Chemical Changes & Reactions

Give an example of an endothermic reaction.

Answer

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

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

Answer

Silver nitrate decomposes in the presence of sunlight, forming silver, nitrogen dioxide and oxygen. Hence, it is kept in coloured reagent bottles.

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

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 + I₂

(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 + I₂ — 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

Give an example of a photochemical decomposition reaction.

Answer

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

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

Name a metallic oxide which on thermal decomposition is reduced to a metal.

Answer

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

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.

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

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.

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.

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) When lead nitrate reacts with sodium chloride, an insoluble white precipitate of lead chloride is formed.

Pb(NO₃)₂ + 2NaCl ⟶ 2NaNO₃ + PbCl₂ ↓

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

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

Give balanced equations for the following:

(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

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

Answer

(a) Nitrogen reacts with hydrogen under specific conditions to form 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.

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

Define a thermal dissociation reaction with a suitable example.

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

Define a displacement reaction with example. State why Zn displaces H₂ from dil. H₂SO₄ but Cu 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

A more electropositive metal displaces a less electropositive metal from its aqueous solution. Therefore, zinc, being more electropositive, is placed above [H] in the activity series and displaces hydrogen from dilute sulphuric acid. Copper, being less electropositive, is placed below [H] in the electrochemical series and cannot displace hydrogen from sulphuric acid.

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:

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

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

With the help of balanced equations - show how precipitation reactions are used for identifying the positive radicals in three different salts, each having a different cation [positive ion].

Answer

Precipitation reactions are used in analytical chemistry for identifying the positive radical in a salt by analysing the colour of the precipitate.

(i) FeSO₄ + 2NaOH ⟶ Na₂SO₄ + Fe(OH)₂ ↓

Identified ion: Fe²⁺; a dirty green precipitate of iron [II] hydroxide is obtained. Hence, it can be identified.

(ii) CuSO₄ + 2NaOH ⟶ Na₂SO₄ + Cu(OH)₂ ↓

Identified ion: Cu²⁺; a pale blue precipitate of copper [II] hydroxide is obtained. Hence, it can be identified.

(iii) Pb(NO₃)₂ + 2NaOH ⟶ 2NaNO₃ + Pb(OH)₂ ↓

Identified ion: Pb²⁺; a chalky white precipitate of lead [II] hydroxide is obtained. Hence, it can be identified.

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

Explain the term energy changes in a chemical change. 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 $\xrightleftharpoons{\enspace\Delta\enspace}$ 2H₂O + 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:
     $\underset{\text{[acidified]}}{2\text{H}_2\text{O}} \xrightarrow[\text{Current}]{\text{Electric}} \underset{\text{[cathode]}}{2\text{H}_2} + \underset{\text{[anode]}}{\text{O}_2}$

Choose the reaction which is not a chemical change under prevalent condition.

1. Fe + CuSO₄ ⟶
2. FeCO₃ $\xrightarrow{\Delta}$
3. Fe $\xrightarrow{\Delta}$
4. Fe + S $\xrightarrow{\Delta}$

Answer

Fe $\xrightarrow{\Delta}$

Reason — Heating iron alone, under prevalent conditions, does not produce a new substance; hence it is not a chemical change. A chemical reaction is a chemical change in which reactants are transformed into new substances called products.

The other reactions are chemical changes:

1. Fe + CuSO₄ → FeSO₄ + Cu — displacement reaction
2. FeCO₃ $\xrightarrow{\Delta}$ FeO + CO₂ — thermal decomposition
3. Fe + S $\xrightarrow{\Delta}$ FeS — direct combination reaction

Choose the reaction which is not a precipitation reaction under prevalent conditions.

1. Pb(NO₃)₂ + NaOH ⟶
2. Pb(NO₃)₂ + Na₂SO₄
3. Pb(NO₃)₂ $\xrightarrow{\Delta}$
4. Pb(NO₃)₂ + NaCl

Answer

Pb(NO₃)₂ $\xrightarrow{\Delta}$

Reason — A precipitation reaction is a double decomposition reaction in which two compounds in aqueous solution react to form two new compounds, one of which is an insoluble precipitate. On heating, Pb(NO₃)₂ undergoes thermal decomposition, not precipitation.

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

The other reactions form insoluble lead salts or lead hydroxide as precipitates:

1. Pb(OH)₂
2. PbSO₄
3. PbCl₂

Choose the conversion reaction which is exothermic & takes place by supply of catalytic energy.

1. Hydrogen to HCl
2. Ammonia to nitric acid
3. Nitrogen to nitric oxide
4. Oxygen to ozone.

Answer

Ammonia to nitric acid

Reason — The conversion of ammonia to nitric acid involves the catalytic oxidation of ammonia. In this reaction, ammonia reacts with oxygen in the presence of a platinum catalyst and the reaction is exothermic, as heat is evolved.

4NH₃ + 5O₂ $\xrightarrow[\text{800°C}]{\text{Pt}}$ 4NO + 6H₂O + heat

Hydrogen to HCl :

H₂ + Cl₂ $\xrightarrow{\text{sunlight}}$ 2HCl

In this reaction, light is required for initiation of the reaction.

Nitrogen to nitric oxide :

N₂ + O₂ $\leftrightharpoons$ 2NO - $\Delta$

This is endothermic reaction and needs very high temperature.

Oxygen to ozone :

3O₂ $\xrightarrow{\text{ultraviolet light}}$ 2O₃

This is endothermic and requires electrical discharge.

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

Match the conversion in Column A with correct type of chemical change or reaction .

Answer

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.

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

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

Differentiate between the following :

Synthesis reaction & a substitution reaction.

Answer

Differentiate between the following :

Electrolytic decomposition & photochemical decomposition

Answer

Differentiate between the following :

Thermal decomposition & thermal dissociation.

Answer

Differentiate between the following :

Decomposition reaction & a double decomposition reaction

Answer

Differentiate between the following :

Neutralization reaction & a precipitation reaction.

Answer

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

Answer

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: