Q: Question 11Give balanced equations for thermal decomposition of :(a) lead carbonate(b) lead nitrate(c) ammonium dichromate(d) mercury [II] oxide(e) calcium hydroxide

(a) PbCO3 →Δ\xrightarrow{\enspace\Delta\enspace}Δ PbO + CO2(b) 2Pb(NO3)2 →Δ\xrightarrow{\enspace\Delta\enspace}Δ 2PbO + 4NO2 + O2(c) (NH4)2Cr2O7 →Δ\xrightarrow{\enspace\Delta\enspace}Δ Cr2O3 + 4H2O + N2(d) 2HgO →Δ\xrightarrow{\enspace\Delta\enspace}Δ 2Hg + O2(e) Ca(OH)2 →Δ\xrightarrow{\enspace\Delta\enspace}Δ CaO + H2O

Q: Question 18Supply of energy maybe required to initiate a reaction. State the different forms with a suitable example of reactions initiated by supply of energy.

Different forms of energy :(i) Heat energy: N2+O2→3000°C2NO\text{N}_2 + \text{O}_2 \xrightarrow {3000\degree \text{C}} 2\text{NO}N2+O23000°C2NO(ii) Light energy: H2+Cl2→Sunlight2HCl\text{H}_2 + \text{Cl}_2 \xrightarrow {\text{Sunlight}} 2\text{HCl}H2+Cl2Sunlight2HCl(iii) Electricity: 2H2O[acidified]→CurrentElectric2H2[cathode]+O2[anode]\underset{\text{[acidified]}}{2\text{H}_2\text{O}} \xrightarrow[\text{Current}]{\text{Electric}} \underset{\text{[cathode]}}{2\text{H}_2} + \underset{\text{[anode]}}{\text{O}_2}[acidified]2H2OElectricCurrent[cathode]2H2+[anode]O2(iv) Pressure : N2+3H2⇌200 atmos. pressureFe/450-500 °C 2NH3\text{N}_2 + 3\text{H}_2 \xrightleftharpoons[\text{200 atmos. pressure}]{\text{Fe/450-500 \degree C }} \text{2NH}_3N2+3H2Fe/450-500 °C 200 atmos. pressure2NH3(v) Catalyst : 4NH3+5O2→Pt./800°C4NO+6H2O4\text{NH}_3 + 5\text{O}_2 \xrightarrow {\text{Pt.}/800\degree \text{C}} 4\text{NO} + 6\text{H}_2\text{O}4NH3+5O2Pt./800°C4NO+6H2O(vi) Sound Energy: C2H2[acetylene]→EnergySound2C+H2\underset{\text{[acetylene]}}{\text{C}_2\text{H}_2} \xrightarrow[\text{Energy}]{\text{Sound}} 2\text{C} + \text{H}_2[acetylene]C2H2SoundEnergy2C+H2

Question 1

Question 6Chemical reactions may proceed with evolution or absorption of heat. Give an example of each.

Accepted Answer

Chemical reaction which proceed with evolution of heat [Exothermic] —Nitrogen and hydrogen:N2 + 3H2 ⇌Δ\xrightleftharpoons{\enspace\Delta\enspace}Δ​ 2NH3 + ΔTChemical reaction which proceed by absorption of heat [Endothermic] —Nitrogen and oxygen :N2 + O2 →Δ\xrightarrow{\enspace\Delta\enspace}Δ​ 2NO - ΔT

Question 2

Question 7Define the following types of chemical changes or reactions with a suitable example of each.(a) Direct combination reaction or synthesis(b) Decomposition reaction(c) Displacement reaction or substitution reaction(d) Double decomposition reaction

Accepted Answer

(a) A chemical reaction in which two or more elements or compounds react to form one new compound is called a Direct combination reaction or synthesis.For example,Hydrogen burns in air to give water:  2H2 [g] + O2 [g] ⟶ 2H2O [l](b) A chemical reaction in which a chemical compound decomposes into two or more simpler substances (elements and/or compounds) is called a Decomposition reaction.For example,Calcium carbonate on heating decomposes to Calcium Oxide and Carbon dioxide:  CaCO3 →Δ\xrightarrow{\enspace\Delta\enspace}Δ​ CaO + CO2(c) A chemical reaction in which an element or radical has replaced another element in a compound is known as Displacement reaction or Substitution reaction.For example,Magnesium displaces Copper from Copper [II] sulphate solution:Mg + CuSO4 ⟶ MgSO4 + Cu(d) A chemical reaction in which both reactants [compounds] are decomposed to give two new compounds by exchanging their radicals is known as Double decomposition reaction.For example,Silver nitrate + Potassium chloride ⟶ Silver chloride + Potassium nitrateAgNO3 + KCl ⟶ AgCl + KNO3

