Chemical Changes and Reactions

(a) What is a chemical reaction?

(b) State the conditions necessary for a chemical change or reaction.

Answer

(a) A chemical reaction is a process of breaking chemical bonds of the reacting substances (reactants) and making new bonds to form new substances (products).

(b) Conditions necessary for a chemical change or reaction are :

1. Mixing (close contact) — Some chemical reactions take place when two substances are mixed in their solid state.
   Pb(NO₃)₂ (s) + 2KI (s) ⟶ 2KNO₃ + PbI₂ (s)

2. Solution — In some cases, a chemical reaction occurs when substances are mixed either in molten or aqueous state.
   AgNO₃ + NaCl ⟶ AgCl ↓ + NaNO₃

3. Heat — Some chemical reactions occur only on heating.
   CuCO₃ (s) $\xrightarrow{\Delta}$ CuO (s) + CO₂ ↑ (g)

4. Light — Some chemical reactions take place by the action of light. They are called photochemical reaction.
   2AgNO₃ $\xrightarrow{\text{sunlight}}$ 2Ag + 2NO₂ + O₂ ↑

5. Electricity — Some reactions are caused or accompanied by the passage of an electric current. They are called electrochemical reaction.
   2NaCl $\xrightarrow{\text{electricity}}$ 2Na + Cl₂ ↑

6. Pressure — Some chemical reaction take place only when the involved substances are subjected to high pressure.
   N₂ + 3H₂ $\xrightleftharpoons{\text{above 200 atm}}$ 2NH₃ ↑

7. Catalyst — Some chemical reactions need a catalyst to accelerate or decelerate the rate at which they occur.
   2H₂O₂ $\xrightarrow{\text{MnO}_2}$ 2H₂O + O₂ ↑

Define the following terms:

(a) Chemical change

(b) Chemical bond

(c) Effervescence

(d) Precipitate

Answer

(a) A chemical change is a permanent change in which the chemical composition of a substance is changed and one or more new substances with different chemical compositions and different properties are formed.

(b) A chemical bond is the force that holds the atoms of a molecule together, as in a compound.

(c) Effervescence is the formation of gas bubbles in a liquid during a reaction.

(d) Certain chemical reactions are characterized by the formation of insoluble solid substances called precipitate.

Give an example of a reaction where the following are involved:

(a) Heat

(b) Light

(c) Electricity

(d) Close contact

(e) Solution

(f) Pressure

(g) Catalyst

Answer

(a) Heat

CuCO₃ (s) $\xrightarrow{\Delta}$ CuO (s) + CO₂ ↑ (g)

(b) Light

2AgNO₃ $\xrightarrow{\text{sunlight}}$ 2Ag + 2NO₂ ↑ + O₂ ↑

(c) Electricity

2NaCl $\xrightarrow{\text{electricity}}$ 2Na + Cl₂ ↑

(d) Close contact

Pb(NO₃)₂ (s) + 2KI (s) ⟶ 2KNO₃ + PbI₂ (s)

(e) Solution

NaCl (aq.) + AgNO₃ (aq.) ⟶ AgCl (white ppt.) ↓ + NaNO₃ (aq.)

(f) Pressure

N₂ + 3H₂ $\xrightleftharpoons{\text{above 200 atm}}$ 2NH₃ ↑

(g) Catalyst

2H₂O₂ $\xrightarrow{\text{MnO}_2}$ 2H₂O + O₂ ↑

Define :

(a) Photochemical reaction

(b) Electrochemical reaction.

Give one example in each case.

Answer

(a) A chemical reaction that takes place by the action of light is called a photochemical reaction.

Example —

2AgNO₃ $\xrightarrow{\text{sunlight}}$ 2Ag + 2NO₂ ↑ + O₂ ↑

(b) Reactions that are caused or accompanied by the passage of an electric current are called electrochemical reaction.

Example — On passing current through through molten sodium chloride, sodium and chloride are obtained separately.

2NaCl $\xrightarrow{\text{electricity}}$ 2Na + Cl₂ ↑

Give an example of each of the following chemical changes.

(a) A photochemical reaction involving:

1. silver salt
2. water

(b) A reaction involving:

1. blue solution
2. formation of a dirty green precipitate

(c) Two gases combine to form a white solid.

(d) Two solids combine to form a liquid.

(e) A reaction where color change is noticed.

Answer

(a) A photochemical reaction involving

1. silver salt
   2AgNO₃ $\xrightarrow{\text{sunlight}}$ 2Ag + 2NO₂ ↑ + O₂ ↑

2. water
   Cl₂ + H₂O $\xrightarrow{\text{sunlight}}$ HCl + HClO

(b) A reaction involving

1. blue solution

$\underset{\text{ iron}}{\text{Fe}} + \underset{\text{ blue sol.}}{\text{CuSO}_4} \longrightarrow \underset{\text{ green sol.}}{\text{FeSO}_4} + \underset{\text{ copper-brown red deposit}}{\text{Cu}}$

2. formation of a dirty green precipitate

$\underset{\text{ ferrous sulphate}}{\text{FeSO}_4} + \underset{\text{ sodium hydroxide}}{2\text{NaOH}} \longrightarrow \underset{\text{ dirty green ppt. }}{\text{Fe(OH)}_2}↓ + \underset{\text{ sodium sulphate}}{\text{Na}_2\text{SO}_4}$

(c) Two gases combine to form a white solid.

NH₃ (g) + HCl (g) ⟶ NH₄Cl (s)

(d) Two solids combine to form a liquid.

C (s) + 2S (s) ⟶ CS₂ (l)

(e) A reaction where color change is noticed.

$\underset{\text{ iron}}{\text{Fe}} + \underset{\text{ blue sol.}}{\text{CuSO}_4} \text{ (aq.)} \longrightarrow \underset{\text{ green sol.}}{\text{FeSO}_4} + \underset{\text{ copper-brown red deposit}}{\text{Cu}}$

Write the chemical reaction where the following changes are observed.

(a) Gas is evolved

(b) Colour change is noticed

(c) Precipitate is formed

(d) Physical state of reactants is changed

Answer

(a) Gas is evolved

Zn(s) + H₂SO₄ (aq) ⟶ ZnSO₄ (aq) + H₂ (g)

(b) Colour change is noticed

$\underset{\text{ iron}}{\text{Fe}} + \underset{\text{ blue sol.}}{\text{CuSO}_4} \text{ (aq.)} \longrightarrow \underset{\text{ green sol.}}{\text{FeSO}_4} + \underset{\text{ copper-brown red deposit}}{\text{Cu}}$

(c) Precipitate is formed

$\underset{\text{ ferrous sulphate}}{\text{FeSO}_4 \text{ (aq.)}} + \underset{\text{ sodium hydroxide}}{2\text{NaOH}\text{ (aq.)}} \longrightarrow \underset{\text{ dirty green ppt.}}{\text{Fe(OH)}_2↓} + \underset{\text{ sodium sulphate}}{\text{Na}_2\text{SO}_4\text{ (aq.)}}$

(d) Physical state of reactants is changed

C (s) + 2S (s) ⟶ CS₂ (l)

Give reason for the following :

(a) Silver nitrate solution is kept in coloured bottles.

(b) Molybdenum is used in the manufacture of ammonia.

(c) Blue solution of copper sulphate changes to green when a piece of iron is added to this solution.

(d) Colourless concentrated sulphuric acid in a test tube changes to blue on adding a small piece of copper to it.

Answer

(a) Silver nitrate solution is kept in brown bottles in the laboratory because it decomposes in the presence of light. It becomes black and forms silver, nitrogen dioxide and oxygen.

2AgNO₃ $\xrightarrow{\text{sunlight}}$ 2Ag + 2NO₂ ↑ + O₂ ↑

(b) Molybdenum acts as a promoter and increases the efficiency of the catalyst iron, hence it is used in the manufacture of ammonia.

