# NCERT Exemplar Problems and their Solutions

Reviewed By:
Krishna Kant Majee
M.Sc., B.Ed.

### Chapter 1-Chemical Reactions and Equations

Q.19. Write the balanced chemical equations for the following reactions and identify the type of reaction in each case.
(a) Nitrogen gas is treated with hydrogen gas in the presence of a catalyst at 773 K to form ammonia gas.
(b) Sodium hydroxide solution is treated with acetic acid to form sodium acetate and water.
(c) Ethanol is warmed with ethanoic acid to form ethyl acetate in the presence of concentrated $H_{2}SO_{4}.$
(d) Ethane is burnt in the presence of oxygen to form carbon dioxide, water and releases heat and light.

(a) $\ce{ N_{2}+ 3H_{2}(g) ->[{Catalyst}][{773K}] 2NH_{3}(g)}$

This is an example of combination reaction.
(b) $NaOH (aq) + CH_{3}COOH (l) \rightarrow CH_{3}COONa (aq) + H_{2}O (l)$
This is an example of neutralization reaction as well as double displacement reaction.
(c) $\ce{ C2H5OH(l) + CH3COOH(l) ->[{Conc H_{2}SO_{4}}]CH3COOC2H5 +H2O(l)}$

This is an example of double displacement reaction and also called as esterification.
(d)$\ce{2C2H6(g) + 7O2(g) → 4CO2(g) + 6H2O(g)}$
This is an example of combustion reaction.

Q.20. Write the balanced chemical equation for the following reactions and identify the type of reaction in each case.
(a) In Thermite reaction, iron (III) oxide reacts with aluminium and gives molten iron and aluminium oxide.
(b) Magnesium ribbon is burnt in an atmosphere of nitrogen gas to form solid magnesium nitride.
(c) Chlorine gas is passed in an aqueous potassium iodide solution to form potassium chloride solution and solid iodine.
(d) Ethanol is burnt in air to form carbon dioxide, water and releases heat.

(a)$\ce{ Fe2O3(S) + 2Al ->[{Heat}] 2Fe(l) + Al2O3}$
This is an example of displacement and Redox reaction
(b)$\ce{3Mg(S) + N2(g)->[{Heat}] Mg3N2(S)}$
This is an example of combination reaction
(c)$\ce{2KI(aq) + Cl2(g) -> 2KCl(aq) + I2(s)}$

This is an example of Redox and displacement reaction
(d)$\ce{C2H5OH(l) + 3O2(g)-> 2CO2(g) +3H2O + Heat}$
This is an example of combustion reaction

Q.21. Complete the missing components/variables given as X and Y in the following.
(a) $Pb(NO_{3})_{2}(aq)\rightarrow PbI_{2}(X) + 2KNO_{3}(Y)$
(b) $Cu(s) + 2AgNO_{3}(aq)\rightarrow Cu(NO_{3})_{2}(aq) +X(s)$
(c) $Zn(s) + H_{2}SO_{4}(aq)\rightarrow ZnSO_{4}(X)+H_{2}(Y)$
(d) $\ce{CaCO3(s)->[{X}]CaO(s) +CO2}$

(a) $Pb(NO_{3})_{2}(aq)\rightarrow PbI_{2}(s) + 2KNO_{3}(aq)$
(b) $Cu(s) + 2AgNO_{3}(aq)\rightarrow Cu(NO_{3})_{2}(aq) +2Ag(s)$
(c) $Zn(s) + H_{2}SO_{4}(aq)\rightarrow ZnSO_{4}(aq)+H_{2}(s)$
(d) $\ce{CaCO3(s)->[{Heat}]CaO(s) +CO2}$

Q.22. Which among the following changes are exothermic or endothermic in nature?
(a) Decomposition of ferrous sulphate
(b) Dilution of sulphuric acid
(c) Dissolution of sodium hydroxide in water
(d) Dissolution of ammonium chloride in water

(a) Decomposition of ferrous sulphate absorbs heat during reaction therefore it is an example of endothermic reaction.
(b) When we dissolve sulphuric acid in water, large amount of heat is liberated so it is an exothermic reaction.
(c) When we dissolve sodium hydroxide in water, large amount of heat is liberated so it is an exothermic reaction.’
(d) Mixing of ammonium chloride in water absorbs heat from reaction mixture therefore it is an example of endothermic reaction.

Q.23 Identify the reducing agent in the following reactions.
(a) $\ce{4NH3 + 5O2 -> 4NO + 6H2O}$
(b) $\ce{2H2O + 2F2 -> 4HF + O2}$
(c) $\ce{Fe2O3 + 3CO -> 2Fe + 3CO2}$
(d) $\ce{2H2 + O2-> 2H2O}$

(a) $\ce{NH3}$ is the reducing agent as it gives hydrogen to $O_{2}$.
(b) $H_{2}O$ is the reducing agent.
(c) CO is a reducing agent.
(d) H2 is the reducing agent.

