CBSE Class 10 Science HOTs Chemical Reactions and Equations Set 02

Question. Which of the following is redox reaction?
(a) \( NaOH + HCl \longrightarrow NaCl + H_2O \)
(b) \( 4Na + O_2 \longrightarrow 2Na_2O \)
(c) \( CaCO_3 \xrightarrow{heat} CaO + CO_2 \)
(d) \( Pb(NO_3)_2 + 2KI \longrightarrow PbI_2 + 2KNO_3 \)

Answer: (b) \( 4Na + O_2 \longrightarrow 2Na_2O \)

Question. Which of the following is not balanced?
(a) \( Cu + HNO_3 \longrightarrow Cu(NO_3)_2 + NO_2 + H_2O \)
(b) \( Zn + H_2SO_4 \longrightarrow ZnSO_4 + H_2 \)
(c) \( 2KOH + H_2SO_4 \longrightarrow K_2SO_4 + 2H_2O \)
(d) \( (NH_4)_2Cr_2O_7 \xrightarrow{\Delta} N_2+ Cr_2O_3 + 4H_2O \)

Answer: (a) \( Cu + HNO_3 \longrightarrow Cu(NO_3)_2 + NO_2 + H_2O \)

Question. Which of the following is endothermic combination reaction?
(a) \( C + O_2 \longrightarrow CO_2 \)
(b) \( N_2 + O_2 \longrightarrow 2NO \)
(c) \( N_2 + 3H_2 \longrightarrow 2NH_3 \)
(d) \( CaO + H_2O \longrightarrow Ca(OH)_2 \)

Answer: (b) \( N_2 + O_2 \longrightarrow 2NO \)

Question. Which of the following are correct about the reaction \( Zn(s) + 2HCl(dil.) \longrightarrow ZnCl_2 + H_2 \)
I. It is displacement reaction.
II. It is exothermic.
III. Zinc is more reactive than hydrogen.
IV. Zinc is less reaction than \( H_2 \).
(a) I, II
(b) I, II, III
(c) II and III
(d) II and IV

Answer: (b) I, II, III

Direction: In the following Questions, the Assertion and Reason have been put forward. Read the statements carefully and choose the correct alternative from the following:
(a) Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.
(b) The Assertion and the Reason are correct but the Reason is not the correct explanation of the Assertion.
(c) Assertion is true but the Reason is false.
(d) The statement of the Assertion is false but the Reason is true.

Question. Assertion: All combustion reactions are oxidation reactions.
Reason: Combustion reactions are exothermic.
(a) Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.
(b) The Assertion and the Reason are correct but the Reason is not the correct explanation of the Assertion.
(c) Assertion is true but the Reason is false.
(d) The statement of the Assertion is false but the Reason is true.

Answer: (b) The Assertion and the Reason are correct but the Reason is not the correct explanation of the Assertion.

Question. Assertion: Electrolysis of acidified water liberates \( H_2 \) at cathode.
Reason: Cathode is positive electrode.
(a) Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.
(b) The Assertion and the Reason are correct but the Reason is not the correct explanation of the Assertion.
(c) Assertion is true but the Reason is false.
(d) The statement of the Assertion is false but the Reason is true.

Answer: (c) Assertion is true but the Reason is false.

Question. What happens when
(i) Magnesium burns in presence of \( N_2 \)
(ii) Zinc carbonate is heated.
Write balanced chemical equations.

Answer: (i) When magnesium burns in the presence of nitrogen, it forms magnesium nitride.
\( 3Mg(s) + N_2(g) \longrightarrow Mg_3N_2(s) \)
(ii) When zinc carbonate is heated, it undergoes thermal decomposition to form zinc oxide and carbon dioxide gas.
\( ZnCO_3(s) \xrightarrow{heat} ZnO(s) + CO_2(g) \)

Question. Define redox reaction. Illustrate it with an example.

Answer: A chemical reaction in which both oxidation (loss of electrons or addition of oxygen) and reduction (gain of electrons or removal of oxygen) occur simultaneously is called a redox reaction.
Example:
\( CuO + H_2 \longrightarrow Cu + H_2O \)
In this reaction, \( CuO \) is reduced to \( Cu \) (removal of oxygen) and \( H_2 \) is oxidized to \( H_2O \) (addition of oxygen).