(c) When a piece of iron is added to a blue coloured copper sulphate solution, the blue colour of the solution fades and eventually turns into light green due to the formation of ferrous sulphate.

$\underset{\text{ iron}}{\text{Fe}} + \underset{\text{ blue sol.}}{\text{CuSO}_4}\text{ (aq.)} \longrightarrow \underset{\text{ green sol.}}{\text{FeSO}_4} + \underset{\text{ copper-brown red deposit}}{\text{Cu}}$

(d) When a small piece of copper is added to concentrated sulphuric acid, a blue solution of copper sulphate is formed.

$\underset{\text{ brown}}{\text{Cu (s)}} + \underset{\text{ conc.}}{2\text{H}_2\text{SO}_4\text{(aq)}} \longrightarrow \underset{\text{ blue}}{\text{CuSO}_4\text{(aq)}} + 2\text{H}_2\text{O (l)} + \text{SO}_2\text{(g)}$

State the changes observed in the following chemical reactions.

(a) A small piece of sodium is dropped in water.

(b) Few iron filings are added to blue copper sulphate solution.

(c) Water is added to quick lime (CaO).

(d) Hydrochloric acid is added to metal sulphide.

Answer

(a) Sodium reacts vigorously, producing bubbles of hydrogen gas, moving rapidly on the water surface, and often melting into a small ball. The reaction is highly exothermic and may produce a flame.
2Na + 2H₂O ⟶ 2NaOH + H₂ ↑

(b) The blue colour of copper sulphate solution gradually changes to light green due to the formation of ferrous sulphate. A reddish-brown deposit of copper is formed on the iron filings.
Fe + CuSO₄ ⟶ FeSO₄ + Cu

(c) Quick lime reacts with water with the evolution of a large amount of heat. A hissing sound may be heard and a white, powdery/pasty substance called slaked lime, Ca(OH)₂, is formed.
CaO + H₂O ⟶ Ca(OH)₂ + Δ

(d) A gas with a rotten egg smell is released, indicating the formation of hydrogen sulphide gas during the reaction.
FeS + 2HCl ⟶ FeCl₂ + H₂S ↑

State the type of reactions and give one equation in each case.

(a) Two or more reactants combine to form a single product.

(b) A single reactant breaks up into two or more products.

Answer

(a) Combination reaction or Synthesis
     2SO₂ (g) + O₂ (g) ⟶ 2SO₃(s)

(b) Decomposition reaction
     2H₂O (l) $\xrightarrow[\text{current}]{\text{electric}}$ 2H₂ (g) + O₂ (g)

Give an example of each:

Combination reaction in which:

(i) An element of valency 3 combines with nitrogen.

(ii) A non-metal oxide reacting with water to form an acid.

(iii) A metal oxide reacting with water to form its hydroxide.

Answer

(i) 2Al + N₂ ⟶ 2 AlN

(ii) SO₂ + H₂O ⟶ H₂SO₃ (sulphurous acid)

(iii) K₂O + H₂O ⟶ 2KOH (Caustic potash)

Give an example of each:

Write the decomposition reactions for the following:

(i) Metallic oxide on heating forms metallic oxide and oxygen.

(ii) A compound on heating gives a gas which turns lime water milky.

(iii) A white hydroxide on heating gives a yellow residue and water vapours.

Answer

(i) $\underset{\text{Mercuric oxide}}{2\text{HgO}} \text{(s)} \xrightarrow{\Delta} \underset{\text{ Mercury}}{2\text{Hg}} \text{(l)} + \underset{\text{ Oxygen}}{\text{O}_2} \text{(g)}$

(ii) ZnCO₃ ⟶ ZnO + CO₂ ↑

(iii) Zn(OH)₂ ⟶ ZnO + H₂O

Write the reaction for the decomposition of a nitrate which does not yield nitrogen dioxide on heating.

Answer

2KNO₃ ⟶ 2KNO₂ + O₂

Give two balanced equations to show thermal dissociation.

Answer

Two balanced equations showing thermal dissociation:

1. NH₄Cl ⇌ NH₃ ↑ + HCl ↑

2. N₂O₄ $\xrightleftharpoons[\text{cool}]{\text{heat}}$ 2NO₂

Name:

(a) two coloured oxides that do not decompose on heating.

(b) a carbonate that is stable to heat

(c) an oxide formed during thunder and lighting.

(d) the compound formed when phosphorus and oxygen combines.

Answer

(a) Two coloured oxides that do not decompose on heating are

1. Copper(II) oxide (CuO) – Black
2. Iron(III) oxide (Fe₂O₃) – Reddish-brown

(b) Potassium Carbonate (K₂CO₃)

(c) Nitric oxide (NO)

(d) Phosphorus pentoxide (P₂O₂)

Give one example each of :

(a) Thermal decomposition.

(b) Electrolytic decomposition.

(c) Photochemical decomposition.

Answer

(a) Calcium carbonate decomposes on heating.
CaCO₃ $\xrightarrow{\Delta}$ CaO + CO₂

(b) Water decomposes into hydrogen and oxygen.
2H₂O $\xrightarrow{\text{electric current}}$ 2H₂ + O₂

(c) Silver chloride decomposes in the presence of light.
2AgCl $\xrightarrow{\text{sunlight}}$ 2Ag + Cl₂

Write balanced equations for the following and identify the type of chemical reaction.

(a) Iron reacts with chlorine to form iron (III) chloride.

(b) Calcium oxide combines with silicon dioxide to form calcium silicate.

(c) Hydrogen peroxide produces water and oxygen.

Answer

(a) 2Fe + 3Cl₂ ⟶ 2FeCl₃
     Combination reaction

(b) CaO + SiO₂ ⟶ CaSiO₃
     Combination reaction

(c) 2H₂O₂ ⟶ 2H₂O + O₂
     Decomposition reaction

A silvery white metal A is in the form of a ribbon. It burns with a dazzling white flame to form a white powder B. When water is added to powder B, it partially dissolves to form a substance C.

(a) What is metal A ?

(b) Write the equation for the conversion of A to B and state the type of reaction.

(c) Write a balanced equation for the conversion of B to C and state the type of reaction.

Answer

(a) Metal A is magnesium.

(b) 2Mg + O₂ ⟶ 2MgO
Type of reaction: Combination reaction

(c) MgO + H₂O ⟶ Mg(OH)₂
Type of reaction: Combination reaction

Which of the following is not a characteristic of a chemical change?

1. It is irreversible

2. No net energy change is involved

3. New substance is formed

4. Involves absorption or liberation of energy

Answer

No net energy change is involved

Reason — Chemical reactions always involve energy changes either energy is absorbed or released.

Characteristics of a chemical change are :

1. It's irreversible.

2. A new substance is formed.

3. Involves absorption or liberation of energy.

A reaction of a type: AB + CD ⟶ AD + CB, involves

1. no chemical change

2. decomposition of AB and CD

3. exchange of ions of AB and CD

4. combination of AB and CD

Answer

exchange of ions of AB and CD

Reason — In the type of reaction called double displacement two compounds exchange their positive and negative radicals.

Example : AB + CD ⟶ AD + CB

A new set of substances (AD and CB) is formed, so a chemical change definitely occurs. Decomposition means breaking a compound into simpler substances, which is not happening here. Combination means two substances combine to form one product, but here we get two new products.

The reaction:

BaCl₂ (aq) + H₂SO₄ (aq) ⟶ BaSO₄ (s) + 2HCl (aq) is

1. displacement reaction

2. neutralisation reaction

3. decomposition reaction

4. double displacement reaction

Answer

Double displacement reaction

Reason — A chemical reaction in which both reactants [compounds] are decomposed to give two new compounds by exchanging their radicals is called a Double decomposition or double displacement reaction.

It is represented as XY + AB ⟶ XB + AY

In a displacement reaction, one element replaces another. In neutralisation acid reacts with base to form salt and water. Here, no water is formed. Decomposition involves one compound breaking into simpler substances.