Q.24 Identify the oxidizing agent (oxidant) in the following reactions.
(a) $\ce{Pb3O4 + 8HCl -> 3PbCl2 + Cl2 + 4H2O}$
(b) $\ce{2Mg + O2 -> 2MgO}$
(c) $\ce{CuSO4 + Zn -> Cu + ZnSO4}$
(d) $\ce{V2O5 + 5Ca -> 2V + 5CaO}$
(e) $\ce{3Fe + 4H2O -> Fe3O4 + 4H2}$
(f) $\ce{CuO + H2 -> Cu + H2O}$

(a) Since $Pb_{3}O_{4}$ gives oxygen so it is an oxidizing agent (oxidant).
(b) Since $O_{2}$ provides oxygen so it is oxidant.
(c) $CuSO_{4}$ is the oxidizing agent because it gets reduces to Cu.
(d) $V_{2}O_{5}$ has been reduced to V so it will be the oxidant.
(e) $H_{2}O$ is the oxidant as it is reduced to $H_{2}$.
(f) CuO is the oxidant as it is reduced to Cu.

Q.25 Write the balanced chemical equations for the following reactions.
(a) Sodium carbonate on reaction with hydrochloric acid in equal molar concentrations gives sodium chloride and sodium hydrogen carbonate.
(b) Sodium hydrogen carbonate on reaction with hydrochloric acid gives sodium chloride, water and liberates carbon dioxide.
(c) Copper sulphate on treatment with potassium iodide precipitates cuprous iodide $(Cu_{2}I_{2}), liberates iodine gas and also forms potassium sulphate. Answer. (a)$\ce{Na2CO3(s) + HCl (aq) -> NaCl (aq) + NaHCO3 (aq)}$(b)$\ce{NaHCO3 (s) + HCl (aq) -> NaCl (aq) + H2O(l) + CO2 (g)}$(c) 4\ce{CuSO4 (s) + 4KI (aq) -> Cu2I2 (s) + I2 (g) + 2K2SO4 (aq)}$

Q.26 A solution of potassium chloride when mixed with silver nitrate solution, an insoluble white substance is formed. Write the chemical reaction involved and also mention the type of the chemical reaction.

The solution of KCl with $AgNO_{3}$ form white precipitate of AgCl. This is a double displacement reaction.
$\ce{KCl (aq) + AgNO3 (aq) -> AgCl (s) + KNO3 (aq)}$

Q.27 Ferrous sulphate decomposes with the evolution of a gas having a characteristic odour of burning sulphur. Write the chemical reaction involved and identify the type of reaction.

$\ce{2FeSO4 (s) -> Fe2O3 (s) + SO2 (g) + SO3 (g)}$
The heating of ferrous sulphate gives ferric oxide and mixture of sulphur dioxide and sulphur trioxide gas. This is an example of thermal decomposition reaction and the odour of burning sulphur is due to $SO_{2}$ gas.

Q.28. Why do fire flies glow at night?

Fire flies have a luminous bag which contains a protein; luciferin. The oxidation of luciferin occurs in the presence of luciferare enzyme with emission of light which occurs due to oxidation of magnesium. Hence they glow at night.

Q.29. Grapes hanging on the plant do not ferment but after being plucked from the plant can be fermented. Under what conditions do these grapes ferment? Is it a chemical or a physical change?

After plucking grapes from plants, fermentation of sugar is carried out in the presence of yeast which changes sugar to ethanol and carbon dioxide. This process occurs in the absence of oxygen means in anaerobic conditions.
Whereas grapes attach to plant involve in aerobic respiration and no fermentation can be possible under aerobic conditions.
Here fermentation is a chemical change as it results the formation of new substances; alcohol and carbon dioxide.

Q.30. Which among the following are physical or chemical changes?
(a) Evaporation of petrol
(b) Burning of Liquefied Petroleum Gas (LPG)
(c) Heating of an iron rod to red hot
(d) Curdling of milk
(e) Sublimation of solid ammonium chloride

(a) Physical change
(b) Chemical change
(c) Physical change
(d) Chemical change
(e) Physical change

Q.31. During the reaction of some metals with dilute hydrochloric acid, following observations were made.
(a) Silver metal does not show any change.
(b) The temperature of the reaction mixture rises when aluminium (Al) is added.
(c) The reaction of sodium metal is found to be highly explosive.
(d) Some bubbles of a gas are seen when lead (Pb) is reacted with the acid Explain these observations giving suitable reasons.

(a) The reactivity of silver is very low as it is placed below hydrogen in reactivity series so it does not react with dilute hydrochloric acid.
(b) Aluminium reacts with dilute HCl to form Aluminium chloride with hydrogen gas. It is an exothermic reaction so temperature of reaction mixture rises.
(c) Sodium is an alkali metal which is one of the most reactive metals and readily reacts with dilute HCl to form NaCl and hydrogen gas. The evolution of hydrogen gas cause explosion.
(d) Reaction of lead metal with dilute HCl forms lead (II) chloride and releases hydrogen gas in the form of bubbles. Since reaction is quite slow due to less reactivity of lead, only bubbles of $H_{2}$ are seen to evolve.

