# Chemical Changes & Reactions

## Questions

#### Question 1(1986)

Explain : 'Silver nitrate solution is kept in coloured reagent bottles in the laboratory.'

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.

$2\text{AgNO}_3\xrightarrow{\Delta} 2\text{Ag} + 2\text{NO}_2 + \text{O}_2$

#### Question 1(1987)

Give an example of an endothermic reaction.

N2 + O2 $\xrightarrow{\enspace\Delta\enspace}$ 2NO - ΔT

#### Question 1(1989)

Reactions can be classified as : Direct combination, decomposition, simple displacement, double decomposition, Redox reactions. State which of the following types, takes place in the reactions given below

(i) Cl2 + 2KI ⟶ 2KCl + l2

(ii) 2Mg + O2 ⟶ 2MgO

(iii) SO2 + 2H2O + Cl2 ⟶ 2HCl + H2SO4

(iv) AgNO3 + HCl ⟶ AgCl + HNO3

(v) 4HNO3 ⟶ 4NO2 + 2H2O + O2

(i) Cl2 + 2KI ⟶ 2KCl + l2Simple displacement

(ii) 2Mg + O2 ⟶ 2MgO — Direct combination

(iii) SO2 + 2H2O + Cl2 ⟶ 2HCl + H2SO4Redox reaction

(iv) AgNO3 + HCl ⟶ AgCl + HNO3Double decomposition

(v) 4HNO3 ⟶ 4NO2 + 2H2O + O2Decomposition reaction

#### Question 1

Explain the term chemical reaction with special reference to reactants and products.

A chemical reaction is a change, in which matter changes into a new substance or substances. In a chemical reaction the substance or substances taking part in the reaction called reactants are transformed into one or more new substances called the products.

#### Question 2

Give a suitable example with equation to show the representation of a chemical reaction.

Silver nitrate reacts with hydrochloric acid to form silver chloride and nitric acid.

AgNO3 + HCl ⟶ AgCl + HNO3

#### Question 3

A chemical reaction is often accompanied by external indications or characteristics.
Give two examples where a chemical reaction is accompanied by a change in colour of the reactants & products on completion of the reaction.

The two examples are —

1. Heat on Copper [II] Nitrate:
2Cu(NO3)2 $\xrightarrow{\enspace\Delta\enspace}$ 2CuO + 4NO2 + O2
Blue copper [II] nitrate changes to black coloured copper [II] oxide.
2. Heat on Lead [II] Nitrate:
2Pb(NO3)2 $\xrightarrow{\enspace\Delta\enspace}$ 2PbO + 4NO2 + O2
White lead [II] nitrate changes to yellow litharge.

#### Question 4

Give balanced equations for reactions involving evolution of a gas on addition of dilute acid to :

(a) sodium sulphite

(b) calcium carbonate

(a) Na2SO3 + H2SO4 [dil.] ⟶ Na2SO4 + H2O + SO2 [g]
Sulphur dioxide gas is evolved.

(b) CaCO3 + 2HCl [dil.] ⟶ CaCl2 + H2O + CO2 [g]
Carbon dioxide gas is evolved.

#### Question 5

Give a balanced equation for conversion of:

(a) an ammonium salt to a basic gas

(b) a soluble lead salt to an insoluble lead salt – formed as a white precipitate.

(a) Ammonium salt [NH4Cl] on reaction with calcium hydroxide produces NH3(g) which is a basic gas.

2NH4Cl + Ca(OH)2 ⟶ CaCl2 + 2H2O + 2NH3 [g]

(b) Lead nitrate when reacts with sodium chloride insoluble white ppt. of lead chloride is formed.