Thermal decomposition of sodium carbonate will produce

1. carbon dioxide

2. oxygen

3. sodium hydroxide

4. no other product

Answer

Carbon dioxide

Reason — Sodium carbonate, on heating gives sodium oxide and carbon dioxide

Na₂CO₃ $\xrightarrow{\Delta}$ Na₂O + CO₂ ↑

Complete the following statements.

(a) The chemical change involving iron and hydrochloric acid illustrates a ............... reaction.

(b) In the type of reaction called ............... two compounds exchange their positive and negative radicals.

(c) A catalyst either ............... or ............... the rate of a chemical change but itself remains ............... at the end of the reaction.

(d) On heating, hydrated copper sulphate changes its colour from ............... to ...............

Answer

(a) The chemical change involving iron and hydrochloric acid illustrates a displacement reaction.

(b) In the type of reaction called double displacement two compounds exchange their positive and negative radicals.

(c) A catalyst either accelerates or decelerates the rate of a chemical change but itself remains unaffected at the end of the reaction.

(d) On heating, hydrated copper sulphate changes its colour from blue to white.

Match the following :

Answer

When hydrogen burns in oxygen, water is formed; when electricity is passed through water, hydrogen and oxygen are given out. Name the type of chemical changes involved in the two cases.

Answer

When hydrogen burns in oxygen, hydrogen and oxygen combine together and forms water, hence, it is a direct combination reaction whereas, when electricity is passed through water, it breaks to give hydrogen and oxygen, hence, it is a decomposition reaction.

Explain the following chemical changes by giving one example of each:

(a) Double decomposition

(b) Thermal dissociation

(c) Reversible reaction

(d) Displacement

Answer

(a) Double decomposition — This is a type of chemical change in which two compounds in a solution react to form two new compounds by mutual exchange of radicals. Double decomposition reaction is also called double displacement reaction.
AB + CD ⟶ AD + CB

Eg., AgNO₃ + NaCl ⟶ AgCl ↓ + NaNO₃

(b) Thermal dissociation — A reversible decomposition reaction brought about only by heat is called thermal dissociation reaction.

Heat some solid ammonium chloride in a test tube. Two colourless gases, ammonia and hydrogen chloride, are produced. As these gases move up to the upper part of the test tube which is cooler, they combine to form ammonium chloride, which appears as a white sublimate on the upper cooler side of the test tube.

NH₄Cl ⇌ NH₃ ↑ + HCl ↑

(c) Reversible reaction — A reaction that can be reversed by changing the conditions under which the reaction is taking place is called a reversible reaction.

$\underset{\text{ red hot iron}}{\text{3Fe}} + \underset{\text{ steam}}{4\text{H}_2\text{O}} \rightleftharpoons \underset{\text{ magnetic oxide}}{\text{Fe}_3\text{O}_4} + \underset{\text{ hydrogen gas}}{4\text{H}_2}$

(d) Displacement — A chemical change in which a more active element displaces a less active element from its salt solution is called a displacement reaction.

Mg + H₂SO₄ ⟶ MgSO₄ + H₂ ↑

(a) What is synthesis?

(b) What kind of chemical reaction is synthesis? Support your answer with an example.

Answer

(a) A reaction in which two or more substances combine together to form a single substance is called a synthesis or combination reaction.

A + B ⟶ AB

(b) Synthesis is a direct combination reaction.

In the above reaction, substances A and B combine to give a molecule of a new substance, AB (Product).

For example, carbon burns in oxygen to form a gaseous compound, carbon dioxide.

C + O₂ $\xrightarrow{\Delta}$ CO₂ ↑

Decomposition brought about by heat is known as thermal decomposition. What is the difference between thermal dissociation and thermal decomposition?

Answer

Difference between the two is that thermal dissociation is a reversible reaction whereas, thermal decomposition is an irreversible reaction.

(a) What is a decomposition reaction ?

(b) Decomposition reactions can occur by (i) heat (ii) electricity and (iii) sunlight.
Give two balanced reactions for each.

Answer

(a) The chemical reaction in which a compound splits into two or simpler substances (elements or compounds) is called decomposition reaction.

(b) Two balanced chemical equations for:

(i) Heat

2HgO (s) $\xrightarrow{\Delta}$ 2Hg (l) + O₂ (g)

CaCO₃ (s) $\xrightarrow{\Delta}$ CaO (s) + CO₂ (g)

(ii) Electricity

2NaCl (l) $\xrightarrow{\text{electricity}}$ 2Na (l) + Cl₂ (g)

2H₂O (l) $\xrightarrow{\text{electricity}}$ 2H₂ (g) + O₂ (g)

(iii) Light

2AgCl (s) $\xrightarrow{\text{ sunlight}}$ 2Ag (s) + Cl₂ (g)

2H₂O₂ (l) $\xrightarrow{\text{ sunlight}}$ 2H₂O (l) + O₂ (g)

Samples of four metals A, B, C and D are taken and were added to the following solutions one by one. The results obtained have been tabulated as follows :

Solution to which metal is added

Use the table given above to answer the following questions:

(a) Which is the most reactive metal ?

(b) What would you observe when B is added to solution of copper (II) sulphate ?

(c) Arrange the metals A, B, C and D in order of increasing reactivity.

Answer

(a) B is the most reactive metal.
Reason — B displaces iron from iron (II) sulphate solution, showing that B is more reactive than iron. It does not displace zinc from zinc sulphate solution, so it is less reactive than zinc. Among the given metals A, B, C and D, metal B is the most reactive.

(b) The blue colour of copper (II) sulphate solution will fade, and a reddish-brown deposit of copper will be formed.
Reason — Since B is more reactive than iron, it is also more reactive than copper. Hence, it will displace copper from copper (II) sulphate solution.

(c) D < C < A < B
Reason — A metal displaces another metal from its salt solution only if it is more reactive than that metal.

• B displaces iron from iron (II) sulphate, so B is more reactive than iron.
• A displaces copper from copper (II) sulphate but does not displace iron from iron (II) sulphate, so A is less reactive than B.
• C displaces silver from silver nitrate but does not displace copper from copper (II) sulphate, so C is less reactive than A.
• D does not displace silver from silver nitrate, so D is the least reactive.

Hence, the increasing order of reactivity is:
D < C < A < B

(a) Define neutralization reaction with an example.

(b) Give a balanced equation for this reaction.

(c) Give three applications of neutralization reactions.

Answer

(a) The reaction between an acid and a base that forms salt and water only is referred to as Neutralization reaction.

Reaction between an acid and an alkali to form salt and water only is an example of neutralization reaction.

(b) NH₄OH + HCl ⟶ NH₄Cl + H₂O

(c) Applications of neutralization reactions are as follows :

1. When someone is stung by a bee, formic acid enters the skin and causes pain, which can be relieved by rubbing the spot with slaked lime or baking soda, both of which are bases and thus neutralize the acid.
2. The acid which is accidentally spilt on to our clothes can be neutralised with ammonia solution.
3. If soil is somewhat acidic and thus unfavourable for the growth of certain crops, slaked lime is added to neutralise the excess acid.

What do you understand by precipitation reaction? Explain with an example.

Answer

A chemical reaction in which two compounds in their aqueous state react to form an insoluble salt (precipitate) as one of the products is called a precipitation reaction.

Example: Take a solution of silver nitrate in a test tube and add dil. hydrochloric acid. A curdy white ppt. is formed.

AgNO₃ + HCl ⟶ AgCl ↓ + HNO₃

(a) What are double displacement reactions?

(b) Give an example of a double displacement reaction, where gas is evolved.

Answer

(a) Double displacement reactions are those in which two compounds in a solution react to form two new compounds by mutual exchange of radicals. This type of reaction is also known as a double decomposition reaction.

(b) FeS (s) + H₂SO₄ (aq) ⟶ FeSO₄ (aq) + H₂S ↑

State the type of reactions each of the following represent and balance the ones that are not balanced.