Question 3

Question 11Give balanced equations for thermal decomposition of :(a) lead carbonate(b) lead nitrate(c) ammonium dichromate(d) mercury [II] oxide(e) calcium hydroxide

Accepted Answer

(a) PbCO3 →Δ\xrightarrow{\enspace\Delta\enspace}Δ​ PbO + CO2(b) 2Pb(NO3)2 →Δ\xrightarrow{\enspace\Delta\enspace}Δ​ 2PbO + 4NO2 + O2(c) (NH4)2Cr2O7 →Δ\xrightarrow{\enspace\Delta\enspace}Δ​ Cr2O3 + 4H2O + N2(d) 2HgO →Δ\xrightarrow{\enspace\Delta\enspace}Δ​ 2Hg + O2(e) Ca(OH)2 →Δ\xrightarrow{\enspace\Delta\enspace}Δ​ CaO + H2O

Question 4

Question 12Define a thermal dissociation reaction with a suitable example. Give an example of a photochemical decomposition reaction. Name a metallic oxide which on thermal decomposition is reduced to a metal.

Accepted Answer

Thermal dissociation reaction is a decomposition reaction in which a substance dissociates into two or more simpler substances on application of heat.For example, on application of heat, ammonium chloride decomposes into Ammonia and HCl:NH4Cl ⇌coolheat\xrightleftharpoons[\enspace	ext{cool}\enspace]{\enspace	ext{heat}\enspace}heatcool​ NH3 + HClAn example of photochemical decomposition reaction is the decomposition of silver salts in the presence of light:2AgCl →☼\xrightarrow{\enspace☼\enspace}☼​ 2Ag + Cl2HgO is a metallic oxide which reduces to a metal on thermal decomposition as per the equation below:2HgO →Δ\xrightarrow{\enspace\Delta\enspace}Δ​ 2Hg + O2

Question 5

Question 13Define a displacement reaction with a suitable example. State how it is represented. Give a reason why zinc displaces hydrogen from dilute sulphuric acid but copper does not.

Accepted Answer

A chemical reaction which takes place when an element [or radical] has replaced another element in a compound is known as Displacement reactionIt is represented as :X + YZ ⟶ Y + XZe.g.,Magnesium + Copper [II] sulphate ⟶ Magnesium sulphate + CopperMg + CuSO4 ⟶ MgSO4 + CuAs a more electropositive metal displaces a less electropositive metal from it's aq. soln. therefore, zinc being more electropositive is placed above [H] in activity series and displaces hydrogen from dilute sulphuric acid whereas copper being less electropositive is placed below [H] in electrochemical series and cannot displace hydrogen from sulphuric acid.

Question 6

Question 14Explain the term double decomposition precipitation reaction. Give a balanced equation for the preparation of two different insoluble lead salts from their salt solutions by double decomposition precipitation.

Accepted Answer

Reaction between two compounds in aqueous solution state to give two new compounds one of which is insoluble is called double decomposition precipitation reaction.Balanced equations for the preparation of two different insoluble lead salts are given below:Pb(NO3)2Lead nitrate+2NaClsodium chloride⟶2NaNO3sodium nitrate+PbCl2↓Lead [II] chloride\underset{	ext{Lead nitrate}}{	ext{Pb(NO}_3)_2} + \underset{	ext{sodium chloride}}{2	ext{NaCl}} \longrightarrow \underset{	ext{sodium nitrate}}{2{	ext{NaNO}_3}} + \underset{	ext{Lead [II] chloride}}{	ext{PbCl}_2 \downarrow}Lead nitratePb(NO3​)2​​+sodium chloride2NaCl​⟶sodium nitrate2NaNO3​​+Lead [II] chloridePbCl2​↓​Pb(NO3)2Lead nitrate+Na2SO4sodium sulphate⟶2NaNO3sodium nitrate+PbSO4↓Lead sulphate\underset{	ext{Lead nitrate}}{	ext{Pb(NO}_3)_2} + \underset{	ext{sodium sulphate}}{	ext{Na}_2	ext{SO}_4} \longrightarrow \underset{	ext{sodium nitrate}}{2{	ext{NaNO}_3}} + \underset{	ext{Lead sulphate}}{	ext{PbSO}_4 \downarrow}Lead nitratePb(NO3​)2​​+sodium sulphateNa2​SO4​​⟶sodium nitrate2NaNO3​​+Lead sulphatePbSO4​↓​

Question 7

Question 15Explain with the help of balanced equations, how precipitation reactions are used for identifying the positive radicals in three different salts, each having a different cation [positive ion].

