O → NaOH + H_{2}

**Step-1 :** Examine the number of atoms of different elements present in unbalanced equations.

Element | Number of atoms in Reactants | Number of atoms in Products |

Na | 1 | 1 |

H | 2 | 3 |

O | 1 | 1 |

**Step-2 :** Pick an element to balance the equation. In the above equation Na and O are balanced, Hydrogen is not.

Step : To balance Hydrogen on both sides we need to multiply H_{2}O by 2 which makes Hydrogen atoms equal to 4 on the reactants’ side. To make Hydrogen 4 on the products’ side, multiply NaOH by 2. Now oxygen has become 2 on both side. But Sodium atoms has become two on the products’ side. Multiply Na by 2 on the reactants side so that they become equal on both side. The steps are as follows :

(i) Na + 2 H_{2}O → NaOH + H_{2}

(ii) Na + 2 H_{2}O → 2NaOH + H_{2}

(iii) 2 Na + 2 H_{2}O → 2NaOH + H_{2}

The equation is now balanced.

**Example 2:** Fe + H_{2}O → Fe_{3}O_{4} + H_{2}

**Step-1 :**

Element | Number of atoms in Reactants | Number of atoms in Products |

Fe | 1 | 3 |

H | 2 | 2 |

O | 1 | 4 |

**Step-2** **: **Pick up the compound which has the maximum number of atoms whether a reactant or a product, and in that compound select the elements which has the highest number of atoms, e.g., we select Fe_{3}O_{4} in the above equation :

To balance oxygen atoms,

In reactants | In products | |

Initial To balance | 1 (in H_{2}O)1 × 4 | 4 (in Fe 4 × 1 |

To equalise the number of atoms, we put the coefficient on the left side of the formula.

A coefficient is a small whole number, like coefficients used in algebraic equations.

You must keep in mind that we can put coefficients but we cannot change the subscripts in the formula, i.e., to balance Oxygen atoms, we can put the coefficient 4 as 4 H_{2}O and not H_{2}O_{4} or (H_{2}O)_{4}. Now the partly balanced equation becomes as follows :

Fe(s) + 4 H_{2}O(g) → Fe_{3}O_{4}(s) + H_{2}(g)

(Partly balanced)

**Step-3 : **Pick up the second element to balance this partly balanced equation. Let us try to balance hydrogen atoms.

In partly balanced equation. Atoms of Hydrogen.

In reactants | In products | |

Initial To balance | 8 (in 8 H_{2}O)8 × 1 | 2 (in H 2 × 4 |

To equalise the number of Hydrogen atoms, we use 4 as the coefficient of H_{2} in the products.

Fe(s) + 4 H_{2}O(g) → Fe_{3}O_{4}(s) + 4 H_{2}

**Step-4 :** Pick up third element to be balanced. The element which is left to be balanced is Fe.

In reactants | In products | |

Initial To balance | 1 (in Fe) 1 × 3 | 3 (in Fe 3 × 1 |

To equalise, we use 3 as coefficient of Fe in reactants.

3Fe + 4H_{2}O → Fe_{3}O_{4} + 4H_{2}

Atoms | In reactants | In products |

Fe H O | 3 8 4 | 3 8 4 |

The equation is balanced because atoms of all the elements are equal on both sides.

This method of balancing equation is known as hit and trial method.