Chemical Changes & Reactions

Question 1

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

Answer

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.

Answer

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.

Answer

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

Answer

(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.

Answer

(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.

Answer

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

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 $\xrightarrow{\enspace\Delta\enspace}$ 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

Answer

(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.

Answer

(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.

Answer

(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 :

(a) lead carbonate

(b) lead nitrate

(c) ammonium dichromate

(d) mercury [II] oxide

(e) calcium hydroxide

Answer

(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.

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 $\xrightleftharpoons[\enspace\text{cool}\enspace]{\enspace\text{heat}\enspace}$ 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.

Answer

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.

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:

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

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

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].

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 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.

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 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.

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 $\xrightleftharpoons{\enspace\Delta\enspace}$ 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.

Answer

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:

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

Question 1(1986)

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

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.

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

Question 1(1987)

Give an example of an endothermic reaction.

Answer

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

Answer

(i) Cl₂ + 2KI ⟶ 2KCl + l₂ — Simple displacement

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

(iii) SO₂ + 2H₂O + Cl₂ ⟶ 2HCl + H₂SO₄ — Redox reaction

(iv) AgNO₃ + HCl ⟶ AgCl + HNO₃ — Double decomposition

(v) 4HNO₃ ⟶ 4NO₂ + 2H₂O + O₂ — Decomposition reaction

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]

Answer

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.

Answer

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.

Answer

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.

Answer

Question 4.2

Differentiate between the following :

Electrolytic decomposition & photochemical decomposition

Answer

Question 4.3

Differentiate between the following :

Thermal decomposition & thermal dissociation.

Answer

Question 4.4

Differentiate between the following :

Decomposition reaction & a double decomposition reaction

Answer

Question 4.5

Differentiate between the following :

Neutralization reaction & a precipitation reaction.

Answer

Question 5

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

Answer

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.
4. Lead nitrate.
5. Calcium hydroxide.

Answer

Below table lists the solid residual product formed along with it's colour for the given substances: