CBSE Class 10 Science HOTs Chemical Reactions and Equations Set 04

Case Based Questions

Read the passage given below and answer the following questions:
In decomposition reactions, a single reactant breaks down to form two or more products. Decomposition reaction is opposite to combination reaction. Thermal decomposition reactions use the energy in form of heat for decomposition of reactants. Electrolytic decomposition reactions involve the use of electrical energy for the decomposition of reactant molecules. Photolysis or photochemical decomposition involves the use of light energy for the purpose of decomposition.

Question. Give one example of decomposition reaction.
Answer: \( CaCO_{3(s)} \xrightarrow{\Delta} CaO_{(s)} + CO_{2(g)} \)

Question. \( 2Pb(NO_3)_2 \rightarrow 2PbO + nA + O_2 \) Assuming n as coefficient and A as product, what is nA in the given reaction?
Answer: \( nA = 4NO_2 \)

Question. Amino acid is formed by the decomposition of which component of our diet? Is decomposition reaction exothermic or endothermic?
Answer: Proteins in our diet get broken down into amino acids. All decomposition reactions are endothermic as heat is used up to break down the reactant.

Question. OR
What happens when AgCl is exposed to sunlight? What type of chemical reaction takes place when electricity is passed through water?
Answer: Photodecomposition of AgCl takes place: \( 2AgCl_{(s)} \xrightarrow{Sunlight} 2Ag_{(s)} + Cl_{2(g)} \). Electrolysis of water is an electrolytic decomposition: \( 2H_2O \xrightarrow{Current} 2H_2 + O_2 \)

Read the passage given below and answer the following questions :
Oxidation has damaging effect on metals as well as on food. The damaging effect of oxidation on metal is studied as corrosion and that on food is studied as rancidity. The phenomenon due to which metals are slowly eaten away by the reaction of air, water and chemicals present in atmosphere, is called corrosion. For example, iron articles are shiny when new, but get coated with a reddish brown powder when left for sometime. This process is known as rusting of iron. Rancidity is the process of slow oxidation of oil and fat (which are volatile in nature) present in the food materials resulting in the change of smell and taste in them.

Question. Rancidity can be prevented by
(a) adding antioxidants
(b) packaging oily food in nitrogen gas
(c) both (a) and (b)
(d) None of the options
Answer: (c) both (a) and (b)

Question. Combination of phosphorus and oxygen is an example of
(a) oxidation
(b) reduction
(c) decomposition
(d) None of the options
Answer: (a) oxidation

Question. A science teacher wrote the following statements about rancidity :
(I) When fats and oils are reduced, they become rancid.
(II) In chips packet, rancidity is prevented by oxygen.
(III) Rancidity is prevented by adding antioxidants.
Select the correct option.
(a) (I) only
(b) (II) and (III) only
(c) (III) only
(d) (I), (II) and (III)
Answer: (c) (III) only

Question. Two statements are given below regarding rusting of iron.
(I) The rusting of iron is a redox reaction and reaction occurs as, \( 4Fe + 3O_2 \rightarrow 4Fe^{3+} + 6O^{2-} \)
(II) The metallic iron is oxidised to \( Fe^{2+} \) and \( O_2 \) is reduced to \( O^{2-} \).
Select the correct statement(s).
(a) I only
(b) II only
(c) Both I and II
(d) None of the options
Answer: (a) I only

Question. Which of the following measures can be adopted to prevent or slow down rancidity?
(I) Food materials should be packed in air tight container.
(II) Food should be refrigerated.
(III) Food materials and cooked food should be kept away from direct sunlight.
(a) Only II and III
(b) Only I and III
(c) Only I and II
(d) I, II and III
Answer: (d) I, II and III

A & R Questions

In the following questions a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following.
(a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
(b) Assertion and reason both are correct statements but reason is not correct explanation for assertion.
(c) Assertion is correct statement but reason is wrong statement.
(d) Assertion is wrong statement but reason is correct statement.

