Chemical Reactions And Equations

Which of the following is not a physical change?

• Boiling of water to give water vapour

• Melting of ice toto give water

• Dissolution of salt in water

• Combustion of Liquefied Petroleum Gas (LPG)

Answer explanation

Answer:

Combustion of Liquefied Petroleum Gas (LPG) is not a physical change. It is a chemical change because combustion of LPG produces new substances. A lot of heat is produced along with carbon dioxide and water vapour during this reaction. It is also irreversible in nature.

The following reaction is an example of a

4NH3 (g) +5O2 (g) → 4NO (g) +6H2O (g)

1. displacement reaction

2. combination reaction

3.redox reaction

4. neutralisation reaction

• 1 and 4

• 2 and 3

• 1 and 3

• 3 and 4

Answer explanation

This is a redox reaction because oxidation and reduction both are happening simultaneously. It is also a displacement reaction because H of NH3 is displaced by oxygen. A displacement reaction is also a redox reaction as it is always followed by oxidation as well as reduction.

Which of the following statements about the given reaction is correct?

3Fe(s) + 4H2O→ Fe3O4 + 4H2

a. Iron metal is getting oxidised.

b. Water is getting reduced.

c. Water is acting as reducing agent.

d. Water is acting as oxidising agent.

• A, B and D

• A, C and D

• B, C and D

• C, B, A

Answer explanation

Iron is getting oxidized and forming ferrous ferric oxide (Fe3O4)4 Water molecule is getting reduced to hydrogen ions. Water is acting as an oxidizing agent by donating its oxygen atom to Fe.

Which of the following are exothermic processes?

1. Reaction of water with quick time

2. Dilution of an acid

3. Evaporation of water

4. Sublimation of camphor (crystals)

• 1 and 2

• 2 and 3

• 1 and 4

• 3 and 4

Answer explanation

When quick lime reacts with water, calcium hydroxide is produced and a large amount of heat is released. Similarly, when an acid or a base is diluted with water, heat is released making it a highly exothermic reaction.

Three beakers labelled as A, B and C each containing 25 mL of water were taken. A small amount of Na0H, anhydrous CuSO2 and NaCl were added to the beakers A, B and C respectively. It was observed that there was an increase in the temperature of the solutions contained in beakers A and B, whereas in case of beaker C, the temperature of the solution falls. Which one of the following statement (s) is (are) correct?

1. In beakers A and B, exothermic process has occurred.

2. In breakers A and B, endothermic process has occurred.

3. In breaker C, exothermic process has occurred.

4. In beaker C, endothermic process has occurred.

• Only 1

• Only 2

• 1 and 4

• 2 and 3

Answer explanation

Chemical reaction in beakers A and B gives out heat. Hence the temperature of the solution increases. Thus, we can call it an exothermic reaction. In beaker C, heat is absorbed from water instead and hence the temperature decreases. Thus, we can call it

an endothermic process.

A dilute ferrous sulphate solution was gradually added to the beaker containing acidified permanganate solution. The light purple colour of the solution fades and finally disappears. Which of the following is the correct explanation for the observation?

• KMnO4 is an oxidising agent, it oxidises FeSO4

• FeSO4 acts as an oxidizing agent and oxidizes KMnO4

• The color disappears due to dilution; no reaction is involved

• KMnO4 is an unstable compound and decomposes in presence of FeSO4 to a colorless

  compound.

Answer explanation

Which among the following are double displacement reactions?

(i) Pb + CuCl2 PbCl2+Cu

(ii) Na2SO4+BaCl2 BaSO4+2NaCl

(iii) C+O2 CO2

(iv) CH4+2O2 CO2+2H2O

• (i) and (iv)

• Only (ii)

• (i) and (ii)

• (iii) and (iv)

Answer explanation

Double displacement reactions are such chemical reactions in which one component of each reactant exchange to form products. The cations and anions of two compounds exchange their ions to form two new compounds. Here only in reaction (ii), ions of both reactants are exchanged and two entirely different compounds are formed.

Solid calcium oxide reacts vigorously with water to form calcium hydroxide accompanied by the liberation of heat. This process is called slaking of lime water. Which among the following is/are true about slaking of lime water and the solution formed?

(i) It is an endothermic reaction.

(ii) It is exothermic reaction.

(iii) The pH of the resulting solution will be more than seven.

(iv)The pH of the resulting solution will be less than seven.

• (i) and (ii)

• (ii) and (iii)

• (i) and (iv)

• (ii) and (iv)

Answer explanation

Slaking of lime is an exothermic reaction because a large amount heat is produced during the reaction. The heat can be felt by touching the beaker from outside. The resulting compound Ca(OH)2, which is also called slaked lime, turns red litmus solution to blue and hence proves it to be a basic solution. Thus, the pH of this solution will be more than