Q.32. A substance X, which is an oxide of a group 2 element, is used intensively in the cement industry. This element is present in bones also. On treatment with water it forms a solution which turns red litmus blue. Identify X and also write the chemical reactions involved.

The substance X is calcium oxide (CaO) which is also known as quick lime. Reaction of quick lime with water forms calcium hydroxide. It is an alkaline solution and easily turns red litmus to blue.
$\ce{CaO(s)+ H2O(l) -> Ca(OH)2 }$

Q.33. Write a balanced chemical equation for each of the following reactions and also classify them.
(a) Lead acetate solution is treated with dilute hydrochloric acid to form lead chloride and acetic acid solution.
(b) A piece of sodium metal is added to absolute ethanol to form sodium ethoxide and hydrogen gas.
(c) Iron (III) oxide on heating with carbon monoxide gas reacts to form solid iron and liberates carbon dioxide gas.
(d) Hydrogen sulphide gas reacts with oxygen gas to form solid sulphur and liquid water.

(a)$Pb(CH_{3}COO)_{2} (aq) + 2HCl (dil) \rightarrow PbCl_{2}(s)\downarrow + 2CH_{3}COOH (aq)$
This is a double displacement and a precipitation reaction.
(b) $2C_{2}H_{5}OH (l) + 2Na (s) \rightarrow 2C_{2}H_{5}O^{-}Na^{+} + H_{2}\uparrow$
This is a displacement reaction.
(c) $Fe_{2}O_{3}(s) + 3CO(g) \rightarrow 2Fe (s) + 3CO_{3}(g)\uparrow$
This is an example of Redox reaction.
(d) $2H_{2}S (g) + O_{2} (g) \rightarrow 2S (s) + 2H_{2}O (l)$
This is an example of Redox reaction.

Q.34. Why do we store silver chloride in dark coloured bottles?

Silver chloride can decompose in the presence of light to form silver metal therefore it must store in dark coloured bottles which can interrupt the path of light and prevent the decomposition of silver chloride.
The photochemical decomposition of silver chloride is shown below;
$\ce{2AgCl(s) ->[{Sunlight}] 2Ag(s) + Cl2(g)}$

Q.35. Balance the following chemical equation and identify chemical reaction the type of chemical reaction.
(a) $Mg(s) + Cl_{2}(g)\rightarrow MgCl_{2}(s)$
(b) $\ce{HgO(s)->[{Heat}]Hg + O2}$
(c) $\ce{Na(s)+ S(s)->[{Fuse}] Na2S(s)}$
(d) $TiCl_{4}(vap) + Mg(l)\rightarrow MgCl_{2}(l) + Ti(s)$
(e) $CaO + SiO_{2}(s)\rightarrow CaSiO_{3}(s)$
(f) $\ce{H2O2(l)->[{uv}] H2O(l)+ O2(g)}$

(a) $Mg(s) + Cl_{2}(g)\rightarrow MgCl_{2}(s)$ (Combination reaction)
(b) $\ce{HgO(s)->[{Heat}]Hg + O2}$ (Thermal decomposition reaction)
(c) $\ce{Na(s)+ S(s)->[{Fuse}] Na2S(s)}$ (Combination reaction)
(d) $TiCl_{4}(vap) + Mg(l)\rightarrow MgCl_{2}(l) + Ti(s)$ (Displacement reaction)
(e) $CaO + SiO_{2}(s)\rightarrow CaSiO_{3}(s)$ (Combination reaction)
(f) $\ce{H2O2(l)->[{uv}] H2O(l)+ O2(g)}$ (Photochemical reaction)

Q.36. A magnesium ribbon is burnt in oxygen to give a white compound X accompanied by emission of light. If the burning ribbon is now placed in an atmosphere of nitrogen, it continues to burn and forms a compound Y.
(a) Write the chemical formulae of X and Y.
(b) Write a balanced chemical equation, when X is dissolved in water.

Answer: Reaction of Mg with oxygen forms MgO
$2Mg(s) + O_{2}\rightarrow 2MgO(s) + Light$
Reaction of Mg with atmospheric nitrogen forms $Mg_{3}N_{2}$
$3Mg(s) + N_{2}(g)\rightarrow Mg_{3}N_{2}$
(a) The chemical formula of X = MgO; $Y= Mg_{3}N_{2}$
(b) $MgO + H_{2}O \rightarrow Mg(OH)_{2}$

Q.37. Zinc liberates hydrogen gas when reacted with dilute hydrochloric acid, whereas copper does not. Explain, why?

$Cu + HCl \rightarrow No reaction$
(b) The black substance that is formed by the reaction of hydrogen sulphide and atmospheric oxygen is silver sulphide $(Ag_{2}S).$
$4Ag + O_{2} + 2H_{2}S \rightarrow 2Ag_{2}S + 2H_{2}O$