Question. (a) \( PbO_2 + 4HCl \longrightarrow PbCl_2 + Cl_2 + 2H_2O \)
Identify
(i) Oxidising agent
(ii) Reducing agent
(iii) Substance getting oxidised
(iv) Substance getting reduced
(b) Why does a apple turn brown when kept in air?

Answer: (a) (i) Oxidising agent: \( PbO_2 \)
(ii) Reducing agent: \( HCl \)
(iii) Substance getting oxidised: \( HCl \) (to \( Cl_2 \))
(iv) Substance getting reduced: \( PbO_2 \) (to \( PbCl_2 \))
(b) Apples contain an enzyme called polyphenol oxidase. When the apple is cut and exposed to oxygen in the air, this enzyme catalyzes the oxidation of phenolic compounds in the apple, forming melanin, which has a brown colour.

Question. Translate the following into balanced chemical equation.
(i) Potassium permanganate on heating gives potassium manganate, manganese dioxide and oxygen gas.
(ii) Sulphur burns in presence of oxygen to form sulphur dioxide.
(iii) Potassium reacts with water to form potassium hydroxide and hydrogen gas.

Answer: (i) \( 2KMnO_4 \xrightarrow{heat} K_2MnO_4 + MnO_2 + O_2 \)
(ii) \( S(s) + O_2(g) \longrightarrow SO_2(g) \)
(iii) \( 2K(s) + 2H_2O(l) \longrightarrow 2KOH(aq) + H_2(g) \)

Question. Complete the following reactions and balance them.
(i) \( Mg + O_2 \longrightarrow \)
(ii) \( MnO_2 + HCl \longrightarrow \)
(iii) \( NH_3 + HCl \longrightarrow \)
(iv) \( CuO + H_2 \longrightarrow \)
(v) \( CO_2 + H_2O \xrightarrow[Chlorophyll]{Sunlight} \)

Answer: (i) \( 2Mg + O_2 \longrightarrow 2MgO \)
(ii) \( MnO_2 + 4HCl \longrightarrow MnCl_2 + Cl_2 + 2H_2O \)
(iii) \( NH_3 + HCl \longrightarrow NH_4Cl \)
(iv) \( CuO + H_2 \longrightarrow Cu + H_2O \)
(v) \( 6CO_2 + 6H_2O \xrightarrow[Chlorophyll]{Sunlight} C_6H_{12}O_6 + 6O_2 \)

Question. Hydrogen peroxide (\( H_2O_2 \)) is a colourless liquid which can act as oxidising as well as reducing agent. When it acts as oxidising agent, only water is formed. When it acts as reducing agent, oxygen gas is formed. It acts as rocket fuel. Its dilute solution acts as antiseptic and bleaching agent.
(a) What are products formed when \( H_2O_2 \) undergoes decomposition reaction?
(b) With \( FeCl_3(aq) \), \( H_2O_2 \) acts as oxidising or reducing agent?
(c) Complete the following
(i) \( PbS + 4H_2O_2 \longrightarrow \)
(ii) \( 2NaOH + H_2O_2 \longrightarrow \)
Or
(c) Why does \( H_2O_2 \) act as both oxidising as well as reducing agent? Explain.

Answer: (a) When \( H_2O_2 \) undergoes decomposition, it forms water and oxygen gas.
\( 2H_2O_2 \longrightarrow 2H_2O + O_2 \)
(b) With \( FeCl_3(aq) \), \( H_2O_2 \) acts as a reducing agent.
(c) (i) \( PbS + 4H_2O_2 \longrightarrow PbSO_4 + 4H_2O \)
(ii) \( 2NaOH + H_2O_2 \longrightarrow Na_2O_2 + 2H_2O \)
Or
(c) In \( H_2O_2 \), oxygen is in an intermediate oxidation state of -1. It can be oxidized to oxygen gas (\( O_2 \), oxidation state 0), thereby acting as a reducing agent. It can also be reduced to water (\( H_2O \), oxidation state of O is -2), thereby acting as an oxidising agent. Since it can both lose and gain electrons to reach more stable oxidation states, it acts as both.

