Chapter 7, Problem 30A

Interpretation Introduction

Interpretation :

While balancing a chemical equation change of the subscripts are not permitted. The reason must be explained. The effect of changing the subscripts must also be explained.

Concept Introduction :

A balanced chemical equation represents a chemical reaction in which number of atoms is same on both the side of the equation. This is done by changing the coefficients of the reactants and products.

Answer to Problem 30A

Change of number of subscripts is not permitted while balancing a chemical equation because that will change the chemical formula of the species.

Explanation of Solution

  ${\text{CaCl}}_{\text{2}}{\text{(aq)+AgNO}}_{\text{3}}\text{(aq)}\to \text{AgCl(s)}\downarrow {\text{+Ca(NO}}_{\text{3}}{\text{)}}_{\text{2}}\text{(aq)}\text{.}$

To balance the above equation the coefficients must be taken in such a way that number of each atoms become same without changing the formula of any species involved in the reaction.

The balanced chemical equation is given below.

  ${\text{CaCl}}_{\text{2}}{\text{(aq)+2AgNO}}_{\text{3}}\text{(aq)}\to \text{2AgCl(s)}\downarrow {\text{+Ca(NO}}_{\text{3}}{\text{)}}_{\text{2}}\text{(aq)}\text{.}$

Another way to balance the equation is given below which is not permitted.

  ${\text{CaCl}}_{\text{2}}{\text{(aq)+2AgNO}}_{\text{3}}\text{(aq)}\to {\text{Ag}}_{\text{2}}{\text{Cl}}_{\text{2}}\text{(s)}\downarrow {\text{+Ca(NO}}_{\text{3}}{\text{)}}_{\text{2}}\text{(aq)}\text{.}$

In this case formula of AgCl is changed to Ag₂Cl₂ which is not allowed.

Conclusion

Number of atoms is counted in both the side of an equation to check that a reaction is balanced.