(a) Cl₂ + 2KBr ⟶ 2KCl + Br₂

(b) NaOH + HCl ⟶ NaCl + H₂O

(c) 2HgO ⟶ 2Hg + O₂ ↑

(d) Fe + CuSO₄ ⟶ FeSO₄ + Cu

(e) PbO₂ + SO₂ ⟶ PbSO₄

(f) 2KClO₃ ⟶ 2KCl + 3O₂ ↑

(g) 2H₂O₂ ⟶ 2H₂O + O₂ ↑

(h) KNO₃ + H₂SO₄ ⟶ HNO₃ + KHSO₄

(i) CuO + H₂ ⟶ Cu + H₂O

(j) CaCO₃ ⟶ CaO + CO₂ ↑

(k) NH₄Cl ⟶ NH₃ + HCl

(l) PbO + 2HNO₃ ⟶ Pb(NO₃)₂ + 2H₂O

(m) AgNO₃ + NaCl ⟶ AgCl ↓ + NaNO₃

Answer

(a) Cl₂ + 2KBr ⟶ 2KCl + Br₂

Displacement reaction

(b) NaOH + HCl ⟶ NaCl + H₂O

Neutralisation reaction

(c) 2HgO ⟶ 2Hg + O₂ ↑

Decomposition reaction

(d) Fe + CuSO₄ ⟶ FeSO₄ + Cu

Displacement reaction

(e) PbO₂ + SO₂ ⟶ PbSO₄

Combination reaction

(f) 2KClO₃ ⟶ 2KCl + 3O₂ ↑

Decomposition reaction

(g) 2H₂O₂ ⟶ 2H₂O + O₂ ↑

Decomposition reaction

(h) KNO₃ + H₂SO₄ ⟶ HNO₃ + KHSO₄

Double decomposition reaction

(i) CuO + H₂ ⟶ Cu + H₂O

Displacement reaction

(j) CaCO₃ ⟶ CaO + CO₂ ↑

Decomposition reaction

(k) NH₄Cl ⟶ NH₃ ↑ + HCl

Decomposition reaction

(l) PbO + 2HNO₃ ⟶ Pb(NO₃)₂ + H₂O

Neutralisation reaction

(m) AgNO₃ + NaCl ⟶ AgCl ↓ + NaNO₃

Double decomposition reaction

Which of the of following is not a chemical change.

1. Rusting of iron
2. Iron is magnetised
3. Potassium is dissolved in water
4. Magnesium wire is burnt

Answer

Iron is magnetised

Reason — Magnetisation of iron is not a chemical change, as it is a temporary change. Magnetising iron only changes its magnetic properties, no new substance is formed.
The others are chemical change because:

• Iron reacts with oxygen and moisture to form a new substance iron oxide.
• Potassium reacts vigorously with water to form Potassium hydroxide and hydrogen gas. A new substance forms, so it's chemical.
• Magnesium reacts with oxygen to form Magnesium oxide, a new substance, along with bright light and heat.

Heating of zinc nitrate is a ............... reaction.

1. Displacement
2. Combination
3. Decomposition
4. Redox

Answer

Decomposition reaction

Reason — Zinc nitrate undergoes thermal decomposition and produces reddish brown Nitrogen dioxide gas (NO₂), Zinc oxide (ZnO), and Oxygen gas (O₂).

$\text{Zn}(\text{NO}_3)_2 \xrightarrow{\Delta} \text{4NO}_2 + \text{2ZnO} + \text{O}_2 ↑$

It can't be displacement as no element is replacing another. In combination, substances combine to form one product, which is the opposite here. Redox reaction does involve oxidation and reduction, but the question is asking for the type of reaction.

Burning of a metal is an example of :

1. Decomposition reaction
2. Combination reaction between an element and a compound
3. Direct combination reaction between two elements
4. Displacement reaction

Answer

Direct combination reaction between two elements

Reason — A reaction in which two elements react to form a single substance is called a direct combination reaction between two elements.
When a metal, for example Magnesium, is burnt, it reacts with oxygen to form a new compound magnesium oxide.

2Mg + O₂ ⟶ 2MgO

Hence, burning of a metal is a direct combination reaction between two elements.

Decomposition reaction involves breaking down a compound, not forming one. In displacement reaction no element is replacing another. No compound is present here, therefore combination reaction between an element and a compound is incorrect.

Formation of white precipitate on mixing sodium chloride solution and silver nitrate solution is an example of ............... reaction.

1. Combination
2. Decomposition
3. Displacement
4. Double decomposition

Answer

Double decomposition

Reason — A chemical reaction in which both reactants [compounds] are decomposed to give two new compounds by exchanging their radicals is called a Double decomposition or double displacement reaction. Eg.,

AgNO₃ + NaCl ⟶ AgCl ↓ + NaNO₃

A brown substance A on heating in air turns black forming another substance B. Substances A and B are:

1. A = Fe and B = FeO
2. A = Zn and B = ZnO
3. A = Cu and B = CuO
4. A = Pb and B = PbO

Answer

A = Cu and B = CuO

Reason — Brown copper on heating forms black copper oxide.

2Cu + O₂ ⟶ 2CuO

Which one of these is a chemical change?

1. Water changes to steam
2. Dissolution of sugar in water
3. Combustion of LPG
4. Liquefying ammonia

Answer

Combustion of LPG

Reason — A chemical change is a permanent change and combustion of LPG is a permanent change.
Others are not a chemical change because:

• Water only changes its state from liquid to gas. No new substance is formed.
• Sugar dissolves in water, but its chemical identity remains the same. It can be recovered by evaporation.
• Ammonia only changes its state from gas to liquid. No new substance is formed.

The reaction of neutralization is a :

1. Displacement reaction
2. Double decomposition
3. Combination reaction
4. Combination reaction between two compounds

Answer

Double decomposition

Reason — A chemical reaction in which both reactants [compounds] are decomposed to give two new compounds by exchanging their radicals is called a Double decomposition or double displacement reaction.

These reactions are of two types:

1. precipitation reaction
2. neutralization reaction

FeS + H₂SO₄ ⟶ FeSO₄ + H₂S is an example of:

1. Displacement reaction
2. Double decomposition reaction
3. Decomposition reaction
4. Combination reaction

Answer

Double decomposition reaction

Reason — A chemical reaction in which both reactants [compounds] are decomposed to give two new compounds by exchanging their radicals is called a Double decomposition or double displacement reaction.

AB + CD ⟶ AD + CB

No single element is replacing another element, so it can't be displacement reaction. A single compound breaking into simpler substances is not happening, so it can't be decomposition reaction. Two reactants combining into one product is not the case here, so it can't be combination reaction.

Which of the following statements is/are correct with respect to the figure shown alongside?

P — A is HCl, B is iron.

Q — A is Cl₂, B is copper.

R — A is Cl₂, B is iron.

1. Only P

2. Only Q

3. Only R

4. Both P and R

Answer

Only R

Reason — When chlorine gas (Gas A) is passed over heated iron (Compound B), it forms Iron(III) chloride, which is brown in color.
Whereas hydrochloric acid (HCl) will not produce the same reaction, and copper reacting with chlorine gas will produce copper(II) chloride, which is typically green or blue-green, not brown.

Hence A is chlorine gas and B is iron.

Aman performed the following four experiments

The experiments in which the blue colour of the solution will fade are :

1. P and Q

2. P, Q and R

3. Q and R

4. Q, R and S

Answer

Q and R

Reason — Blue colour is because of CuSO₄. In experiment Q, zinc displaces copper from copper sulphate solution because zinc is more reactive and present above copper in reactivity series.

CuSO₄ (aq) + Zn ⟶ ZnSO₄ + Cu ↓

In experiment R, iron displaces copper from copper sulphate solution because iron is also more reactive and present above copper in reactivity series.

