Question 1

Question 3How do the following help in bringing about a chemical change? Explain each with a suitable example.(a) Pressure(b) Light(c) Catalyst(d) Heat

Accepted Answer

(a) Some chemical reactions take place when the reactants are subjected to high pressure.When nitrogen and hydrogen gas are subjected to high pressure, ammonia gas is produced.N2+3H2⇌200 atmos.pressure450 °C 2NH3	ext{N}_2 + 3	ext{H}_2 \xrightleftharpoons[\underset{	ext{pressure}}{	ext{200 atmos.}}]{	ext{450 \degree C }} 	ext{2NH}_3N2​+3H2​450 °C pressure200 atmos.​​2NH3​(b) Some chemical reactions can take place only in the presence of light and are called photochemical reactions.Photosynthesis is a chemical reaction in which glucose is prepared by the green leaves of a plant but light is necessary for the reaction to take place.6CO2[carbon dioxide]+6H2O→SunlightChlorophyllC6H12O6[glucose]+6O2[oxygen]\underset{	ext{[carbon dioxide]}}{6	ext{CO}_2} + 6	ext{H}_2	ext{O} \xrightarrow[	ext{Sunlight}]{	ext{Chlorophyll}} \underset{	ext{[glucose]}}{	ext{C}_6	ext{H}_{12}	ext{O}_6} + \underset{	ext{[oxygen]}}{6	ext{O}_2}[carbon dioxide]6CO2​​+6H2​OChlorophyllSunlight​[glucose]C6​H12​O6​​+[oxygen]6O2​​(c) Some chemical reactions need a catalyst to change the rate of reaction, if the reaction is too slow or too fast. Positive catalyst increases the rate of chemical reaction whereas negative catalyst decreases the rate of reaction.Finely divided iron is used as a positive catalyst in the manufacture of ammonia from hydrogen and oxygen.     N2+3H2⇌200-900 atmos.Fe catalyst/450°C 2NH3	ext{N}_2 + 3	ext{H}_2 \xrightleftharpoons[	ext{200-900 atmos.}]{	ext{Fe catalyst/450\degree C }} 	ext{2NH}_3N2​+3H2​Fe catalyst/450°C 200-900 atmos.​2NH3​(d) Some chemical reactions take place only in the presence of heat.If iron powder and sulphur powder are mixed, they do not react. But they are heated, they react to form iron sulphide.Fe + S →Δ\xrightarrow{\enspace\Delta\enspace}Δ​ FeS

Question 2

Question 4Explain the following types of chemical reactions giving two examples for each of them:(a) combination reaction(b) decomposition reaction(c) displacement reaction(d) double displacement reaction

Accepted Answer

(a) Combination Reaction — A reaction in which two or more substances combine to form a single substance is called a combination reaction. It is also called a synthesis reaction.Examples:(i) Two elements combine to form a compound.When iron and sulphur (both elements) are heated together they combine to form a compound iron sulphide.Fe (s) + S (s) →Δ\xrightarrow{\enspace\Delta\enspace}Δ​ FeS (s)(ii) An element and a compound can also combine to form a product.Carbon monoxide, a compound, burns in presence of oxygen, an element, to form a single product, carbon dioxide.2CO (g) + O2 (g) →Δ\xrightarrow{\enspace\Delta\enspace}Δ​ 2CO2 (g)(b) Decomposition Reaction — A reaction in which a compound breaks up on heating into two or more simpler substances is called a decomposition reaction.Examples:(i) Calcium carbonate decomposes on strong heating to form two compounds, calcium oxide and carbon dioxide.CaCO3 (s) →Δ\xrightarrow{\enspace\Delta\enspace}Δ​ CaO (s) + CO2 (g)(ii) Mercuric oxide when heated decomposes to form mercury and oxygen.2HgO (s) →Δ\xrightarrow{\enspace\Delta\enspace}Δ​ 2Hg (s) + O2 (g)(c) Displacement Reaction — A reaction in which a more active element displaces a less active element from a compound is called a displacement reaction.Examples:(i) Zinc is more reactive than copper so it displaces copper from copper sulphate solution to form zinc sulphate.Zn (s) + CuSO4 (aq) ⟶ ZnSO4 (aq) + Cu (s)(ii) Chlorine is more reactive than bromine, so it displaces bromine from potassium bromide solution.2KBr + Cl2 ⟶ 2KCl + Br2(d) Double Displacement Reaction — A chemical reaction in which two compounds in their aqueous state exchange their ions or radicals to form new compounds is called a double decomposition reaction.AB + CD ⟶ CB + ADHere AB and CD are reactants. They exchange their ions to form CB and AD which are the products.Examples:(i) When a solution of silver nitrate is added to a solution of sodium chloride a precipitate of silver chloride is formed.AgNO3 (aq) + NaCl (aq) ⟶ NaNO3 (aq) + AgCl ↓(ii) When ferrous sulphate solution is added to sodium hydroxide solution, a dirty green precipitate of ferrous hydroxide is formed.FeSO4 (aq) + 2NaOH (aq) ⟶ Fe(OH)2 ↓ + Na2SO4 (aq)

Question 3

Question 6How will you obtain?(a) Magnesium oxide from magnesium.(b) Silver chloride from silver nitrate.(c) Nitrogen dioxide from lead nitrate.(d) Zinc chloride from zinc.(e) Ammonia from nitrogen.Also give balanced equations for the reactions.

