WBBSE Solutions For Class 8 School Science Chapter 2 Element Compound And Chemical Reaction Short Answer Type Questions

Chapter 2 Element Compound And Chemical Reaction Chemical Reaction SAQs

Question 1. What do you mean by a chemical reaction? What are reactants and products? Describe a suitable example.
Answer:

Chemical Reaction:

A chemical reaction means a permanent rearrangement or regrouping between the atoms or radicals of the combining substances (i.e., the reactants) to form one or more substances of different properties.

Chemicals participating in a reaction are called reactants.

Chemical substances which are produced due to a permanent rearrangement or regrouping between the reactants are called products.

For example, \( \mathrm{N}_2+3 \mathrm{H}_2 \rightarrow 2 \mathrm{NH}_3\) . In this reaction, nitrogen and hydrogen are reactants, and due to the reaction between them, ammonia is produced. The chemical properties of ammonia are different from nitrogen and hydrogen.

Question 2. What are the factors we need to induce, influence and regulate chemical changes?
Answer:

The factors we heed to induce, influence and regulate chemical changes are physical contact, heat, light, pressure, the presence of ‘solvent, electricity and catalyst.

Question 3. Why heat is needed to increase the rate of a chemical reaction?
Answer:

Heat is a form of energy. So when heated, the reactants gain energy. The kinetic energy of the reactant molecules is increased. These energized substances then react at a much faster rate.

Question 4. Give one example to show that a chemical reaction is initiated when the reactants are heated.
Answer:

Heat is one of the factors which is needed to initiate a chemical reaction. For example, when blue crystals of hydrated cupric nitrate are heated, the evolution of brown fumes of nitrogen dioxide (NO₂)takes place and a black residue of cupric oxide (CuO) is left. In absence of heating no such reaction occurs,

⇒ \(\begin{gathered}
\text { heat } \\
2 \mathrm{Cu}\left(\mathrm{NO}_3\right)_2 \rightarrow 2 \mathrm{CuO}+4 \mathrm{NO}_2+\mathrm{O}_2
\end{gathered}\)

Question 5. Give one example to show a chemical reaction which is initiated by pressure.
Answer:

Industrially ammonia gas is produced from nitrogen and hydrogen gas at a pressure as high as 200 times the normal atmospheric pressure.

⇒ \(\mathrm{N}_2+3 \mathrm{H}_2 \rightarrow 2 \mathrm{NH}_3\)

Question 6. Give one example to show that a chemical reaction which is initiated by sound.
Answer:

Sound is sometimes a factor which initiates a chemical reaction. For example, when a loud sound is made near a gas jar filled with acetylene gas (C₂H₂), the gas dissociates into carbon and hydrogen.

⇒ \(\mathrm{C}_2 \mathrm{H}_2 \rightarrow 2 \mathrm{C}+\mathrm{H}_2\)

Question 7. Give one example to show a chemical reaction which is initiated by electricity.
Answer:

If electricity is passed through water (through two platinum electrodes), containing a little amount of sulphuric acid, then hydrogen and oxygen gas are separately evolved at two electrodes. In this case, the water undergoes electrolysis. The dissociation of water is achieved by passing electricity through it.

⇒ \(2 \mathrm{H}_2 \mathrm{O} \rightarrow 2 \mathrm{H}_2+\mathrm{O}_2\)

Question 8. In a photography shop, there is a dark room. Why?
Answer:

In a photography shop, there is a dark room

In film cameras, photographic films are used. Films are coated with various substances which remain unchanged when stored in dark.

When exposed to light, the photo-sensitive substances undergo photochemical reactions. To avoid exposure to light, a dark room is necessary.

Question 9. Give a suitable example to indicate the significance of the presence of a solvent during a chemical reaction.
Answer:

The presence of solvent is very important for many of the reactions.

For example, when dry baking soda (NaHCO₃) and dry crystals of tartaric acid are mixed, no visible change occurs, indicating no reaction is taking place.

But when an aqueous solution of baking soda and an aqueous solution of tartaric acid are mixed with each other, bubbles of carbon dioxide evolved from the reaction mixture.

NaHCO₃ and tartaric acid are both soluble in water. When they are dissolved in water, then sufficient contact between the reactants is achieved and the reaction takes place.

Question 10. If ether or benzene is used as a solvent during the reaction between baking soda and tartaric acid, no reaction takes place. Why?
Answer:

If ether or benzene is used as a solvent during the reaction between baking soda and tartaric acid, no reaction takes place.