Chemical Equations

Question. Sodium reacts with water to form sodium hydroxide and hydrogen gas. The balanced equation which represents the above reaction is
(a) \( Na_{(s)} + 2H_2O_{(l)} \rightarrow 2NaOH_{(aq)} + 2H_{2(g)} \)
(b) \( 2Na_{(s)} + 2H_2O_{(l)} \rightarrow 2NaOH_{(aq)} + H_{2(g)} \)
(c) \( 2Na_{(s)} + 2H_2O_{(l)} \rightarrow NaOH_{(aq)} + 2H_{2(g)} \)
(d) \( 2Na_{(s)} + H_2O_{(l)} \rightarrow 2NaOH_{(aq)} + 2H_{2(g)} \)
Answer: (b) \( 2Na_{(s)} + 2H_2O_{(l)} \rightarrow 2NaOH_{(aq)} + H_{2(g)} \)

Question. It is important to balance the chemical equations to satisfy the law of conservation of mass. Which of the following statements of the law is incorrect?
(a) The total mass of the elements present in the reactants is equal to the total mass of the elements present in the products.
(b) The number of atoms of each element remains the same, before and after a chemical reaction.
(c) The chemical composition of the reactants is the same before and after the reaction.
(d) Mass can neither be created nor can it be destroyed in a chemical reaction.
Answer: (c) The chemical composition of the reactants is the same before and after the reaction.

Question. In which of the following, the identity of initial substance remains unchanged?
(a) Curdling of milk
(b) Formation of crystals by process of crystallisation
(c) Fermentation of grapes
(d) Digestion of food 
Answer: (b) Formation of crystals by process of crystallisation

Question. Identify ‘x’, ‘y’ and ‘z’ in the following reaction :
\( 2KClO_{3(x)} \xrightarrow{y} 2KCl_{(x)} + O_{2(z)} \)
(a) x = gas; y = reaction condition; z = gas
(b) x = solid; y = liquid; z = gas
(c) x = number of moles of \( KClO_3 \); y = reaction condition; z = number of molecules of oxygen
(d) x = physical state of \( KClO_3 \) and \( KCl \); y = reaction condition, z = physical state of \( O_2 \). 
Answer: (d) x = physical state of \( KClO_3 \) and \( KCl \); y = reaction condition, z = physical state of \( O_2 \).

Question. Assertion (A) : Following is a balanced chemical equation for the action of steam on iron :
\( 3Fe + 4H_2O \rightarrow Fe_3O_4 + 4H_2 \)
Reason (R) : The law of conservation of mass holds good for a chemical equation.
(a) Both (A) and (R) are true and reason (R) is the correct explanation of the assertion (A).
(b) Both (A) and (R) are true, but reason (R) is not the correct explanation of the assertion (A).
(c) (A) is true, but (R) is false.
(d) (A) is false, but (R) is true. 
Answer: (a) Both (A) and (R) are true and reason (R) is the correct explanation of the assertion (A).

Question. What is a balanced chemical equation ? 
Answer: A chemical equation in which the number of atoms of each element on the reactant side is equal to the number of atoms of the same element on the product side is called a balanced chemical equation.

Question. Translate the following statement into a balanced chemical equation :
‘‘Barium chloride reacts with aluminium sulphate to give aluminium chloride and barium sulphate.’’ 
Answer: \( 3BaCl_2 + Al_2(SO_4)_3 \rightarrow 2AlCl_3 + 3BaSO_4 \)

Question. Give the chemical name of the reactants as well as the products of the following chemical equation:
\( HNO_3 + Ca(OH)_2 \rightarrow Ca(NO_3)_2 + H_2O \) 
Answer: Reactants: Nitric acid and Calcium hydroxide; Products: Calcium nitrate and Water.

