Short Answer -Class 10 – Science -Chemistry -Chapter 1 – Chemical Reaction and Equation

Class 10 – Science -Chemistry -Short Answer -Chapter 1 – Chemical Reaction and Equation

Q1. In the electrolysis of water:

a. Name the gas collected at the cathode and anode respectively.

b. Why is volume of gas collected at one electrode double than that at the other? Name this gas.

c. How will you test this gas? [CBSE 2012]

Ans : a. Hydrogen is collected at the cathode, oxygen is collected at the anode.

b. It is because H2 O contains hydrogen and oxygen in the ratio 2 : 1.

c. Bring a burning matchstick near the gas, if the gas burns with ‘pop’ sound, the gas is H2 .

Q2. Write balanced chemical equations for the following reactions: [CBSE 2011]

a. Hydrogen sulphide gas burns in air to give water and sulphur dioxide.

b. Barium chloride reacts with zinc sulphate to give zinc chloride and barium sulphate.

c. Natural gas bums in air to form carbon dioxide and water.

$$H_{2}S(g) + 3O_{2}(g) \rightarrow  2H_{2}O(l) + 2SO_{2}(g)$$

$$BaCl_{2}(aq)+ZnSO_{4}(aq)\rightarrow BaSO_{4}(s)+ZnCl_{2}(aq)$$

$$CH_{4}(g)+2O_{2}\rightarrow CO_{2}(g)+2H_{2} O(l)$$

Q3. [CBSE 2011]

a. Why is respiration considered an exothermic reaction?

b. Define the terms oxidation and reduction,

c. Identify the substance that is oxidised and reduced in the following reaction:

$$CuO(s)+Zn(s)\overset{heat}{\rightarrow} Cu(s)+ZnO(s) $$

Ans :  a. In respiration, energy is released, therefore, it is considered an exothermic reaction.

b. Oxidation involves addition of O2 or loss of electrons.  Reduction involves addition of H2 or gain of electrons.

c. Zn is getting oxidised, CuO is getting reduced.

Q4 [CBSE 2011]

a. Can combination reaction be an oxidation reaction?

b. How will you test whether the gas evolved in a reaction is hydrogen?

 c. Why does copper not evolve hydrogen on reacting with dilute sulphuric acid?

 Ans :  a. Yes, combination reaction can be called a oxidation reaction.

b. Bring a burning splinter near the gas, if it burns with pop sound, it is hydrogen gas.

c. It is because copper is less reactive than hydrogen.

Q5. Write the balanced chemical equations for the following chemical reactions: a. Hydrogen + Chlorine \rightarrow  Hydrogen Chloride

b. Lead + Copper Chloride —-> Lead chloride + Copper

c. Zinc oxide + Carbon ——-> Zinc + Carbon Monoxide

Ans :

$$a. H2 (g) + Cl2 (g) \rightarrow 2HCl(g)$$

$$b. Pb(s) + CuCl2 (aq) \rightarrow PbCl2 (s) + Cu(s)$$

$$c. ZnO(s) + C(s) \rightarrow Zn(s) + CO(g)$$

Q6. An aqueous solution of metal nitrate ‘P’ reacts with sodium bromide solution to form yellow precipitate ‘Q’ which is used in photography. ‘Q’ on exposure to sunlight undergoes decomposition to form metal present along with a reddish brown gas. Identify ‘P’ and ‘Q’ write the balanced chemical equation for the chemical reaction. List the two categories in which reaction can be placed. [CBSE 2012]

Ans :  ‘P’ is silver nitrate (AgNO3 )

$$AgNO_{3}(aq) + NaBr(ag) \rightarrow  AgBr(s) + NaNO_{3}(aq)$$

This reaction is categorised as double displacement as well as precipitation reaction.

$$2AgBr(s) \overset{sunlight}{\rightarrow} 2Ag(s) + Br_{2}(g)$$

The above reaction is photochemical decomposition reaction.

 Q7. What is balanced chemical equation?

Why should chemical equation be balanced?

Ans. The reaction in which the number of atoms of each element is equal on the reactant side and product side is called balanced equation. Chemical reaction should be balanced because only a balanced equation tells us the relative quantities of different reactants and products involved in the reaction.

Q8. (a)Define Rusting

(b) Why do you apply paint an iron article?

 Ans. (a) The process of slow eating of the surface of metal iron when exposed to air for a longer period of time is called rusting.

(b)We apply paint on iron articles because it  forms a protective coating on the surface of iron and we can protect it against rusting.

Q9 (a) Can a displacement reaction be a redox reaction? Explain with the help of an example.

(b) Write the type of chemical reaction in the following:

(i) Reaction between an acid and a base

(ii) Rusting of iron.

Ans: Consider the following displacement reaction:

(a)  Zn(s)+ CuSO4(aq) → ZnSO4(aq) + Cu(s)

(b)  Here, Zn has changed into ZnSO4 (i.e., Zn2+ ions) by loss of electrons. Hence, Zn has been oxidised. CuSO4 (i.e., Cu2+) has changed into Cu by gain of electrons. Hence, CuSO4 has been reduced. Thus, the above reaction is a displacement reaction as well as a redox reaction.

(i) Neutralisation reaction     (ii) Oxidation reaction.

Q10. What happens when food materials containing fats and oils are left for a long time? List two observable changes and suggest three ways by which this phenomenon can be prevented.

Ans: Food materials containing fats and oils change their taste and smell due to a process called rancidity. Rancidity is a process in which air reacts with fats and oils which changes the smell and taste of food.

Methods of prevention:

Vacuum packing,

refrigeration of food materials,

placing of food materials, away from direct sunlight.

Q11. “We need to balance a skeletal chemical equation.” Give reason to justify the statement.

Ans:

Skeletal chemical equation is unbalanced. We need to balance chemical equation because of law of conservation of mass. It states that ‘matter can neither be created nor be destroyed’. Therefore, chemical equation must be balanced in each and every chemical reaction.

 Q12. What is rancidity? Mention any two ways by which rancidity can be prevented.

Ans: The process in which taste and smell of food gets spoiled is called rancidity. It happens due to oxidation.

Prevention from rancidity:

(i) Antioxidants are added to fatty acids to prevent oxidation, e.g. chips are packed in presence of nitrogen gas which prevents spoilage by oxidation.

(ii)Food should be kept in airtight container in refrigerator.

Q13. Write the chemical equation of the reaction in which the following changes have taken place with an example of each:

(i) Change in colour

(ii) Change in temperature

(iii) Formation of precipitate

Ans:.

(i)Cu (s) + 2AgNO3 (aq)———–> Cu(NO3)2(aq) + 2Ag

The solution will become blue in colour and shiny silver metal will be deposited.

(ii) NaOH + HCl ———–> NaCl +  H2O+ heat

The temperature will increase because heat will be evolved.

(iii) Pb(NO3)2 (aq) + 2KI (aq)———–> Pbl2 (s) + 2KNO3 (aq)

Yellow ppt

Yellow precipitate of Pbl2will be formed.