Question. Assertion : Zinc reacts with sulphuric acid to form zinc sulphate and hydrogen gas and it is a displacement reaction.
Reason : Zinc reacts with oxygen to form zinc oxide.
(a) A
(b) B
(c) C
(d) D
Answer: (b) Assertion and reason both are correct statements but reason is not correct explanation for assertion.

Question. Assertion : In a balanced chemical equation, total mass of the reactants is equal to the total mass of the products.
Reason : Mass can neither be created nor destroyed during a chemical change.
(a) A
(b) B
(c) C
(d) D
Answer: (a) Assertion and reason both are correct statements and reason is correct explanation for assertion.

Question. Assertion : Rusting of iron metal is the most common form of corrosion.
Reason : The effect of rusting of iron can be reversed if they are left open in sunlight.
(a) A
(b) B
(c) C
(d) D
Answer: (c) Assertion is correct statement but reason is wrong statement.

Question. Assertion : Iron articles are painted so as to prevent them from rusting.
Reason : When the surface of iron is coated with paint, its surface does not come in contact with oxygen and moisture therefore rusting does not take place.
(a) A
(b) B
(c) C
(d) D
Answer: (a) Assertion and reason both are correct statements and reason is correct explanation for assertion.

Question. OR
Assertion : When calcium carbonate is heated, it decomposes to give calcium oxide and carbon dioxide.
Reason : The decomposition reaction takes place on application of heat, therefore, it is an endothermic reaction.
(a) A
(b) B
(c) C
(d) D
Answer: (b) Assertion and reason both are correct statements but reason is not correct explanation for assertion.

Multiple Choice Questions

Question. Consider the following reaction :
\( 2H_2S_{(g)} + SO_{2(g)} \rightarrow 2H_2O_{(l)} + 3S_{(s)} \)
(I) and (II) are respectively
(a) oxidation and reduction
(b) reduction and oxidation
(c) oxidation and oxidation
(d) reduction and reduction.
Answer: (b) reduction and oxidation

Question. Which reaction does not produce carbon dioxide gas?
(a) Burning of fuel
(b) Reacting baking soda with vinegar
(c) Thermal decomposition of sugar
(d) Respiration
Answer: (c) Thermal decomposition of sugar

Question. Which of the following informations about the reaction of quick lime with water is not true?
(a) Quick lime reacts with water vigorously.
(b) During reaction the test tube becomes hot.
(c) Quick lime reacts with water to form slaked lime.
(d) During the reaction dazzling white light is produced.
Answer: (d) During the reaction dazzling white light is produced.

Question. Which of the following statements is correct?
Rusting of iron is a chemical change because
(a) a new substance with new properties is produced
(b) chemical composition of reactant is changed
(c) change is permanent and cannot be reversed easily
(d) All of the options
Answer: (d) All of the options

VSA Type Questions

Question. What do you mean by ‘skeletal equation’?
Answer: An equation which shows the reactants and products taking part in a chemical reaction symbolically, but the number of atoms of one or more elements may or may not be equal on the two sides of the equation is called a skeletal (or unbalanced) equation.

Question. Suggest two methods to prevent rusting
Answer: (i) Painting the iron article. (ii) Greasing or oiling the iron article.

SA I Type Questions

Question. Why do we apply paint on iron articles?
Answer: We apply paint on iron articles so as to prevent it from rusting. When the surface of iron is coated with paint, its surface does not come in contact with oxygen and moisture, therefore, rusting does not take place.

Question. Why is respiration considered as an exothermic reaction? Explain.
Answer: In the process of respiration, glucose combines with oxygen in the cells of our body and provides energy. Thus, respiration is an exothermic process.
\( C_6H_{12}O_{6(aq)} + 6O_{2(g)} \rightarrow 6CO_{2(g)} + 6H_2O_{(l)} + Energy \)

Question. Identify the substances that are oxidized and the substances that are reduced in the following reactions :
(i) \( 4Na_{(s)} + O_{2(g)} \rightarrow 2Na_2O_{(s)} \)
(ii) \( CuO_{(s)} + H_{2(g)} \rightarrow Cu_{(s)} + H_2O_{(l)} \)
Answer: (i) Sodium (\( Na \)) is oxidised and \( O_2 \) is reduced.
(ii) \( H_2 \) is oxidised and \( CuO \) is reduced.