Pb(NO3)2 + 2NaCl ⟶ 2NaNO3 + PbCl2

#### Question 6

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

Chemical reaction which proceed with evolution of heat [Exothermic]

• Nitrogen and hydrogen:
N2 + 3H2 $\xrightleftharpoons{\enspace\Delta\enspace}$ 2NH3 + ΔT

Chemical reaction which proceed by absorption of heat [Endothermic]

• Nitrogen and oxygen :
N2 + O2 $\xrightarrow{\enspace\Delta\enspace}$ 2NO - ΔT

#### Question 7

Define 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

(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 $\xrightarrow{\enspace\Delta\enspace}$ 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 nitrate

AgNO3 + KCl ⟶ AgCl + KNO3

#### Question 8

Give a balanced equation for a direct combination reaction involving :

(a) Two elements — one of which is a neutral gas and the other a yellow non-metal

(b) Two elements – one of which is a neutral gas and the other a monovalent metal

(c) Two compounds – resulting in formation of a weak acid

(a) Direct Combination of two elements one of which is a neutral gas and the other a yellow non-metal:

S + O2 [g] ⟶ SO2

(b) Direct Combination of two elements one of which is a neutral gas and the other a monovalent metal

4Na + O2 [g] ⟶ 2Na2O

(c) Formation of a weak acid:

CO2 [g] + H2O ⟶ H2CO3

#### Question 9

Give balanced equations for the following reactions of synthesis involving formation of :

(a) An acid – from sulphur dioxide gas

(b) An alkali – from a basic oxide – sodium oxide

(c) A salt – from a trivalent metal and a coloured gas.

(a) SO2 + H2O ⟶ H2SO3

(b) Na2O + H2O ⟶ 2NaOH

(c) 2Al + 3Cl2 ⟶ 2AlCl3

#### Question 10

Convert — (a) nitrogen to ammonia (b) hydrogen to hydrogen chloride – by a direct combination reaction.

(a) Nitrogen reacts with hydrogen under specific conditions liberating ammonia

N2 + 3H2 ⇌ 2NH3 + Δ

Conditions :

Temperature : 450-500°C [Optimum temperature]

Pressure : 200 to 900 atmospheres [Optimum pressure]

Catalyst : Finely divided iron [Fe]

(b) Moist hydrogen gas combines with chlorine in presence of diffused sunlight.

H2 + Cl2 ⟶ 2HCl [g]

Conditions — Diffused Sunlight, reaction is explosive in direct sunlight. Moisture acts as a catalyst.

#### Question 11

Give balanced equations for thermal decomposition of :

(c) ammonium dichromate

(d) mercury [II] oxide

(e) calcium hydroxide

(a) PbCO3 $\xrightarrow{\enspace\Delta\enspace}$ PbO + CO2

(b) 2Pb(NO3)2 $\xrightarrow{\enspace\Delta\enspace}$ 2PbO + 4NO2 + O2

(c) (NH4)2Cr2O7 $\xrightarrow{\enspace\Delta\enspace}$ Cr2O3 + 4H2O + N2

(d) 2HgO $\xrightarrow{\enspace\Delta\enspace}$ 2Hg + O2

(e) Ca(OH)2 $\xrightarrow{\enspace\Delta\enspace}$ CaO + H2O

#### Question 12

Define 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.

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 $\xrightleftharpoons[\enspace\text{cool}\enspace]{\enspace\text{heat}\enspace}$ NH3 + HCl

An example of photochemical decomposition reaction is the decomposition of silver salts in the presence of light:

2AgCl $\xrightarrow{\enspace☼\enspace}$ 2Ag + Cl2

HgO is a metallic oxide which reduces to a metal on thermal decomposition as per the equation below:

2HgO $\xrightarrow{\enspace\Delta\enspace}$ 2Hg + O2

#### Question 13

Define 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.

A chemical reaction which takes place when an element [or radical] has replaced another element in a compound is known as Displacement reaction

It is represented as :

X + YZ ⟶ Y + XZ

e.g.,

Magnesium + Copper [II] sulphate ⟶ Magnesium sulphate + Copper

Mg + CuSO4 ⟶ MgSO4 + Cu

As 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 14

Explain 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.

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:

$\underset{\text{Lead nitrate}}{\text{Pb(NO}_3)_2} + \underset{\text{sodium chloride}}{2\text{NaCl}} \longrightarrow \underset{\text{sodium nitrate}}{2{\text{NaNO}_3}} + \underset{\text{Lead [II] chloride}}{\text{PbCl}_2 \downarrow}$

$\underset{\text{Lead nitrate}}{\text{Pb(NO}_3)_2} + \underset{\text{sodium sulphate}}{\text{Na}_2\text{SO}_4} \longrightarrow \underset{\text{sodium nitrate}}{2{\text{NaNO}_3}} + \underset{\text{Lead sulphate}}{\text{PbSO}_4 \downarrow}$

#### Question 15

Explain 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].