$\underset{\text{ iron}}{\text{Fe}} + \underset{\text{ blue sol.}}{\text{CuSO}_4}\text{ (aq.)} \longrightarrow \underset{\text{ green sol.}}{\text{FeSO}_4} + \underset{\text{ copper-brown red deposit}}{\text{Cu}}$

Hence, in experiment Q and R, copper is displaced by more reactive metal. So, the blue colour of the solution will fade.

The figure given below demonstrates the preparation of oxygen.

2KClO₃ (s) $\xrightarrow{\text{MnO}{_2}/{\text {heat}}}$ 2KCl (s) + 3O₂ (g)

(I) Which of the following statements is/are correct about the reaction?

1. It is a decomposition reaction and is exothermic.

2. It is a decomposition reaction and is endothermic.

3. It is a photochemical decomposition reaction.

4. It is a combination reaction.

(II) Potassium chlorate is:

1. an oxidising agent.

2. a reducing agent.

3. both reducing as well as an oxidising agent.

4. all options are correct.

(III) In the reaction, manganese dioxide:

1. takes part in the reaction.

2. acts as a catalyst.

3. decreases the speed of reaction.

4. helps in producing more oxygen.

Answer

(I) It is a decomposition reaction and is endothermic.

Reason — When potassium chlorate (2KClO₃) is heated in the presence of manganese dioxide it will undergo decomposition reaction. Since heat is absorbed during the process it is a endothermic reaction.

(II) an oxidising agent

Reason — Potassium chlorate (KClO₃) provides oxygen, which supports oxidation of other substances. It releases oxygen, hence it can oxidize other materials.

(III) acts as a catalyst.

Reason — Potassium chlorate decomposes only at 700°C and even then the rate of release of oxygen is very slow. But when potassium chlorate is heated in the presence of manganese dioxide, decomposition begins at a much lower temperature, 300°C and manganese dioxide remains unaffected. Thus, in this reaction, manganese dioxide acts as a catalyst

What happens when a solution of an acid is mixed with a solution of a base in a test tube?

P — The temperature of the solution increases.

Q — The temperature of the solution decreases.

R — The temperature of the solution remains the same.

S — Salt formation takes place.

1. Only P

2. P and R

3. Q and R

4. P and S

Answer

P and S

Reason — When a solution of an acid is mixed with a solution of a base, it forms salt and water. This is referred to as neutralization reaction. Neutralization reactions are also exothermic reactions. Hence the temperature of the solution increases.

For example :

NaOH + HCl ⟶ NaCl + H₂O + Heat

Name

(a) a carbonate which does not decompose on heating.

(b) a nitrate which produces oxygen as the only gas.

(c) a compound which produces carbon dioxide on heating

(d) a nitrate which produces brown gas on heating.

Answer

(a) Sodium carbonate

(b) Potassium nitrate

(c) Calcium carbonate

(d) Lead nitrate

Which is the most reactive element in this question :

CuSO₄ + Fe ⟶ FeSO₄ + Cu

Answer

Fe, as Fe is higher in the metal reactivity series. The reactivity of the metal reactivity series decreases as one moves from top to bottom.

Define :

(a) Photochemical reaction

(b) Electrochemical reaction

Give one example in each case.

Answer

(a) Photochemical reaction — It is a reaction that occurs with the absorption of light.

Example : Decomposition of silver nitrate

2AgNO₃ $\xrightarrow{\text{sunlight}}$ 2Ag + 2NO₂ ↑ + O₂ ↑

(b) Electrochemical reaction — It is a reaction that occurs with absorption of electrical energy.

Example : Acidulated water breaks into hydrogen and oxygen when electric current is passed through it.

2H₂O $\xrightarrow[\text{current}]{\text{electric}}$ 2H₂ ↑ + O₂ ↑

Give an example of a reaction where the following are involved

(a) Evolution of heat

(b) Absorption of heat

(c) High pressure is required

Answer

(a) CH₄ + 2O₂ ⟶ CO₂ ↑ + 2H₂O + Heat

(b) C + 2S $\xrightarrow \Delta$ CS₂

(c) N₂ + 3H₂ $\xrightleftharpoons{\text{above 200 atm}}$ 2NH₃ ↑

Why does the blue colour of copper sulphate solution fade on adding iron fillings ?

Answer

Blue colour of copper sulphate fades away as iron displaces copper from copper sulphate solution as iron is more reactive than copper.

Give an example of:

(a) Single displacement reaction

(b) Double displacement reaction

(c) Photochemical reaction

(d) Thermal decomposition reaction

(e) Electrolytic decomposition reaction

(f) Combination reaction between two compounds ?

Answer

(a) Single displacement reaction

CuSO₄ + Zn ⟶ Cu + ZnSO₄

(b) Double displacement reaction

BaCl₂ (aq) + H₂SO₄ (aq) ⟶ BaSO₄ (s) + 2HCl (aq)

(c) Photochemical reaction

2AgNO₃ $\xrightarrow{\text{sunlight}}$ 2Ag + 2NO₂ ↑ + O₂ ↑

(d) Thermal decomposition reaction

2HgO (s) $\xrightarrow{\Delta}$ 2Hg (l) + O₂ (g)

(e) Electrolytic decomposition reaction

2H₂O $\xrightarrow[\text{current}]{\text{electric}}$ 2H₂ + O₂ ↑

(f) Combination reaction between two compounds ?

NH₃ + HCl ⟶ NH₄Cl

What do you understand by a 'chemical reaction'?

Answer

A chemical reaction is a process of breaking chemical bonds of the reacting substances (reactants) and making new bonds to form new substances (products).

Example : BaCl₂ (aq) + H₂SO₄ (aq) ⟶ BaSO₄ (s) + 2HCl (aq)

Give an example of each of the following chemical changes.

(a) A reaction involving

(i) change of state

(ii) formation of a precipitate

(b) An exothermic and an endothermic reaction involving carbon as one of the reactants.

(c) A reaction where colour change is noticed.

Answer

(a) An example of:

(i) A reaction involving change of state — Ammonia gas reacts with hydrogen chloride gas to produce solid ammonium chloride.

NH₃ (g) + HCl (g) ⇌ NH₄Cl (s)

(ii) A reaction involving formation of a precipitate — Take a solution of silver nitrate in a test tube and add solution of sodium chloride, a curdy white ppt. is formed.

AgNO₃ (aq) + NaCl (aq) ⟶ AgCl ↓ + NaNO₃ (aq)

(b) Exothermic reaction involving carbon — Combustion of methane gives out large amount of energy.

CH₄ + 2O₂ ⟶ CO₂ ↑ + 2H₂O + Heat

Endothermic reaction involving carbon — When carbon is heated with sulphur at high temperature, liquid carbon disulphide is formed.

C + 2S $\xrightarrow \Delta$ CS₂

(c) A reaction where colour change is noticed — When a few pieces of iron are dropped into a blue coloured copper sulphate solution, the blue colour of the solution fades and eventually turns into light green due to the formation of ferrous sulphate.

$\underset{\text{ iron}}{\text{Fe}} + \underset{\text{ blue sol.}}{\text{CuSO}_4} \longrightarrow \underset{\text{ green sol.}}{\text{FeSO}_4} + \underset{\text{ copper-brown red deposit}}{\text{Cu}}$

What is a chemical change? Give two examples of a chemical change?

Answer

A chemical change is a permanent change in which the chemical composition of a substance is changed and one or more new substances with different chemical compositions and different properties are formed.

Examples :

1. Pb(NO₃)₂ (s) + 2KI (s) ⟶ 2KNO₃ + PbI₂ (s)

2. NaCl (aq.) + AgNO₃ (aq.) ⟶ AgCl (white ppt.) ↓ + NaNO₃ (aq.)

Define exothermic and endothermic reactions. Give two examples in each case.

Answer

A chemical reaction in which heat is given out is called an exothermic reaction. It causes a rise in temperature.

Examples:

1. Combustion of methane gives out large amount of energy.
   CH₄ + 2O₂ ⟶ CO₂ ↑ + 2H₂O + Heat

2. When carbon burns in oxygen to form carbon dioxide, lot of heat is produced.
   C + O₂ ⟶ CO₂ ↑ + Heat

A chemical reaction in which heat is absorbed is called an endothermic reaction. It causes a fall in temperature.

Example :

1. When carbon is heated with sulphur at high temperature, liquid carbon disulphide is formed.
   C + 2S $\xrightarrow \Delta$ CS₂

2. Calcium carbonate decomposes to carbon dioxide and calcium oxide when heated to 1000°C
   CaCO₃ (s) $\xrightarrow{\Delta}$ CaO (s) + CO₂ (g)

Why is energy involved in a chemical change?

Answer

A chemical reaction involves the breaking up of chemical bonds between atoms resulting in absorption of energy in the form of heat, and simultaneous formation of bonds with release of energy. These two types of energies are different from each other, i.e., there is either a surplus or a deficit of energy during the reaction.

Therefore, in a chemical reaction, energy is involved.

A metal X (used for galvanization) is immersed in the aqueous solution of a blue salt YSO₄. Metal Y is deposited on the surface of the solution and a strip of metal X dissolves.

(a) Predict the metal X.

(b) What type of reaction is it?

(c) Write the word equation for this reaction and balance it.

Answer

(a) Zinc (Zn)

(b) Displacement reaction

(c) Copper(II) sulphate + Zinc  ⟶  Zinc sulphate + Copper ↓
CuSO₄ (aq.) + Zn ⟶ ZnSO₄ + Cu↓

Explanation — Galvanization involves coating iron with zinc (Zn) to prevent rusting.

So, Metal X  ⟶ Zinc (Zn).

Since Zn displaces Y from its sulphate solution, Zn must be more reactive than Y. The blue salt indicates YSO₄ is CuSO₄ (copper(II) sulphate), which is blue in color.

So, Metal Y  ⟶ Copper (Cu)

YSO₄  ⟶ CuSO₄

A more reactive metal (zinc) displaces a less reactive metal (copper) from its salt solution.

CuSO₄ (aq.) + Zn ⟶ ZnSO₄ + Cu↓

State the main characteristics of chemical reactions. Give at least one example in each case.

Answer

The main characteristics of chemical reactions are :

1. Evolution of gas
   Zn(NO₃)₂ $\xrightarrow{\Delta}$ 4NO₂ + 2ZnO + O₂ ↑

2. Change of colour
   $\underset{\text{ iron}}{\text{Fe}} + \underset{\text{ blue sol.}}{\text{CuSO}_4} \text{ (aq.)} \longrightarrow \underset{\text{ green sol.}}{\text{FeSO}_4} + \underset{\text{ copper-brown red deposit}}{\text{Cu}}$

3. Change of state
   2H₂O (l) $\xrightarrow[\text{current}]{\text{electric}}$ 2H₂ (g) + O₂ (g)

4. Formation of precipitate
   NaCl (aq.) + AgNO₃ (aq.) ⟶ AgCl (white ppt.) ↓ + NaNO₃ (aq.)

State the effects of endothermic and exothermic reactions on the surroundings with examples.

Answer

In exothermic reactions heat is given out, hence it causes a rise in temperature.

CH₄ + 2O₂ ⟶ CO₂ ↑ + 2H₂O + Heat

In endothermic reactions heat is absorbed, hence it causes a fall in temperature.

C + 2S $\xrightarrow \Delta$ CS₂

Complete and balance the following reactions:

(a) NaCl (aq.) + AgNO₃ (aq.) ⟶

(b) Pb(NO₃)₂ + KI ⟶

(c) CuCO₃ $\xrightarrow{\Delta}$

(d) Pb(NO₃)₂ $\xrightarrow{\Delta}$

(e) CO₂ + H₂O $\xrightarrow[\text{sunlight}]{\text{Chlorophyll}}$

Answer

(a) NaCl (aq.) + AgNO₃ (aq.) ⟶ AgCl (white ppt.) ↓ + NaNO₃ (aq.)

(b) Pb(NO₃)₂ (s) + 2KI (s) ⟶ 2KNO₃ + PbI₂ (s)

(c) CuCO₃ (s) $\xrightarrow{\Delta}$ CuO (s) + CO₂ (g)

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

(e) 6CO₂ + 6H₂O $\xrightarrow[\text{sunlight}]{\text{Chlorophyll}}$ C₆H₁₂O₆ + 6O₂

What do you observe in the following cases?

(a) Lead nitrate is heated.

(b) Silver chloride is exposed to sunlight.

(c) Hydrogen peroxide is exposed to sunlight

(d) H₂S gas is passed through copper sulphate solution

(e) Barium chloride is added to sodium sulphate solution.

(f) Water is added to quick lime

(g) Sodium chloride solution is added to silver nitrate solution.

Answer

(a) When white crystalline solid Lead nitrate is heated, it decomposes into a buff yellow residue of lead monoxide. Reddish brown gas Nitrogen dioxide and colourless gas Oxygen are evolved.

$\underset{\text{[lead nitrate (white)]}}{2\text{Pb(NO}_3)_2} \overset{\Delta}{\longrightarrow} \underset{\text{[litharge - buff yellow]}}{2\text{PbO}} + \underset{\text{[Nitrogen dioxide]}}{4\text{NO}_2} + \underset{\text{[Oxygen]}}{\text{O}_2}$

(b) When silver chloride (white) is exposed to sunlight, it undergoes photochemical decomposition forming a grey metal, silver and a greenish yellow gas, chlorine.

2AgCl (s) $\xrightarrow{\text{ sunlight}}$ 2Ag (s) + Cl₂ (g)

(c) When hydrogen peroxide is exposed to sunlight, it changes to water and colourless, odourless oxygen gas is evolved.

2H₂O₂ $\xrightarrow{\text{ sunlight}}$ 2H₂O + O₂ ↑

(d) When H₂S is passed through copper sulphate solution, a black precipitate of copper sulphide is obtained

CuSO₄ + H₂S ⟶ CuS [black] + H₂SO₄

(e) White coloured precipitate of barium sulphate is formed when sodium sulphate is mixed with barium chloride.

Na₂SO₄ (aq.) + BaCl₂ (aq.) ⟶ 2NaCl (aq.) + BaSO₄ [white ppt.] ↓

(f) When calcium oxide (quick lime) combines with water, a vigorous reaction takes place with the liberation of a large amount of heat (exothermic reaction) and calcium hydroxide is formed. Heat produced boils the water and a hissing sound is produced.

CaO (s) + H₂O (l) ⟶ Ca(OH)₂ (s) + Heat

(g) When sodium chloride is added to the silver nitrate solution, a white curdy precipitate of silver chloride is formed.

NaCl (aq.) + AgNO₃ (aq.) ⟶ AgCl (white ppt.) ↓ + NaNO₃ (aq.)

Discuss the factors that affect a chemical change.