Accepted Answer

(a) When magnesium reacts with oxygen it produces magnesium oxide.2Mg + O2 ⟶ 2MgO(b) When silver nitrate reacts with hydrochloric acid silver chloride is formed.AgNO3 (aq) + HCl ⟶ AgCl↓ + HNO3(c) When lead nitrate is heated strongly, a reddish brown gas nitrogen dioxide is produced.2Pb(NO3)2 →Δ\xrightarrow{\enspace\Delta\enspace}Δ​ 2PbO + 4NO2 (g) + O2 (g)(d) Zinc reacts with dilute HCl to produce zinc chloride.Zn + 2HCl (dil.) ⟶ ZnCl2 + H2 (g)(e) When nitrogen gas reacts with hydrogen gas in presence of finely divided iron as catalyst, subjected to pressure of 200-900 atm and temperature of about 450°C, ammonia gas is produced.     N2+3H2⇌200-900 atmos.Fe catalyst/450°C 2NH3	ext{N}_2 + 3	ext{H}_2 \xrightleftharpoons[	ext{200-900 atmos.}]{	ext{Fe catalyst/450\degree C }} 	ext{2NH}_3N2​+3H2​Fe catalyst/450°C 200-900 atmos.​2NH3​

Question 4

Question 7What do you observe when:(a) Iron nail is kept in copper sulphate solution for sometime?(b) Phenolphthalein is added to sodium hydroxide solution?(c) Blue litmus paper is dipped in dilute hydrochloric acid?(d) Lead nitrate is heated?(e) Magnesium ribbon is burnt in oxygen?(f) Ammonia is brought in contact with hydrogen chloride gas?

Accepted Answer

(a) When iron nail is kept in a blue coloured copper sulphate solution for sometime, a reddish brown coating is seen on the iron nail and the colour of the solution changes gradually from blue to light green. The reason for this observation is that iron being more reactive than copper, displaces copper from the solution and copper is deposited on the iron nail.Fe + CuSO4 (aq) ⟶ FeSO4 + Cu(b) When a few drops of phenolphthalein is added to sodium hydroxide solution, it turns pink indicating that sodium hydroxide solution is basic in nature.(c) Blue litmus paper changes into red colour when dipped in dilute hydrochloric acid confirming its acidic nature.(d) When solid lead nitrate is heated strongly, it decomposes to produce light yellow solid lead monoxide, reddish brown nitrogen dioxide gas and colourless oxygen gas.2Pb(NO3)2 →Δ\xrightarrow{\enspace\Delta\enspace}Δ​ 2PbO + 4NO2 (g) + O2 (g)(e) When magnesium ribbon is burnt in oxygen gas, it burns with a dazzling white light and produces a white powder of magnesium oxide.2Mg + O2 ⟶ 2MgO(f) When ammonia is brought in contact with hydrogen chloride gas, dense white fumes of ammonium chloride are produced.NH3 + HCl ⟶ NH4Cl

Question 5

Question 8bGive reason:Acidic soil is treated with quicklime.

Accepted Answer

Quicklime is a base which neutralizes the acidic soil. So acidic soil is treated with base like quicklime.

Question 6

Question 8cGive reason:Wasp sting is treated with vinegar.

Accepted Answer

Wasp sting is alkaline in nature which is neutralized by vinegar which is a weak acid.

Question 7

Question 9What is meant by metal reactivity series? State its importance (any two points).

Accepted Answer

A list in which the metals are arranged in a decreasing order of their chemical reactivity is called the metal reactivity series.The importance of metal reactivity series are:The series facilitates the comparative study of metals in terms of the degree of their reactivity.The compounds of the metals like oxides, carbonates, nitrates and hydroxides too can be easily compared.

Question 8

Question 10What are oxides? Give two examples of each of the following oxides.(a) Basic oxide(b) Acidic oxide(c) Amphoteric oxide(d) Neutral oxide

Accepted Answer

Oxides are compounds which essentially contains oxygen in its molecule, chemically combined with a metal or a non-metal.(a) Calcium oxide (CaO) and magnesium oxide (MgO) are basic oxides.(b) Carbon dioxide (CO2) and sulphur dioxide (SO2) are acidic oxides.(c) Zinc oxide (ZnO) and aluminium oxide (Al2O3) are amphoteric oxides.(d) Carbon monoxide (CO) and nitric oxide (NO) are neutral oxides.

Question 9

Question 11Define exothermic and endothermic reactions. Give two examples of each.

