Exemplar Questions and Solutions

Important CBSE Questions – Important Long Answer Questions

8)What is a chemical equation and what are the key features of a chemical equation?

Ans) A chemical equation is a symbolic representation of a chemical reaction using the chemical formulae of the reactants and the products involved in the reaction instead of using words. Chemical equations are a way of describing a chemical reaction.

Features of a chemical equation:
For example,

Here, A and B are the reactants and C and D are the products.

• The reactants are written on the left side of the arrow and the products are written on the right side.
• The arrow shows the direction of the reaction (the direction of change from reactants to products).
• The optimum conditions that are required for the reaction to take place are generally written above or below the arrow mark.
• The number of atoms of each element involved in the reaction should remain constant.
• The number of molecules of each chemical is represented by a numerical coefficient, which should be a whole number.
• An arrow pointing downward indicates the formation of a product in the form of a precipitate (↓) and an arrow pointing upward shows the release of a gaseous product (↑).
• The physical state of the chemicals are written near the formula by using short forms such as (s) ––– solid ; (l) ––– liquid ; (g)––– gas or vapor and (aq. ) ––– aqueous solution.

9)What is meant by a balanced chemical equation? What are the features of a balanced chemical equation?

A chemical equation in which the number of atoms of each element are same on the side of reactants and products, is called a balanced chemical equation

Features of a balanced chemical equation:

a)Balanced chemical equations give us an idea about the substances taking part in a chemical reaction (those being used up) and the products formed (those being made).

b)The coefficients of a balanced equation tell us in what ratio the substances react or are produced.

c)It also gives us the number of atoms, molecules or ions of the various reactants and products.

d)We can determine the actual masses of the reactants required in a chemical reaction from a balanced chemical equation. We can also estimate the expected amounts of the products weights of the reactants and products involved in the chemical reaction.

10)Give any three limitations of a chemical equation?

1. A chemical equation does not give us any information about the velocity (fast or slow) of the reaction.
2. The nature of the reaction
3. Some reactions occur with explosion. This is not indicated by the chemical equation.
4. It does not give any information about the completion of the reaction(whether the reaction is complete or incomplete)

When a small piece of sodium metal is added to beaker containing water, it catches fire and the solution in the beaker becomes warm. What does this indicate?

 Ans) Sodium metal reacts violently with water to form a colourless basic solution of sodium hydroxide and hydrogen gas along with the release of large amounts of energy in the form of heat. This heat is enough to cause hydrogen gas to catch fire. The formation of gaseous product(hydrogen) and the release of heat energy(Exothermic reaction) Which makes the solution in the beaker warm clearly indicate that a chemical reaction has taken place.

11)What are the rules that we need to follow while writing balancing a chemical equation?

Ans) The following rules are to be strictly followed while balancing a chemical equation:

1. While balancing a chemical equation, we should never alter the formulae of the reactants and products.
2. The formulae of the reactants and products should be in their molecular form and not in the atomic form
   For eg.,
   KClO3  →  KCl + 3O
   This equation is an atomic equation as oxygen is represented in the form of atoms and not molecules. The correct or the molecular form is
   2KCLO₃   →  2KCl + 3O₂
3. Appropriate numbers must be written before the formulae of the reactants and products.
4. A chemical equation should be balanced by using the smallest possible numbers as coefficients of reactants and products.

When silver is kept open in air that contains even traces of hydrogen sulphide, it gets tarnished or turns black. Silver reacts readily with all sulphur-containing compounds like the white of egg (which has sulphur-containing proteins) to form black silver sulphide . The equation of blackening/tarnishing of silver ornaments is represented by

Ag(s) + H₂S(g) + O₂ (g)   →  Ag₂S(s) + H₂O(l). Balance the given equation.

Ans) The skeletal equation of blackening/tarnishing of silver ornaments is

  Ag(s) + H₂S(g) + O₂ (g)  →  Ag₂S(s) + H₂O(l).

First, balance Ag atoms

2Ag(s) + H₂S(g) + O₂(g)  →  Ag₂S(s) + H₂O(l)

Balance oxygen atoms by placing the coefficient 2 before H₂O

2Ag(s) + H₂S(g) + O₂(g)   →  Ag₂S(s) + 2H₂O(l)

Balance hydrogen atoms by placing the coefficient 2 before H₂S

2Ag(s) +2H₂S(g) + O₂(g)  → Ag₂S(s) + 2H₂O(l)

Balance sulphur atoms by placing the coefficient 2 before Ag₂S

2Ag(s) +2H₂S(g) + O₂(g)  →  2Ag₂S(s) + 2H₂O(l)

Again balance the silver atoms by replacing the coefficient 2 before Ag with the coefficient 4

4Ag(s) +2H₂S(g) + O₂(g)  → 2Ag₂S(s) + 2H₂O(l)

This is now a completely balanced equation.