Answer

1. Certain chemical reactions occur only when electricity is passed through the reactants.
   2NaCl $\xrightarrow{\text{electricity}}$ 2Na + Cl₂ ↑

2. Some chemical reactions occur when the involved substances are subjected to high pressure.
   N₂ + 3H₂ $\xrightleftharpoons{\text{above 200 atm}}$ 2NH₃ ↑

3. Some chemical reactions need a catalyst to accelerate or decelerate the rate at which they occur.
   2H₂O₂ $\xrightarrow{\text{MnO}_2}$ 2H₂O + O₂ ↑

4. Some chemical reactions take place by the action of light.
   2AgCl (s) $\xrightarrow{\text{ sunlight}}$ 2Ag (s) + Cl₂ (g)

5. Some chemical reactions take place when two substances are mixed in their solid state.
   Pb(NO₃)₂ (s) + 2KI (s) ⟶ 2KNO₃ + PbI₂ (s)

6. Some chemical reactions occur only on heating

$\underset{\text{[lead nitrate (white)]}}{2\text{Pb(NO}_3)_2} \overset{\Delta}{\longrightarrow} \underset{\text{[litharge - buff yellow]}}{2\text{PbO}} + \underset{\text{[Nitrogen dioxide]}}{4\text{NO}_2} + \underset{\text{[Oxygen]}}{\text{O}_2}$

Balance these reactions and state what type of reactions they are:

(a) NaNO₃ ⟶ NaNO₂ + O₂

(b) AgNO₃ + Zn ⟶ Zn(NO₃)₂ + Ag

(c) Fe + HCl ⟶ FeCl₂ + H₂

(d) AgNO₃ ⟶ Ag + NO₂ + O₂

(e) NaBr + Cl₂ ⟶ NaCl + Br₂

(f) PbO + C ⟶ Pb + CO₂

(g) KClO₃ ⟶ KCl + O₂

(h) AgNO₃ + HCl ⟶ AgCl ↓ + HNO₃

(i) NH₃ + HCl ⟶ NH₄Cl

(j) N₂ + H₂ ⟶ NH₃

Answer

(a) 2NaNO₃ ⟶ 2NaNO₂ + O₂ ↑

Decomposition reaction

(b) 2AgNO₃ + Zn ⟶ Zn(NO₃)₂ + 2Ag

Displacement reaction

(c) Fe + 2HCl ⟶ FeCl₂ + H₂ ↑

Displacement reaction

(d) 2AgNO₃ ⟶ 2Ag + 2NO₂ ↑ + O₂ ↑

Decomposition reaction

(e) 2NaBr + Cl₂ ⟶ 2NaCl + Br₂

Displacement reaction

(f) 2PbO + C ⟶ 2Pb + CO₂ ↑

Displacement reaction

(g) 2KClO₃ ⟶ 2KCl + 3O₂ ↑

Decomposition reaction

(h) AgNO₃ + HCl ⟶ AgCl ↓ + HNO₃

Double decomposition reaction

(i) NH₃ + HCl ⟶ NH₄Cl

Direct combination reaction

(j) N₂ + 3H₂ ⟶ 2NH₃ ↑

Direct combination reaction

Explain the different types of double decomposition reactions by giving suitable equations.

Answer

Different types of double decomposition reactions are:

1. Precipitation 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, when H₂S is passed through copper sulphate solution, a black precipitate of copper sulphide is obtained.
   CuSO₄ (aq) + H₂S (g) ⟶ CuS [black ppt.] ↓ + H₂SO₄

2. Neutralization reaction — The reaction between an acid and a base that forms salt and water only is referred to as Neutralization reaction.
   For example, Sodium chloride (base) reacts with HCl (acid) to form salt and water only.
   NaOH + HCl ⟶ NaCl + H₂O

Explain thermal dissociation and thermal decomposition reactions with examples. What is the difference between them ?

Answer

A reversible decomposition reaction brought about only by heat is called thermal dissociation reaction.

For example, on application of heat, ammonium chloride decomposes into Ammonia and HCl:

NH₄Cl ⇌ NH₃ + HCl

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 without any recombination on cooling is called a Thermal decomposition reaction.

For example, calcium carbonate decomposes to carbon dioxide and calcium oxide when heated to 1000°C

CaCO₃ (s) $\xrightarrow{\Delta}$ CaO (s) + CO₂ (g)

Difference between the two is that thermal dissociation is a reversible decomposition reaction whereas, thermal decomposition is an irreversible reaction.

Metal X was placed in lead nitrate solution. A thin layer of lead metal deposits on metal X. Which is more reactive metal X or lead ? State the type of reaction and give an example of this type.

Answer

Metal X is more active metal because it displaced the metal lead from lead nitrate. It is a displacement reaction.

CuSO₄ + Zn ⟶ ZnSO₄ + Cu

Zn is more active metal than Cu, and it displaces Cu from copper sulphate.

What type of reaction is respiration ? Explain.

Answer

Respiration is an exothermic reaction as it involves combustion of glucose with oxygen in the cells of the body releasing heat energy that is used by cells to perform various functions such as muscle contraction, nerve impulse transmission, and protein synthesis. The equation for respiration reaction is:

C₆H₁₂O₆ + 6O₂ ⟶ + 6CO₂ ↑ + 6H₂O + Heat energy

Discuss decomposition reactions occurring inside our body.

Answer

Digestion of food in our body is an example of decomposition reaction.

The starch present in the food we eat decomposes into glucose and sugar. Proteins undergo decomposition to form amino acids. Fats and oils are decomposed to fatty acids and finally oxidized by respiration into carbon dioxide and water.

Starch $\xrightarrow{\text{Enzymes}}$ Glucose $\xrightarrow{\text{[O]}}$ CO₂ + H₂O + Energy

Take sodium sulphate solution in a test tube and add barium chloride solution to it dropwise.

(a) What do you observe?

(b) Why is the reaction fast?

(c) Which type of reaction is it?

Answer

(a) A white precipitate of barium sulphate is formed immediately.
Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl

(b) The reaction is fast because both sodium sulphate and barium chloride are in aqueous solution, so their ions are already free and can react immediately.

(c) It is a double displacement reaction. More specifically, it is a precipitation reaction.

NaCl + AgNO₃ ⟶ AgCl + NaNO₃

(a) What will you observe in the above chemical change?

(b) Why is this reaction known as a double displacement reaction?

(c) Which reagent can be used to test the chloride radical?

Answer

(a) A white precipitate is formed due to the formation of insoluble silver chloride.

(b) This reaction is known as a double displacement reaction because the ions of the two reactants exchange partners. Sodium chloride and silver nitrate exchange ions to form silver chloride and sodium nitrate.

(c) The chloride ion (Cl^-) is tested using silver nitrate solution. It gives a white precipitate of silver chloride (AgCl) when chloride ions are present.

K, Na, Ca, Mg, Al, Zn, Fe, Pb, Cu, H, Hg, Ag

Pick an element from the reactivity series given above and write the equations for the the following:

(a) Metal hydroxide on heating forms metal oxide and water vapour.

(b) Metal hydroxide on heating forms metal, oxygen and water vapours.

(c) Metal nitrate decomposes to give two products only.

(d) Metal nitrate on heating forms metal oxide, nitrogen dioxide and oxygen.

(e) Metal nitrate on heating forms metal, nitrogen dioxide and oxygen.

(f) Metal carbonate which is stable to heat.

(g) Metal carbonate which forms metal oxide and carbon dioxide on heating.

(h) The heating effect on bivalent metal hydrogen carbonate.

Answer

(a) Metal hydroxide on heating forms metal oxide and water vapour — Calcium hydroxide

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

(b) Metal hydroxide on heating forms metal, oxygen and water vapours — silver hydroxide

4AgOH $\xrightarrow{\Delta}$ 4Ag + O₂ + 2H₂O

(c) Metal nitrate decomposes to give two products only — potassium nitrate

2KNO₃ $\xrightarrow{\Delta}$ 2KNO₂ + O₂ ↑

(d) Metal nitrate on heating forms metal oxide, nitrogen dioxide and oxygen — Lead nitrate

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

(e) Metal nitrate on heating forms metal, nitrogen dioxide and oxygen — Silver nitrate

2AgNO₃ $\xrightarrow{\Delta}$ 2Ag + 2NO₂ + O₂ ↑

(f) Metal carbonate which is stable to heat — Potassium carbonate

Potassium carbonate is stable to heat

(g) Metal carbonate which forms metal oxide and carbon dioxide on heating — Magnesium carbonate

MgCO₃ $\xrightarrow{\Delta}$ MgO + CO₂ ↑

(h) The heating effect on bivalent metal hydrogen carbonate — Magnesium bicarbonate

Mg(HCO₃)₂ $\xrightarrow{\Delta}$ MgCO₃ + H₂O + CO₂ ↑

KI + Cl₂ ⟶ KCl + I₂

(a) Balance the equation given above.