Accepted Answer

Precipitation reactions finds it's use in analytical chemistry for identifying the positive radical in a salt by analyzing the colour of the precipitate.(i) FeSO4 + 2NaOH ⟶ Na2SO4 + Fe(OH)2 ↓Identified ion: Fe2+ ; Dirty green ppt. of Iron [II] hydroxide is obtained. Hence, it can be identified.(ii) CuSO4 + 2NaOH ⟶ Na2SO4 + Cu(OH)2 ↓Identified ion: Cu2+ ; Pale blue ppt. of Copper [II] hydroxide is obtained. Hence, it can be identified.(iii) Pb(NO3)2 + 2NaOH ⟶ 2NaNO3 + Pb(OH)2 ↓Identified ion: Pb2+ ; Chalky white ppt. of Lead [II] hydroxide is obtained. Hence, it can be identified.

Question 8

Question 16Define the term – double decomposition – neutralization reaction with a suitable representation.Convert :(a) an insoluble base (b) a soluble baseto their respective soluble salts by neutralization reaction.

Accepted Answer

Reaction between a base and an acid to give two new compounds — salt & water — by interchange of radicals is termed double decomposition neutralization reaction.It is represented as :For example,NaOH [Base] + HCl [Acid] ⟶ NaCl [Salt] + H2O [Water](a) Insoluble base [oxide] — Copper [II] oxide reacts with sulphuric acid and forms Copper sulphate :CuO + H2SO4 ⟶ CuSO4 + H2O(b) A soluble base — Sodium hydroxide reacts with HCl and forms soluble salt (sodium chloride) and water:NaOH + HCl ⟶ NaCl + H2O

Question 9

Question 17Explain the term energy changes in a chemical change or reaction. Give an example with a balanced equation, for each of the following reactions:(a) exothermic reaction(b) endothermic reaction(c) photochemical reaction(d) electrochemical reaction.

Accepted Answer

The difference between the chemical energy of the reactants and the products is known as the energy changes in a chemical change or reaction.Example of :(a) exothermic reaction:     2H2 + O2 ⇌Δ\xrightleftharpoons{\enspace\Delta\enspace}Δ​ H2O + ΔT(b) endothermic reaction:     N2 + O2 ⇌Δ\xrightleftharpoons{\enspace\Delta\enspace}Δ​ 2NO - ΔT(c) photochemical reaction:     H2 + Cl2 →sunlight\xrightarrow{	ext{sunlight}}sunlight​ 2HCl     Slow reaction in diffused light     Explosive reaction in direct sunlight(d) electrochemical reaction:     2H2O[acidified]→CurrentElectric2H2[cathode]+O2[anode]\underset{	ext{[acidified]}}{2	ext{H}_2	ext{O}} \xrightarrow[	ext{Current}]{	ext{Electric}} \underset{	ext{[cathode]}}{2	ext{H}_2} + \underset{	ext{[anode]}}{	ext{O}_2}[acidified]2H2​O​ElectricCurrent​[cathode]2H2​​+[anode]O2​​

Question 10

Question 18Supply of energy maybe required to initiate a reaction. State the different forms with a suitable example of reactions initiated by supply of energy.

Accepted Answer

Different forms of energy :(i) Heat energy:     N2+O2→3000°C2NO	ext{N}_2 + 	ext{O}_2 \xrightarrow {3000\degree 	ext{C}} 2	ext{NO}N2​+O2​3000°C​2NO(ii) Light energy:     H2+Cl2→Sunlight2HCl	ext{H}_2 + 	ext{Cl}_2 \xrightarrow {	ext{Sunlight}} 2	ext{HCl}H2​+Cl2​Sunlight​2HCl(iii) Electricity:     2H2O[acidified]→CurrentElectric2H2[cathode]+O2[anode]\underset{	ext{[acidified]}}{2	ext{H}_2	ext{O}} \xrightarrow[	ext{Current}]{	ext{Electric}} \underset{	ext{[cathode]}}{2	ext{H}_2} + \underset{	ext{[anode]}}{	ext{O}_2}[acidified]2H2​O​ElectricCurrent​[cathode]2H2​​+[anode]O2​​(iv) Pressure :     N2+3H2⇌200 atmos. pressureFe/450-500 °C 2NH3	ext{N}_2 + 3	ext{H}_2 \xrightleftharpoons[	ext{200 atmos. pressure}]{	ext{Fe/450-500 \degree C }} 	ext{2NH}_3N2​+3H2​Fe/450-500 °C 200 atmos. pressure​2NH3​(v) Catalyst :     4NH3+5O2→Pt./800°C4NO+6H2O4	ext{NH}_3 + 5	ext{O}_2 \xrightarrow {	ext{Pt.}/800\degree 	ext{C}} 4	ext{NO} + 6	ext{H}_2	ext{O}4NH3​+5O2​Pt./800°C​4NO+6H2​O(vi) Sound Energy:      C2H2[acetylene]→EnergySound2C+H2\underset{	ext{[acetylene]}}{	ext{C}_2	ext{H}_2} \xrightarrow[	ext{Energy}]{	ext{Sound}} 2	ext{C} + 	ext{H}_2[acetylene]C2​H2​​SoundEnergy​2C+H2​

Question 11

Question 1(1986)Explain : 'Silver nitrate solution is kept in coloured reagent bottles in the laboratory.'