Baking soda is an “ionic compound”, and it produces ions when dissolved in water. Ionic substances are soluble in water but not in organic solvents like benzene, ether, etc. So, when organic solvents like benzene are used as the solvent, baking soda remains insoluble, and hence no reaction takes place.

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Question 11. What is meant by a catalyst and a catalytic reaction?
Answer:

Catalyst And a catalytic reaction:

The substance which can influence (i.e., increase or decrease) the rate of a chemical reaction is called a catalyst There are certain catalysts which slow down the rate of a particular chemical reaction. They are termed negative catalysts. The chemical reaction which involves the use of a catalyst is called a catalytic reaction.

Question 12. What is an enzyme?
Answer:

Enzyme:

Enzymes are biological molecules that act as catalysts and help complex reactions occur. The enzymes are basically proteins, but they may be associated with non-protein substances (known as coenzymes or prosthetic groups) that are essential for the action of the enzyme.

Question 13. Why do we get a smell of ammonia in the urinal?
Answer:

Urea (N H₂CON H₂) is present in the urine of mammals. Microbes in urinals decompose urea present in urine to produce ammonia and carbon dioxide, and that is why we get the smell of ammonia in the urinal.

⇒ \(\mathrm{NH}_2 \mathrm{CONH}_2+\mathrm{H}_2 \mathrm{O} \rightarrow 2 \mathrm{NH}_3+2 \mathrm{CO}_2\)

Question 14. Why is the use of protective goggles strongly recommended during welding?
Answer:

Burning of acetylene gas (C₂H₂) produces so much heat that temperature may rise above 2000°C and at this temperature, even iron melts. So this exothermic reaction is utilized for welding.

⇒ \(\mathrm{C}_2 \mathrm{H}_2+5 \mathrm{O}_2 \rightarrow 4 \mathrm{CO}_2+2 \mathrm{H}_2 \mathrm{O}+\text { heat }\)

At this high temperature, light is emitted which contains a significant amount of ultraviolet (UV) radiation which is very harmful to our eyes. So use of a proper spectacle or protective goggles which can shield our eyes from UV radiation is strongly recommended during welding.

Question 15. When water is added to calcium oxide, then steam is evolved. Why?
Answer:

When water is added to calcium oxide, then steam is evolved.

When water is added to calcium oxide (CaO), calcium hydroxide is produced. This is an exothermic reaction. A large amount of heat is liberated and this heat is sufficient to vaporize water. That is why steam evolved from the reaction mixture.

⇒ \(\mathrm{CaO}+\mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{Ca}(\mathrm{OH})_{2-}+\text { heat }(\mathrm{Q})\)

Question 16. How can you recognize a process as a chemical reaction?
Answer:

Primarily if:

1. something is precipitated;
2. some change of colour of the reaction mixture occurs and
3. heat is absorbed or evolved, then we may conclude that a chemical change has taken place.

Question 17. Name three industrial processes where catalysis finds an application. Also, mention the names of the catalysts used.
Answer:

 

 

Question 18. Why is the layer of rust on an iron article unable to protect the underlying surface?
Answer:

Layer of rust on an iron article unable to protect the underlying surface:

The main constituent of rust is hydrated Ferric oxide (Fe₂O₃nH₂O). Moreover, CO₂ is produced in the process of rusting which itself accelerates more rusting in the presence of O₂ and H₂O

Thus a layer of rust cannot prevent further rusting of the underlying surface the brown coating is thickened and iron becomes brittle.

Question 19. Give one example of a redox reaction which is also
Answer: 

Example of a redox reaction which is also

1. A combination reaction
2. A displacement reaction

1. A combination reaction:

 

 

2. A displacement reaction:

 

Question 20. The same substance may act both as an oxidising and a reducing agent-give an example to establish the phenomenon.
Answer:

The same substance may act both as an oxidising and a reducing agent

SO₂ is commonly regarded as a good reducing agent.

It reduces a purple (acidified) solution of potassium permanganate to an almost colourless manganous salt and is itself oxidised to sulphuric acid.

 

 

But during its reaction with hydrogen sulphide, it acts as an oxidising agent and oxidises H₂S to S. In this reaction, SO₂ itself is reduced to sulphur.

Hence, in different reactions, the same substance may sometimes play the role of an oxidant and sometimes that of a reductant.