Question. Explain the significance of photosynthesis. Write the balanced chemical equation involved in the process. 
Answer: Photosynthesis is the process by which green plants synthesize their own food (glucose) using sunlight, chlorophyll, carbon dioxide, and water. It is significant as it provides food for almost all living organisms and releases oxygen into the atmosphere. The balanced chemical equation is:
\( 6CO_{2(g)} + 6H_2O_{(l)} \xrightarrow{\text{Sunlight, Chlorophyll}} C_6H_{12}O_{6(aq)} + 6O_{2(g)} \)

Write balanced chemical equations for the following chemical reactions :

Question. Hydrogen + Chlorine \( \rightarrow \) Hydrogen chloride 
Answer: \( H_2 + Cl_2 \rightarrow 2HCl \)

Question. Lead + Copper chloride \( \rightarrow \) Lead chloride + Copper
Answer: \( Pb + CuCl_2 \rightarrow PbCl_2 + Cu \)

Question. Zinc oxide + Carbon \( \rightarrow \) Zinc + Carbon monoxide 
Answer: \( ZnO + C \rightarrow Zn + CO \)

Types of Chemical Reactions

Question. When aqueous solutions of potassium iodide and lead nitrate are mixed, an insoluble substance separates out. The chemical equation for the reaction involved is
(a) \( KI + PbNO_3 \rightarrow PbI + KNO_3 \)
(b) \( 2KI + Pb(NO_3)_2 \rightarrow PbI_2 + 2KNO_3 \)
(c) \( KI + Pb(NO_3)_2 \rightarrow PbI + KNO_3 \)
(d) \( KI + PbNO_3 \rightarrow PbI_2 + KNO_3 \) 
Answer: (b) \( 2KI + Pb(NO_3)_2 \rightarrow PbI_2 + 2KNO_3 \)

Question. A metal ribbon ‘X’ burns in oxygen with a dazzling white flame forming a white ash 'Y'. The correct description of X, Y and the type of reaction is
(a) X = Ca; Y = CaO; Type of reaction = Decomposition
(b) X = Mg; Y = MgO; Type of reaction = Combination
(c) X = Al; Y = \( Al_2O_3 \); Type of reaction = Thermal decomposition
(d) X = Zn; Y = ZnO; Type of reaction = Endothermic 
Answer: (b) X = Mg; Y = MgO; Type of reaction = Combination

Question. The balanced chemical equation showing reaction between quick lime and water is
(a) \( 2CaO + H_2O \rightarrow 2CaOH + H_2 + \text{Heat} \)
(b) \( CaO + H_2O \rightarrow Ca(OH)_2 + H_2 + \text{Heat} \)
(c) \( CaO + H_2O \rightarrow Ca(OH)_2 + \text{Heat} \)
(d) \( 2CaO + 3H_2O \rightarrow 2Ca(OH)_3 + O_2 + \text{Heat} \) 
Answer: (c) \( CaO + H_2O \rightarrow Ca(OH)_2 + \text{Heat} \)

Question. Study the following chemical reaction:
\( 2Na_{(s)} + 2H_2O_{(l)} \rightarrow 2NaOH_{(aq)} + H_{2(g)} \uparrow \)
The reducing agent in this reaction is
(a) Na (b) \( H_2O \) (c) NaOH (d) \( H_2 \) 
Answer: (a) Na

Question. Assertion (A) : In the following reaction
\( ZnO + C \rightarrow Zn + CO \)
ZnO undergoes reduction.
Reason (R) : Carbon is a reducing agent that reduces ZnO to Zn.
(a) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A)
(b) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A)
(c) Assertion (A) is true, but Reason (R) is False.
(d) Assertion (A) is false, but Reason (R) is true. 
Answer: (a) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A)

Question. Assertion (A) : Reaction of quick lime with water is an exothermic reaction.
Reason (R) : Quicklime reacts vigorously with water releasing a large amount of heat.
(a) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A)
(b) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A)
(c) Assertion (A) is true, but Reason (R) is False.
(d) Assertion (A) is false, but Reason (R) is true. 
Answer: (a) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A)

Question. A student took sodium sulphate solution in a test tube and added barium chloride solution to it. He observed that an insoluble substance has formed. The colour and molecular formula of the insoluble substance is
(a) grey, \( Ba_2SO_4 \)
(b) yellow, \( Ba(SO_4)_2 \)
(c) white, \( BaSO_4 \)
(d) pink, \( BaSO_4 \) 
Answer: (c) white, \( BaSO_4 \)

Question. \( C_6H_{12}O_{6(aq)} + 6O_{2(g)} \rightarrow 6CO_{2(g)} + 6H_2O_{(l)} \)
The above reaction is a/an
(a) displacement reaction
(b) endothermic reaction
(c) exothermic reaction
(d) neutralisation reaction. 
Answer: (c) exothermic reaction

