Chemical Reactions and Equations

(a) and (b)

Reason — Carbon is getting oxidised to carbon dioxide due to gain of oxygen and lead oxide is getting reduced to lead as oxygen is lost.

displacement reaction

Reason —

The above reaction is an example of a displacement reaction because Fe in Fe₂O₃ has been displaced by Al and the reaction is of type : YZ + X ⟶ XZ + Y

Hydrogen gas and iron chloride are produced.

Reason — When dilute hydrochloric acid is added to iron fillings hydrogen gas and iron chloride are produced.

Fe(s) + 2HCl(aq) ⟶ FeCl₂(aq) + H₂ ↑

A balanced equation is one in which the number of atoms of each element of the reactant is equal to the number of atoms of each element of the products.

An equation must be balanced in order to comply with the "Law of Conservation of Mass", which states that mass is neither created nor destroyed in the course of a chemical reaction. An unbalanced equation would imply that atoms have been created or destroyed.

(i) 3H₂ (g) + N₂ (g) ⟶ 2NH₃ (g)

(ii) 2H₂S (g) + 3O₂ (g) ⟶ 2H₂O (l) + 2SO₂ (g)

(iii) 3BaCl₂ (aq) + Al₂(SO₄)₃ (aq) ⟶ 2AlCl₃ (aq) + 3BaSO₄ (s)

(iv) 2K (s) + 2H₂O (l) ⟶ 2KOH (s) + H₂ (g)

(a) 2HNO₃ + Ca(OH)₂ ⟶ Ca(NO₃)₂ + 2H₂O

(b) 2NaOH + H₂SO₄ ⟶ Na₂SO₄ + 2H₂O

(c) NaCl + AgNO₃ ⟶ AgCl + NaNO₃

(d) BaCl₂ + H₂SO₄ ⟶ BaSO₄ + 2HCl

(a) Ca(OH)₂ (aq) + CO₂ (g) ⟶ CaCO₃ (s) + H₂O (l)

(b) Zn (s) + 2AgNO₃ (aq) ⟶ Zn(NO₃)₂ (aq) + 2Ag (s)

(c) 2Al (s) + 3CuCl₂ (aq) ⟶ 2AlCl₃ (aq) + 3Cu (s)

(d) BaCl₂ (aq) + K₂SO₄ (aq) ⟶ BaSO₄ (s) + 2KCl (aq)

(a) 2KBr (aq) + BaI₂ (aq) ⟶ 2KI (aq) + BaBr₂ (s)
Type — Double displacement reaction

(b) ZnCO₃ (s) ⟶ ZnO (s) + CO₂ (g)
Type — Decomposition reaction

(c) H₂ (g) + Cl₂ (g) ⟶ 2HCl (g)
Type — Combination reaction

(d) Mg (s) + 2HCl (aq) ⟶ MgCl₂ (aq) + H₂ (g)
Type — Displacement reaction

Exothermic reactions — Reactions in which heat is released along with the formation of products are called exothermic chemical reactions.
For example, burning of natural gas gives out large amount of heat:
CH₄ (g) + 2O₂ (g) ⟶ CO₂ (g) + 2H₂O (g) + Heat

Endothermic reactions — Reactions in which energy is absorbed are known as endothermic reactions.
For example, when carbon is heated with sulphur at high temperature, liquid carbon disulphide is formed:
C + 2S $\xrightarrow{\text{Heat}}$ CS₂

During digestion, food is broken down into simpler substances such as carbohydrates. These carbohydrates are broken down to form glucose. Glucose combines with oxygen in the cells and provides energy. The special name of this reaction is respiration.

As energy is released in this process, it is an exothermic process.

$\underset{\text{[Glucose]}}{\text{C}_6\text{H}_{12}\text{O}_6}\text{ (aq)} + 6\text{O}_2\text{ (aq)} \longrightarrow 6\text{CO}_2\text{ (aq)} + 6\text{H}_2\text{O}\text{ (l)} + \text{energy}$

In a decomposition reaction, a single reactant breaks down to give simpler products.

CaCO₃ $\xrightarrow{\Delta}$ CaO + CO₂

In a combination reaction, a single product is formed from two or more reactants.

CaO + H₂O ⟶ Ca(OH)₂

Hence, we can say that the decomposition reaction is opposite of combination reaction.