Accepted Answer

Exothermic ReactionA chemical reaction in which heat is given out is called an exothermic reaction.Examples:When carbon burns in oxygen, carbon dioxide is formed and a lot of heat is produced.C + O2 ⟶ CO2 + HeatWhen water is added to quicklime, slaked lime is formed and along with it a large amount of heat energy is produced.CaO + H2O ⟶ Ca(OH)2 + HeatEndothermic ReactionA chemical reaction in which heat is absorbed is called an endothermic reaction.Examples:When nitrogen and oxygen gas are heated together to a temperature of about 3000°C, nitric oxide gas is formed.N2 + O2 →3000°Cheat\xrightarrow[3000\degree	ext{C}]{	ext{heat}}heat3000°C​ 2NOWhen calcium carbonate is heated at 1000°C, it decomposes to give calcium oxide and carbon dioxide.CaCO3 →1000°Cheat\xrightarrow[1000\degree	ext{C}]{	ext{heat}}heat1000°C​ CaO + CO2

Question 10

Question 12State the effect of the following on the surroundings:(a) an endothermic reaction(b) an exothermic reaction

Accepted Answer

(a) An endothermic reaction causes fall in temperature. These reactions absorb heat from the surroundings leading to cooling effect of the surroundings.(b) An exothermic reaction causes rise in temperature. These reactions release heat into surroundings leading to increase in temperature of the surroundings.

Question 11

Question 13What do you observe when:(a) an acid is added to a basic solution?(b) ammonium chloride is dissolved in water?

Accepted Answer

(a) When an acid is added to a basic solution, neutralization reaction takes place and salt and water are produced. For example:NaOH + HCl ⟶ NaCl + H2O(b) When ammonium chloride is dissolved in water, heat is absorbed leading to fall in temperature. So the apparatus in which this reaction occurs becomes colder than earlier.

Question 12

Question 5Give balanced equations for -(i) A direct combination reaction involving two elements, one of which is a non-metal.(ii) A thermal decomposition reaction involving heat on limestone [calcium carbonate].(iii) An electrolytic decomposition reaction involving a neutral liquid.(iv) A displacement reaction involving a metal above hydrogen in the activity series with copper [II] sulphate solution.(v) A double decomposition neutralization reaction involving an acid & a base.(vi) A white precipitate obtained during a double decomposition reaction involving a silver salt with a sodium salt.

Accepted Answer

(i) C + O2 ⟶ CO2(ii) CaCO3 →Δ\xrightarrow{\Delta}Δ​ CaO + CO2(iii) 2H2O →currentelectric\xrightarrow[	ext{current}]{	ext{electric}}electriccurrent​ 2H2 + O2(iv) Zn + CuSO4 ⟶ ZnSO4 + Cu(v) HCl + NaOH ⟶ NaCl + H2O(vi) AgNO3 + NaCl ⟶ AgCl ↓ + NaNO3

Question 13

Question 6State what is meant by 'reactivity series of metals'. With reference to -(a) Water(b) Acidsexplain with suitable examples how the reactivity of the metals could be differentiated.

Accepted Answer

Reactivity series of metals is a series of arrangement of metals in decreasing order of their chemical reactivity. Metal reactivity series is listed below:ActivitySeriesKNaCaMgAlZnFePb*[H]CuHgAgPtAuDifferentiating reactivity of the metals with reference to Water & Acids:ReactionwithMetalsObservationEquationWaterK & NaReact violently with cold water2K + 2H2O ⟶ 2KOH + H2CaReacts less vigorously with cold waterCa + 2H2O ⟶ Ca(OH)2 + H2Mg to FeReact smoothly with water [steam]3Fe + 4H2O ⇌ Fe3O4 + 4H2Below [H]Do not react with water or steam displacing hydrogen from it. AcidsK & NaReact explosively with HCl or dil. H2SO42K + 2HCl ⟶ 2KCl + H2Ca to FeReact less vigorously with decreasing vigour with dil. HCl or dil. H2SO4Ca + 2HCl ⟶ CaCl2 + H2Below [H]Do not react with dil. acids, displacing hydrogen from it.

Question 14

Question 9Certain thermal decomposition reactions, result in formation of oxides. Give balanced equations for the thermal decomposition of the following, which result in formation of a metallic oxide.(a) Limestone(b) Lead carbonate(c) Calcium nitrate(d) Calcium hydroxide.

Accepted Answer

(a) Thermal decomposition of limestoneCaCO3 →Δ\xrightarrow\DeltaΔ​ CaO + CO2(b) Thermal decomposition of lead carbonatePbCO3 →Δ\xrightarrow\DeltaΔ​ PbO + CO2(c) Thermal decomposition of calcium nitrate2Ca(NO3)2 →Δ\xrightarrow\DeltaΔ​ 2CaO + 4NO2 + O2(d) Thermal decomposition of calcium hydroxide.Ca(OH)2 →Δ\xrightarrow\DeltaΔ​ CaO + H2O

Question 15

Question 12Give one example each of -(a) A peroxide(b) A mixed oxide(c) A dioxide.

Accepted Answer

(a) A peroxide — Na2O2(b) A mixed oxide — Pb3O4(c) A dioxide — PbO2

Chemical Reactions

Exercise I

Exercise II

Types of reactions