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 16

Define the term – double decomposition – neutralization reaction with a suitable representation.

Convert :

(a) an insoluble base (b) a soluble base

to their respective soluble salts by neutralization reaction.

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 17

Explain 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.

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 $\xrightleftharpoons{\enspace\Delta\enspace}$ H2O + ΔT

(b) endothermic reaction:
N2 + O2 $\xrightleftharpoons{\enspace\Delta\enspace}$ 2NO - ΔT

(c) photochemical reaction:
H2 + Cl2 $\xrightarrow{\text{sunlight}}$ 2HCl
Slow reaction in diffused light
Explosive reaction in direct sunlight

(d) electrochemical reaction:
$\underset{\text{[acidified]}}{2\text{H}_2\text{O}} \xrightarrow[\text{Current}]{\text{Electric}} \underset{\text{[cathode]}}{2\text{H}_2} + \underset{\text{[anode]}}{\text{O}_2}$

#### Question 18

Supply 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:

$\text{N}_2 + \text{O}_2 \xrightarrow {3000\degree \text{C}} 2\text{NO}$

(ii) Light energy:

$\text{H}_2 + \text{Cl}_2 \xrightarrow {\text{Sunlight}} 2\text{HCl}$

(iii) Electricity:

$\underset{\text{[acidified]}}{2\text{H}_2\text{O}} \xrightarrow[\text{Current}]{\text{Electric}} \underset{\text{[cathode]}}{2\text{H}_2} + \underset{\text{[anode]}}{\text{O}_2}$

(iv) Pressure :

$\text{N}_2 + 3\text{H}_2 \xrightleftharpoons[\text{200 atmos. pressure}]{\text{Fe/450-500 \degree C }} \text{2NH}_3$

(v) Catalyst :

$4\text{NH}_3 + 5\text{O}_2 \xrightarrow {\text{Pt.}/800\degree \text{C}} 4\text{NO} + 6\text{H}_2\text{O}$

(vi) Sound Energy:

$\underset{\text{[acetylene]}}{\text{C}_2\text{H}_2} \xrightarrow[\text{Energy}]{\text{Sound}} 2\text{C} + \text{H}_2$

## Unit Test Paper 2 — Chemical Changes & Rxns

#### Question 1

Complete the statements by filling in the blank with the correct word/s :

1. Direct combination reaction of sulphur dioxide with water gives ............... [H2SO4/H2SO3/H2S2O7].
2. Formation of hydrogen chloride from hydrogen and chlorine is an example of ............... [photochemical reaction / electrochemical reaction].
3. The reaction of hydrogen burning in oxygen to give a neutral liquid is an example of ............... [exothermic/endothermic] reaction.
4. The neutral gas evolved when lead nitrate undergoes thermal decomposition is ............... [nitrogen dioxide/oxygen/nitrogen].
5. The reddish brown precipitate obtained during a double decomposition - precipitation reaction between an iron salt and an alkali is ............... [iron [II] hydroxide / iron [III] hydroxide]

1. Direct combination reaction of sulphur dioxide with water gives H2SO3
2. Formation of hydrogen chloride from hydrogen and chlorine is an example of photochemical reaction.
3. The reaction of hydrogen burning in oxygen to give a neutral liquid is an example of exothermic reaction.
4. The neutral gas evolved when lead nitrate undergoes thermal decomposition is oxygen.
5. The reddish brown precipitate obtained during a double decomposition precipitation reaction between an iron salt and an alkali is iron [III] hydroxide.

#### Question 2.

Select the correct answer from A, B, C, D and E for each statement given below :

A : Ammonia
B : Hydrogen chloride
C : Hydrogen
D : Nitrogen dioxide
E : Nitric oxide

State the gaseous product formed, when -

1. An active metal reacts with dilute sulphuric acid.
2. A metallic nitrate undergoes thermal decomposition giving a coloured gas.
3. Two gases one of them neutral, combines by absorption of light energy.
4. An ammonium salt reacts with an alkali.
5. An exothermic reaction takes place between ammonia and a neutral gas.