Question. A solution of the substance X is used for whitewashing.
(i) Name the substance X and write its formula.
(ii) Write the reaction of the substance X with water.
Answer: (i) The substance X is calcium oxide (also called quick lime). Its formula is \( CaO \).
(ii) \( CaO_{(s)} + H_2O_{(l)} \rightarrow Ca(OH)_{2(aq)} \)

Question. Why does the colour of copper sulphate solution change when an iron nail is dipped in it ?
Answer: In this displacement reaction, a more reactive element iron displaces another less reactive element from its compound copper(II) sulphate. The colour of the solution changes from blue to light green due to the formation of ferrous sulphate (having \( Fe^{2+} \) ions).
\( CuSO_{4(aq)} + Fe_{(s)} \rightarrow FeSO_{4(aq)} + Cu_{(s)} \)

Question. OR
What happens chemically when quick lime is added to water ?
Answer: Calcium hydroxide (or slaked lime) is formed accompanied by a hissing sound. A large amount of heat is evolved during the reaction.
\( CaO_{(s)} + H_2O_{(l)} \rightarrow Ca(OH)_{2(aq)} + heat \)

SA II Type Questions

Question. Can oxidation or reduction take place alone? Why or why not? What are such reactions called?
Answer: Oxidation or reduction cannot take place alone. This is because if one substance loses oxygen (reduction), there must be another substance to take up this oxygen (oxidation). These reactions always take place together and are called redox reactions.

Question. In the following situations, a reaction may or may not take place. If it does, write the balanced molecular, total ionic and net ionic equations illustrating the reaction. Assume all the reactions take place in aqueous solutions.
(a) Some iron nails are placed in a \( CuCl_2 \) solution.
(b) Silver coins are dropped in hydrochloric acid solution.
(c) A copper wire is placed in \( Pb(NO_3)_2 \) solution.
Answer: (a) Molecular equation : \( CuCl_{2(aq)} + Fe_{(s)} \rightarrow FeCl_{2(aq)} + Cu_{(s)} \)
Total ionic equation : \( (Cu^{2+}_{(aq)} + 2Cl^-_{(aq)}) + Fe_{(s)} \rightarrow (Fe^{2+}_{(aq)} + 2Cl^-_{(aq)}) + Cu_{(s)} \)
Net ionic equation : \( Cu^{2+}_{(aq)} + Fe_{(s)} \rightarrow Fe^{2+}_{(aq)} + Cu_{(s)} \)
For (b) and (c) no reaction takes place because silver is less reactive than hydrogen and copper is less reactive than lead.

Question. Below are given two chemical reactions :
(i) \( 2KBr_{(aq)} + Cl_{2(aq)} \rightarrow 2KCl_{(aq)} + Br_{2(aq)} \)
(ii) \( Fe_{(s)} + S_{(s)} \rightarrow FeS_{(s)} \)
Which is combination reaction and which is displacement reaction? Give explanation also.
Answer: (i) In the first reaction, chlorine is displacing bromine from potassium bromide to form potassium chloride and bromine. Thus, it is a displacement reaction.
(ii) In the second reaction, iron combines with sulphur to form iron(II) sulphide. So, it is a combination reaction.