(b) Which type of reaction is it ?

(c) Which element is more reactive ?

(d) What will be the product formed if KCl reacts with I₂ ?

Answer

(a) 2KI + Cl₂ ⟶ 2KCI + I₂

(b) Displacement reaction

(c) Chlorine

(d) No reaction, because Iodine being less reactive than chlorine will not be able to displace chlorine from KCl.

See the diagram given below and answer the following questions:

(a) Write the chemical reaction involved in the process.

(b) Mention the colour of :

(i) copper powder and
(ii) the substance formed after heating it.

(c) Can we reverse the above reaction ? If yes, write the equation for the reverse reaction and state the substance that undergoes oxidation and the substance that undergoes reduction.

Answer

(a) 2Cu + O₂ ⟶ 2CuO

(b)
(i) The colour of copper powder is reddish brown.
(ii) The substance formed after heating, copper(II) oxide, is black in colour.

(c) Yes, the reaction can be reversed by reducing copper(II) oxide with hydrogen.

CuO + H₂ ⟶ Cu + H₂O

In this reaction, hydrogen is oxidised to water, and copper(II) oxide is reduced to copper.

Assertion (A): Zinc can displace copper from aqueous copper sulphate solution.

Reason (R): Copper is placed above zinc in the reactivity series.

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.

Answer

A is true but R is false.

Explanation — Zinc is more reactive than copper. So, zinc is placed above copper in reactivity series. Hence, zinc displaces copper from aqueous copper sulphate solution. Hence A is true.

CuSO₄ (aq) + Zn → ZnSO₄ + Cu ↓

Copper is less reactive than zinc and is placed below zinc in reactivity series. Less reactive elements cannot displace more active elements. Hence, copper cannot displace zinc from its salt solution. Hence R is false.

Therefore, A is true but R is false is the correct option.

Assertion (A): In a redox reaction, the electron gaining species acts as a reducing agent.

Reason (R): Oxidation and reduction reactions occur simultaneously in redox reaction.

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.

Answer

A is false but R is true.

Explanation — The electron gaining species is being reduced, so it acts as an oxidizing agent, not as a reducing agent. The reducing agent is the one that loses electrons. Hence A is false.
A redox reaction is a chemical reaction where both oxidation and reduction occur simultaneously. Hence R is true.

Therefore, A is false but R is true is the correct option.

Assertion (A): The decomposition of vegetable matter into compost is an endothermic reaction.

Reason (R): Decomposition reactions involve the breakdown of a single reactant into simpler products.

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.

Answer

A is false but R is true.

Explanation — The decomposition of vegetable matter (also called composting) is an exothermic reaction. Microbial activity breaks down the organic material and releases heat. Hence A is false.
A chemical reaction in which a compound splits into two or more simpler substances (elements or compounds) is called decomposition reaction. Hence R is true.

Therefore, A is false but R is true is the correct option.

Assertion (A): The decomposition of silver bromide is utilized in black and white photography.

Reason (R): Light provides energy for this endothermic reaction.

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.

Answer

Both A and R are true and R is the correct explanation of A.

Explanation — Silver bromide (AgBr) decomposes when exposed to light. The formation of black metallic silver is what creates the image in black and white photography. Hence A is true.

2AgBr $\xrightarrow{\text{light}}$ 2Ag + Br

R is true as above reaction is an endothermic reaction where heat is absorbed. Light provides energy for this decomposition of silver bromide.

Therefore, Both A and R are true and R is the correct explanation of A is the correct option.

Assertion (A): Chlorine can displace iodide ions from KI solution.

Reason (R): It is an example of displacement reaction.

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.

Answer

Both A and R are true but R is not the correct explanation of A.

Explanation — Chlorine is more active non-metal when compared to iodine. In the reactivity series of non-metals, chlorine is placed above iodine. Hence, chlorine displaces iodine from potassium iodide solution. Hence, A is true.

2KI + Cl₂ ⟶ 2KCI + I₂

Above reaction is displacement reaction in which a more active element chlorine displaces a less active element iodine from its salt solution. Hence R is true.

However, reason (R) does not explain about reactivity of chlorine and position of chlorine and iodine in reactivity series. Hence, Both A and R are true but R is not the correct explanation of A is the correct option.

Assertion (A): Silver is displaced by copper from the aqueous solution of silver nitrate.

Reason (R): Copper is placed below silver in the activity series.

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.

Answer

A is true but R is false.

Explanation — A is true as silver is displaced by copper from the aqueous solution of silver nitrate because copper is more reactive than silver. Hence the assertion is true.
Copper is placed above silver in the activity series. So copper is more reactive and can displace silver. Hence the R is false.

Therefore, A is true but R is false is the correct option.

Riya was revising different types of chemical reactions studied in class. She recalled that:

$\bullet$ A double displacement reaction generally takes place in an aqueous solution where the ions of two compounds exchange places.

$\bullet$ If one of the products formed is an insoluble solid, the reaction is called a precipitation reaction.

$\bullet$ A reaction in which an acid or acidic oxide reacts with a base or basic oxide to form salt and water is known as a neutralisation reaction.

To understand these reactions better, Riya observed the following experiments.

(a) When hydrogen sulphide gas (H₂S) was passed through a blue solution of copper sulphate, a black solid was formed along with sulphuric acid.

(i) Identify the type of reaction taking place.
(ii) Name the black solid formed.

(b) Sodium hydroxide reacts with hydrochloric acid to form sodium chloride and water.
Explain why this reaction is considered a double displacement reaction.

(c) Complete and balance the following chemical equation ;

Pb(NO₃)₂ (aq.) + KI ⟶ ...... + ......

(d) State one observable change that indicates the reaction in part (c) has taken place.

Answer

(a) (i) This is a double displacement reaction.

(ii) The black solid formed is copper sulphide.

(b) NaOH + HCl ⟶ NaCl + H₂O

Na is combining with Cl which is provided by HCl. H combines with OH which is provided by NaOH. Here, ions exchanged their partners. Therefore, this is called double displacement reaction.

(c) Pb(NO₃)₂ (aq.) + KI ⟶ PbI₂ + 2KNO₃

(d) A bright yellow precipitate is formed which indicates the formation of lead iodide.

An aqueous solution of metal nitrate M reacts with sodium chloride to form a white precipitate of a compound N. N undergoes photochemical reaction (decomposition). Identify and name M and N. Write the chemical equation for the reaction involved and also identify the type of reaction.

Answer

M is silver nitrate.
N is silver chloride.

AgNO₃ + NaCl ⟶ AgCl + NaNO₃

This is double displacement reaction.

2AgCl ⟶ 2Ag + Cl₂

This is decomposition reaction.

A water insoluble substance A on reacting with dilute H₂SO₄ releases a gas which turns a paper dipped in acidified potassium dichromate solution green. When the gas is passed through lime water, the solution obtained turns blue litmus red. On bubbling the gas through lime water, it initially became turbid and the turbid nature disappeared when the gas is passed in excess.

(a) Identify the substance 'A'.

(b) Write the chemical equations of the reactions involved.

(c) State the type of reactions that are involved.

Answer

(a) The substance A is sodium sulphite.

(b) The reactions involved are:

1. SO₂ formation
   Na₂SO₃ + H₂SO₄ ⟶ Na₂SO₄ + H₂O + SO₂

2. Reaction with potassium dichromate
   K₂Cr₂O₇ + 3SO₂ + H₂SO₄ ⟶ K₂SO₄ + Cr₂(SO₄)₃ + H₂O

3. Reaction with lime water
   Ca(OH)₂ + SO₂ ⟶ CaSO₃ + H₂O

4. Excess SO₂
   CaSO₃ + SO₂ + H₂O ⟶ Ca(HSO₃)₂

(c)

1. SO₂ formation - Double displacement reaction

2. Reaction with potassium dichromate - Redox reaction

3. Reaction with lime water - Combination reaction

4. Excess SO₂ - Combination reaction