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.