Exemplar Questions and Solutions

Important CBSE Questions – Important Long Answer Questions

12)In photosynthesis, carbon dioxide gas and water are taken up by plants to form glucose and oxygen gas is liberated. Write a chemical equation to represent this statement and balance it(Formula of glucose is C₆H₁₂O₆).

Ans) The skeletal equation for the process of photosynthesis is-

CO₂(g) + H₂O(l)  →   C₆H₁₂O₆(aq)  + O₂(g)

Let us first balance the carbon atoms. For that let us place 6 before carbon di oxide as glucose(C₆H₁₂O₆) contains 6 carbon while CO₂ contains only one carbon atom

6 CO₂(g) + H₂O(l)  →  C₆H₁₂O₆(aq)   + O₂(g)

Now that carbon is balanced, let us try to balance the hydrogen atoms. As we know glucose contains 12 hydrogen atoms while water(H₂O) has only 2. So by placing the coefficient 6 before water molecules, we can balance the hydrogen atoms.

6 CO₂(g) + 6 H₂O(l) → C₆H₁₂O₆ (aq)  + O₂(g)

Let us now balance the oxygen atoms, we can find that there are 18 oxygen atoms on the LHS(reactants, ie., 6x 2 + 6= 18) while RHS has only 8 oxygen atoms, so by putting the coefficient 6 in front of O₂ (RHS) we get,

6 CO₂(g) + 6 H₂O(l) → C₆H₁₂O₆ (aq) + 6O₂(g)

Now, we can say that the above equation is balanced as we have equal number of C, H, and O atoms on LHS and RHS.

13)Write the difference between exothermic and endothermic:

Exothermic reaction	Endothermic reaction
an exothermic reaction expels heat	an endothermic reaction pulls heat into an object or area
Exothermic Reactions make their surroundings hotter	Endothermic Reactions Cool their Surroundings
Products of exothermic reactions have less energy than the reactants	Products of endothermic reactions have more energy than the reactants
Exothermic reactions are spontaneous reactions	Endothermic reactions are comparatively slower
Rusting of iron, combustion, formation of ice cubes, respiration, etc.	photosynthesis, evaporation, cooking of an egg, baking bread, etc

14)Give an example of a decomposition reaction. Describe an activity to illustrate such a reaction by heating.

Ans) When lead(II) nitrate is heated, it decomposes into lead (II) oxide, nitrogen dioxide and oxygen. The balanced chemical equation for this reaction is 

Activity– Take about 3 g of solid lead nitrate in a boiling tube. Note the colour of the compound(Colourless). Heat it in the flame of the Bunsen burner. Observe the change taking place.

Observation– A pungent smelling brown gas (NO₂) starts evolving from it and a brown residue (PbO) is left in the test tube.

Inference– Here, lead nitrate breaks down or decomposes to form three new substances (lead (II) oxide, nitrogen dioxide and oxygen). Hence, it is an example of decomposition reactions. Decomposition reactions often involve an energy source such as heat, light, or electricity that breaks apart the bonds of compounds. In case of lead nitrate, heat is used. So, thermal decomposition has taken place.

15) Explain the trial and error method of balancing a chemical equation with an example?

Ans) In the trial and error method of balancing, there are no definite rules that need to be followed. This method is applicable for simple chemical reaction. Some simple tips that are to be kept in mind while balancing are-

• Determining the reactants and products in a reaction.
• Writing down the symbols and formulae of the reactant and products in the form of a skeletal equation.
• Any elementary gas (O₂, H₂, N₂ ,F₂ and Cl₂) appearing on either side of the skeletal equation, is written in the atomic state.
•  Counting the number of atoms of each on both sides.
• Selecting the elements that occur for the least number of times in the equation.
•  Changing the coefficient of the molecules of reactants or products as required.
• Atoms of elementary gases are balanced at the last.
• When the balancing is complete, the equation is converted to the molecular form by multiplying the whole equation by 2.

16)Balance the chemical equation, Hydrogen and oxygen React to form water using it and trial method

Ans)

• Determining the reactants and products in a reaction. Here Hydrogen and oxygen are reactants. Water is the product.
• Skeletal equation. 

                       H₂ +O₂   →  H₂O

• Start counting the number of atoms each on both sides. We have 2 hydrogen atoms on RHS and LHS whereas we have 2 oxygen atoms on the LHS and one on the RHS.
• Start balancing O atoms.   LHS contains 2 and RHS has 1. So, Oxygen atoms need to be balanced.
  Changing the coefficient of the molecules of reactants or products as required. Balance the oxygen atoms by placing coefficient 2 in front of  H₂O.
• Now there are 2 oxygen atoms on the reactant side and 2 oxygen atoms on the products side.
  H₂ +O₂  →  2 H₂O
• But the number of hydrogen atoms are not balanced now. To balance hydrogen atoms place the coefficient 2 on the left-hand side of hydrogen in reactants side.
  2H₂ +O₂   →   2 H₂O
• Thus, the atoms of each element on the left and right hand side of the equations are balanced according to the law of conservation of mass

17)Balance the following chemical equations and identify the type of chemical reaction:

a)Al(s) + Cl₂(g)   →  AlCl₃(s)

Ans) 2Al(s) + 3Cl₂(g)  →  2AlCl₃(s). It is a combination reaction

b)

Ans)

This is an example of thermal decomposition reaction

c)CaO(s) + SiO₂(s)   →   CaSiO₃(s)

Ans) CaO(s) + SiO₂(s)  →  CaSiO₃(s). This is a balanced equation and this reaction is an example of combination reaction

d)

Ans)

This is an example of photolytic decomposition reaction.

e)

Ans)

This is a balanced reaction. It is an example of double displacement reaction and precipitation reaction.