1. An active metal reacts with dilute sulphuric acid — C : Hydrogen
2. A metallic nitrate undergoes thermal decomposition giving a coloured gas — D : Nitrogen dioxide
3. Two gases one of them neutral, combines by absorption of light energy — B : Hydrogen chloride
4. An ammonium salt reacts with an alkali — A: Ammonia
5. An exothermic reaction takes place between ammonia and a neutral gas — E : Nitric oxide

#### Question 3

Give a balanced equation for each of the following types of reactions :

1. A direct combination reaction between phosphorus and a neutral gas.
2. A soluble salt of lead formed from an insoluble base by double decomposition – neutralization.
3. A thermal decomposition reaction of a salt – which results in the formation of nitrogen gas.
4. A synthesis reaction between a metal & a non-metal resulting in formation of an insoluble salt of iron.
5. A decomposition reaction of a salt which leaves behind a silvery metal.

1. 4P + O2 ⟶ 2P2O5
2. PbO + 2HNO3 ⟶ Pb(NO3)3 + H2O
3. (NH4)2Cr2O7 ⟶ Cr2O3 + 4H2O + N2
4. Fe + S ⟶ FeS
5. 2AgCl $\xrightarrow{\enspace☼\enspace}$ 2Ag + Cl2

#### Question 4.1

Differentiate between the following :

Synthesis reaction & a substitution reaction.

Synthesis ReactionSubstitution Reaction
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 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 ⟶ water
2H2 [g] + O2 [g] ⟶ 2H2O [l]
For example:
Magnesium + Copper [II] sulphate ⟶ Magnesium sulphate + Copper
Mg + CuSO4 ⟶ MgSO4 + Cu

#### Question 4.2

Differentiate between the following :

Electrolytic decomposition & photochemical decomposition

Electrolytic decompositionPhotochemical decomposition
A 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 4.3

Differentiate between the following :

Thermal decomposition & thermal dissociation.

Thermal decompositionThermal dissociation
A chemical reaction in which a compound decomposes to give two new elements / a new compound & an element / two new compounds on application of heat is called a Thermal decomposition reaction.A decomposition reaction in which a substance dissociates into two or more simpler substances on application of heat is called a Thermal dissociation reaction.
For example:
2Ca(NO3)2 ⟶ 2CaO + 4NO2 + O2
For example:
NH4Cl ⇌ NH3 + HCl

#### Question 4.4

Differentiate between the following :

Decomposition reaction & a double decomposition reaction

Decomposition reactionDouble decomposition reaction
A chemical reaction in which a compound decomposes to give two new elements / a new compound & an element / two new compounds is called a Decomposition reaction.A chemical reaction in which both reactants [compounds] are decomposed to give two new compounds by exchanging their radicals is called a Double decomposition reaction.
It is represented as XY + AB ⟶ XB + AY
For example:
2HgO ⟶ 2Hg + O2
For example:
CaCl2 + Na2CO3 ⟶ 2NaCl + CaCO3

#### Question 4.5

Differentiate between the following :

Neutralization reaction & a precipitation reaction.

Neutralization reactionPrecipitation reaction
A 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 + H2O
For example:
Pb(NO3)2 + 2NaCl ⟶ 2NaNO3 + PbCl2

#### Question 5

Match the chemical reactions in List I with the appropriate answer in List II.

List IList II
AB ⇌ A + B (on heating)A : Double decomposition
AB ⟶ A + BB : Thermal dissociation
X+Y- + A+B- ⟶ X+B- + A+Y-C : Endothermic reaction
X + YZ ⟶ XZ + YD : Displacement reaction
X + Y ⟶ XY - Δ (on heating)E : Decomposition reaction

List IList II
AB ⇌ A + B (on heating)B : Thermal dissociation
AB ⟶ A + BE : Decomposition reaction
X+Y- + A+B- ⟶ X+B- + A+Y-A : Double decomposition
X + YZ ⟶ XZ + YD : Displacement reaction
X + Y ⟶ XY - Δ (on heating)C : Endothermic reaction

#### Question 6

Name the solid residual product formed in each reaction and state it's colour during – thermal decomposition of the following substances.

1. Copper nitrate.
2. Ammonium dichromate.
3. Zinc carbonate.
5. Calcium hydroxide.