Accepted Answer

As silver nitrate gets decomposed by sunlight to produce the respective metal, nitrogen dioxide and oxygen gas, hence it is kept in coloured reagent bottles.2AgNO3→Δ2Ag+2NO2+O22	ext{AgNO}_3\xrightarrow{\Delta} 2	ext{Ag} + 2	ext{NO}_2 + 	ext{O}_22AgNO3​Δ​2Ag+2NO2​+O2​

Question 12

Question 1(1987)Give an example of an endothermic reaction.

Accepted Answer

N2 + O2 →Δ\xrightarrow{\enspace\Delta\enspace}Δ​ 2NO - ΔT

Question 13

Question 3Give a balanced equation for each of the following types of reactions :A direct combination reaction between phosphorus and a neutral gas.A soluble salt of lead formed from an insoluble base by double decomposition – neutralization.A thermal decomposition reaction of a salt – which results in the formation of nitrogen gas.A synthesis reaction between a metal & a non-metal resulting in formation of an insoluble salt of iron.A decomposition reaction of a salt which leaves behind a silvery metal.

Accepted Answer

4P + O2 ⟶ 2P2O5PbO + 2HNO3 ⟶ Pb(NO3)3 + H2O(NH4)2Cr2O7 ⟶ Cr2O3 + 4H2O + N2Fe + S ⟶ FeS2AgCl →☼\xrightarrow{\enspace☼\enspace}☼​ 2Ag + Cl2

Question 14

Question 4.1Differentiate between the following :Synthesis reaction & a substitution reaction.

Accepted Answer

Synthesis ReactionSubstitution ReactionA chemical reaction in which two or more elements or compounds react to form one new compound is called a Direct Combination Reaction or Synthesis Reaction.A chemical reaction in which an element or radical has replaced another element in a compound is called Displacement or Substitution Reaction.For example:hydrogen + oxygen ⟶ water2H2 [g] + O2 [g] ⟶ 2H2O [l]For example:Magnesium + Copper [II] sulphate ⟶ Magnesium sulphate + CopperMg + CuSO4 ⟶ MgSO4 + Cu

Question 15

Question 4.2Differentiate between the following :Electrolytic decomposition & photochemical decomposition

Accepted Answer

Electrolytic decompositionPhotochemical decompositionA decomposition reaction which is brought about by passage of electric current is known as Electrolytic decomposition.A decomposition reaction which occurs in the presence of light is known as Photochemical decomposition.For example:2H2O ⟶ 2H2 + O2 (On Passage of Electric Current)For example:2AgCl ⟶ 2Ag + Cl

Question 16

Question 4.5Differentiate between the following :Neutralization reaction & a precipitation reaction.

Accepted Answer

Neutralization reactionPrecipitation reactionA reaction between two compounds - base and acid to give two new compounds - salt and water - by interchange of radicals is called a Neutralization reaction.A reaction between two compounds in aqueous solution state to give two new compounds one of which is insoluble (precipitate) is called a Precipitation reaction.For example:NaOH + HCl ⟶ NaCl + H2OFor example:Pb(NO3)2 + 2NaCl ⟶ 2NaNO3 + PbCl2

Question 17

Question 5Match the chemical reactions in List I with the appropriate answer in List II.List IList IIAB ⇌ A + B (on heating)A : Double decompositionAB ⟶ A + BB : Thermal dissociationX+Y- + A+B- ⟶ X+B- + A+Y-C : Endothermic reactionX + YZ ⟶ XZ + YD : Displacement reactionX + Y ⟶ XY - Δ (on heating)E : Decomposition reaction

Accepted Answer

List IList IIAB ⇌ A + B (on heating)B : Thermal dissociationAB ⟶ A + BE : Decomposition reactionX+Y- + A+B- ⟶ X+B- + A+Y-A : Double decompositionX + YZ ⟶ XZ + YD : Displacement reactionX + Y ⟶ XY - Δ (on heating)C : Endothermic reaction

Question 18

Question 6Name the solid residual product formed in each reaction and state it's colour during – thermal decomposition of the following substances.Copper nitrate.Ammonium dichromate.Zinc carbonate.Lead nitrate.Calcium hydroxide.

Accepted Answer

Below table lists the solid residual product formed along with it's colour for the given substances:Sl.No.SubstanceResidualProductColour1Copper nitrateCopper [II] oxideBlack2Ammonium dichromateChromic oxideGreen3Zinc carbonateZinc oxideWhite4Lead nitrateLead oxideYellow5Calcium hydroxideCalcium oxideWhite

Chemical Changes & Reactions

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Unit Test Paper 2 — Chemical Changes & Rxns