Question. Which of the following statement about the reaction given below are correct?
\( MnO_2 + 4HCl \rightarrow MnCl_2 + 2H_2O + Cl_2 \)
(i) HCl is oxidized to \( Cl_2 \).
(ii) \( MnO_2 \) is reduced to \( MnCl_2 \).
(iii) \( MnCl_2 \) acts as an oxidizing agent.
(iv) HCl acts as an oxidizing agent.
(a) (ii), (iii) and (iv)
(b) (i), (ii) and (iii)
(c) (i) and (ii) only
(d) (iii) and (iv) only 
Answer: (c) (i) and (ii) only

Question. Which one of the following reactions is categorised as thermal decomposition reaction?
(a) \( 2H_2O_{(l)} \rightarrow 2H_{2(g)} + O_{2(g)} \)
(b) \( 2AgBr_{(s)} \rightarrow 2Ag_{(s)} + Br_{2(g)} \)
(c) \( 2AgCl_{(s)} \rightarrow 2Ag_{(s)} + Cl_{2(g)} \)
(d) \( CaCO_{3(s)} \rightarrow CaO_{(s)} + CO_{2(g)} \) 
Answer: (d) \( CaCO_{3(s)} \rightarrow CaO_{(s)} + CO_{2(g)} \)

Question. Assertion (A) : Burning of natural gas is an endothermic process.
Reason (R) : Methane gas combines with oxygen to produce carbon dioxide and water.
(a) Both (A) and (R) are true and (R) is the correct explanation of (A).
(b) Both (A) and (R) are true but (R) is not the correct explanation of (A).
(c) (A) is true, but (R) is false.
(d) (A) is false, but (R) is true. 
Answer: (d) (A) is false, but (R) is true.

Question. Consider the following processes
I. Dilution of sulphuric acid
II. Sublimation of dry ice
III. Condensation of water vapours
IV. Dissolution of ammonium chloride in water
The endothermic process(es) is/are
(a) I and III
(b) II only
(c) III only
(d) II and IV 
Answer: (d) II and IV

Question. When lead nitrate powder is heated in boiling tube, we observe
(a) brown fumes of nitrogen dioxide
(b) brown fumes of lead oxide
(c) yellow fumes of nitrogen dioxide
(d) brown fumes of nitric oxide. 
Answer: (a) brown fumes of nitrogen dioxide

Question. Assertion (A) : Silver salts are used in black and white photography.
Reason (R) : Silver salts do not decompose in the presence of light.
(a) Both (A) and (R) are true and (R) is the correct explanation of (A).
(b) Both (A) and (R) are true but (R) is not the correct explanation of (A).
(c) (A) is true, but (R) is false.
(d) (A) is false, but (R) is true. 
Answer: (c) (A) is true, but (R) is false.

Consider the reaction:
\( CaO_{(s)} + H_2O_{(l)} \rightarrow Ca(OH)_{2(aq)} \)
This reaction can be classified as
(A) combination reaction
(B) exothermic reaction
(C) endothermic reaction
(D) oxidation reaction.
Question. Which of the following is a correct option?
(a) (A) and (C)
(b) (C) and (D)
(c) (A), (C) and (D)
(d) (A) and (B) 
Answer: (d) (A) and (B)

Question. When hydrogen sulphide gas is passed through a blue solution of copper sulphate, a black precipitate of copper sulphide is obtained and the sulphuric acid so formed remains in the solution. The reaction is an example of a
(a) combination reaction
(b) displacement reaction
(c) decomposition reaction
(d) double displacement reaction. 
Answer: (d) double displacement reaction.

Question. In a double displacement reaction such as the reaction between sodium sulphate solution and barium chloride solution :
(A) exchange of atoms takes place
(B) exchange of ions takes place
(C) a precipitate is produced
(D) an insoluble salt is produced
The correct option is
(a) (B) and (D)
(b) (A) and (C)
(c) only (B)
(d) (B), (C) and (D) 
Answer: (d) (B), (C) and (D)

Question. \( CH_4 + Cl_2 \xrightarrow{\text{Sunlight}} CH_3Cl + HCl \)
The above reaction is an example of
(a) displacement reaction
(b) addition reaction
(c) substitution reaction
(d) double displacement reaction. 
Answer: (c) substitution reaction