Chemical Changes and Reactions

(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 ↑

(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.

(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 ↑

(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 ↑

(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.

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

(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)

(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.

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

No net energy change is involved

Reason — Characteristics of a chemical change are :

1. It's irreversible.

2. A new substance is formed.

3. Involves absorption or liberation of energy.

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

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

Carbon dioxide

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

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

(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.

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.

(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) 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 ↑

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

(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.

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) 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 ↑

(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)

(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

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.)

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

(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

(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.)

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

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

(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

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

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 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.

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

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

Iron is magnetised

Reason — Magnetisation of iron is not a chemical change, as it is a temporary change.

Decomposition reaction

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

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

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.

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 = Cu and B = CuO

Reason — Brown copper on heating forms black copper oxide.

2Cu + O₂ ⟶ 2CuO

Combustion of LPG

Reason — A chemical change is a permanent change and combustion of LPG is a permanent change.

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

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

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.

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.

(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

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

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 the assertion (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 the reason (R) is false.

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 the assertion (A) is false.
A redox reaction is a chemical reaction where both oxidation and reduction occur simultaneously. Hence the reason (R) is true.

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 the assertion (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 the reason (R) is true.

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 the assertion (A) is true.

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

Above reaction is an endothermic reaction where heat is absorbed. Light provides energy for this decomposition of silver bromide. Hence the reason (R) is true and reason (R) is a correct explanation of assertion (A).

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, the assertion (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 the reason (R) is true.

However reason (R) does not explain about reactivity of chlorine and position of chlorine and iodine in reactivity series. Hence reason (R) is not a correct explanation of assertion (A).

A is true but R is false.

Explanation — 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 reason (R) is false.

(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 ↑

(a) CH₄ + 2O₂ ⟶ CO₂ ↑ + 2H₂O + Heat

(b) C + 2S $\xrightarrow \Delta$ CS₂

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

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

(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

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)

(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.

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

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.)

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)

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) 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↓

(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 ↑

(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.

(a) Sodium carbonate

(b) Potassium nitrate

(c) Calcium carbonate

(d) Lead nitrate

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.

(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)

(i) 2Al + N₂ ⟶ 2 AlN

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

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

(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

2KNO₃ ⟶ 2KNO₂ + O₂

Two balanced equations showing thermal dissociation:

1. NH₄Cl ⇌ NH₃ ↑ + HCl ↑

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

(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)