1. CaCO₃ (s) $\xrightarrow{\text{Heat}}$ CaO (s) + CO₂ (g)

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

3. 2H₂O (l) $\xrightarrow{\text{electricity}}$ 2H₂ (g) + O₂ (g)

Cu (s) + 2AgNO₃ (aq) ⟶ Cu(NO₃)₂ (aq) + 2Ag (s)

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

Some examples of precipitation reactions are:

1. Take a solution of silver nitrate in a test tube and add dil. hydrochloric acid. A curdy white ppt. of AgCl is formed.
   AgNO₃ (aq) + HCl (l) ⟶ AgCl (s) ↓ + HNO₃ (g)

2. When sodium sulphate reacts with barium chloride, a white ppt. of barium sulphate appears.

Na₂SO₄ (aq) + BaCl₂ (aq) ⟶ BaSO₄ (s) ↓ + 2NaCl (aq)

(a) Oxidation is gain of oxygen by a substance.

For example,

1. Copper is oxidised to Copper oxide:
   2Cu + O₂ $\xrightarrow{\text{Heat}}$ 2CuO

2. Carbon is oxidised to Carbon dioxide
   C + O₂ ⟶ CO₂

(b) Reduction is loss of oxygen from a substance.

For example,

1. Copper (II) oxide is reduced to Copper
   CuO + H₂ $\xrightarrow{\text{Heat}}$ Cu + H₂O

2. Zinc (II) oxide is reduced to Zinc.
   ZnO + C ⟶ Zn + CO

Element 'X ' is copper. On heating in air it forms Copper (II) oxide, which is black in colour.

2Cu (s) + O₂ (g) $\xrightarrow{\text{Heat}}$ 2CuO (s)

Paint is applied on iron articles primarily to protect them from corrosion and rust formation by creating a barrier between the iron surface and moisture and oxygen in the air.

Oil and fat containing food items are flushed with nitrogen to extend their shelf life by displacing oxygen, which can lead to oxidative rancidity and spoilage. Nitrogen helps create an oxygen-free environment that maintains the quality and freshness of the food items.

(a) Corrosion — When a metal is attacked by substances around it such as moisture, acids, etc, it is said to corrode and this process is called corrosion.
For example, iron articles are shiny when new, but get coated with a reddish brown powder when left for some time due to rusting.

(b) Rancidity — When taste and smell of food items containing fat and oil change due to oxidation, then this condition is called rancidity.
For example, when butter is left exposed to air for a long time, it becomes rancid, making it unsuitable for consumption and cooking.

As magnesium is a very reactive metal hence it reacts with oxygen and a layer of magnesium oxide is formed on its surface, which prevents further reaction of magnesium with oxygen. Therefore, magnesium ribbon is cleaned before burning in air.

(i) H₂ + Cl₂ ⟶ 2HCl

(ii) 3BaCl₂ + Al₂(SO₄)₃ ⟶ 3BaSO₄ + 2AlCl₃

(iii) 2Na + 2H₂O ⟶ 2NaOH + H₂ ↑

(i) BaCl₂ (aq) + Na₂SO₄ (aq) ⟶ BaSO₄(s) + 2NaCl (aq)

(ii) NaOH (aq) + HCl (aq) ⟶ NaCl (aq) + H₂O (l)

(i) The substance 'X' is calcium oxide (Quick lime). Its chemical formula is CaO.

(ii) Calcium oxide reacts vigorously with water to form calcium hydroxide (slaked lime).

CaO(s) + H₂O (l) ⟶ Ca(OH)₂ (aq)

In Activity 1.7, electrolysis of water is carried out and the products obtained are hydrogen and oxygen. As water contains 2 parts of hydrogen element as compared to only 1 part of oxygen element, hence, the amount of hydrogen gas collected in one test tube is double of the amount of oxygen gas collected in the other test tube.

When an iron nail is dipped in a copper sulphate solution, iron being on top of copper in the metal reactivity series displaces copper from copper sulphate solution and forms iron sulphate, which is green in colour.

Fe (s) + CuSO₄ (aq) ⟶ FeSO₄ (aq) + Cu (s)

Hence, the colour of copper sulphate solution changes when an iron nail is dipped in it.

Lead nitrate and sodium chloride react to form sodium nitrate and lead chloride.

Pb(NO₃)₂ + 2NaCl ⟶ 2NaNO₃ + PbCl₂ ↓

(i) 4Na (s) + O₂ (g) ⟶ 2Na₂O (s)

• Sodium (Na) is oxidised to Na₂O as it gains oxygen.
• Oxygen (O₂) is reduced to Na₂O as one atom of oxygen is removed from O₂.

(ii) CuO (s) + H₂ (g) ⟶ Cu (s) + H₂O (l)

• Copper oxide (CuO) is reduced to copper (Cu) as oxygen is lost.
• Hydrogen (H₂) is oxidised to water as it gains oxygen.