Question. Explain the following in terms of gain or loss of oxygen with two examples each.
(i) Oxidation (ii) Reduction
Answer: (i) Oxidation is a reaction which involves addition or gain of oxygen.
Examples: \( 4Na + O_2 \rightarrow 2Na_2O \); \( 2Cu + O_2 \xrightarrow{Heat} 2CuO \)
(ii) Reduction is a reaction which involves loss or removal of oxygen.
Examples: \( CuO + H_2 \xrightarrow{Heat} Cu + H_2O \); \( ZnO + C \rightarrow Zn + CO \)

Question. OR
Differentiate between displacement and double displacement reactions giving equations for these reactions.
Answer: Displacement reaction is a chemical reaction in which a more reactive element displaces a less reactive element from its salt solution.
Example: \( Zn_{(s)} + CuSO_{4(aq)} \rightarrow ZnSO_{4(aq)} + Cu_{(s)} \)
Double displacement reactions are those in which two compounds react to form two other compounds by mutual exchange of ions.
Example: \( BaCl_{2(aq)} + H_2SO_{4(aq)} \rightarrow BaSO_{4(s)} + 2HCl_{(aq)} \)

LA Type Questions

Question. Classify the given reactions into different types:
(a) Barium chloride solution is mixed with copper sulphate solution and a white precipitate is observed.
(b) On heating copper powder in air in a China dish, the surface of copper powder turns black.
(c) On heating green coloured ferrous sulphate crystals, reddish brown solid is left and smell of a gas having odour of burning sulphur is observed.
(d) Iron nails when left dipped in blue copper sulphate solution become brownish in colour and the blue colour of copper sulphate fades away.
(e) Quick lime reacts vigorously with water releasing a large amount of heat.
Answer: (a) Double displacement and precipitation reaction.
(b) Combination reaction (and oxidation).
(c) Decomposition reaction.
(d) Displacement reaction.
(e) Combination and exothermic reaction.

Question. Write balanced equations by predicting the products of the following reactions. Include the physical state of each element or compound.
(a) The combination of barium and iodine.
(b) The neutralization of aqueous rubidium hydroxide with hydrobromic acid.
(c) A single-replacement reaction of calcium metal with a nitric acid solution.
(d) The combustion of solid naphthalene (\( C_{10}H_8 \)).
(e) Decomposition of aluminium hydroxide into solid aluminium oxide and gaseous water.
Answer: (a) \( Ba_{(s)} + I_{2(s)} \rightarrow BaI_{2(s)} \)
(b) \( RbOH_{(aq)} + HBr_{(aq)} \rightarrow RbBr_{(aq)} + H_2O_{(l)} \)
(c) \( Ca_{(s)} + 2HNO_{3(aq)} \rightarrow Ca(NO_3)_{2(aq)} + H_{2(g)} \)
(d) \( C_{10}H_{8(s)} + 12O_{2(g)} \rightarrow 10CO_{2(g)} + 4H_2O_{(l)} \)
(e) \( 2Al(OH)_{3(s)} \rightarrow Al_2O_{3(s)} + 3H_2O_{(g)} \)

Question. (a) Why cannot a chemical change be normally reversed?
(b) Why is it always essential to balance a chemical equation?
(c) Why do diamond and graphite, the two allotropic forms of carbon evolve different amounts of heats on combustion?
(d) Why does milkiness disappear when carbon dioxide is passed through lime water in excess?
(e) List two conditions essential for rusting.
Answer: (a) In a chemical change, the products formed have entirely different properties from the reactants.
(b) To satisfy the law of conservation of mass which states that mass can neither be created nor destroyed.
(c) They have different arrangements of carbon atoms (structures).
(d) Due to the formation of soluble calcium hydrogen carbonate: \( CaCO_{3(s)} + CO_{2(g)} + H_2O_{(aq)} \rightarrow Ca(HCO_3)_{2(aq)} \)
(e) Presence of moisture (water) and presence of air (oxygen).

Question. Explain in detail giving examples:
(a) Corrosion
(b) Rancidity
Answer: (a) Corrosion is the process of slow deterioration of metals when exposed to air and moisture. Example: Rusting of iron.
(b) Rancidity is the oxidation of fats and oils in food resulting in unpleasant smell and taste. It can be prevented by adding antioxidants or using nitrogen gas.