NCERT CHAPTER1 CHEMICAL REACTIONS AND EQUATIONS EXPLANATION

 CHAPTER 1: CHEMICAL REACTION AND EQUATIONS

Chemical reaction

To explain the chemical reaction, it is necessary to understand the terms related to it. So, the terms related to a chemical reaction are

(i) Reactants

Materials required to start a chemical reaction are called reactants. Example: When we add water to the dry limestone, it releases heat. Here, the limestone and the water are reactants. 

(ii) Products

 When two or more reactants mix or react with each other, they form other substances/materials known as products.

In the above example, the gas is coming out; the gas is the product.

What happens during a chemical reaction

1. New products are formed

2. The physical state of the products can be changed

3. Temperature changes

4. Evolution of Gas or not

Writing a chemical reaction

Let the "A" and "B" react to each other and form "C" and "D." This condition can be written in the symbolic form as

A + B → C + D + 🛆 (heat)

Example 1: When a magnesium ribbon burns in air, it forms magnesium oxide. 

To start this reaction, firstly, the magnesium ribbon is cleaned properly because there is a firm layer of magnesium oxide over the surface. The coating of oxide acts as a barrier and does not allow the magnesium surface to react with air. 

 It reacts with oxygen and produces heat. It is a kind of exothermic reaction (as it releases heat).

It burns in the air with the dazzling white flame. The reaction can be written as

2Mg + O₂ ➡ 2MgO (Magnesium oxide as product)

(Reactants—magnesium and oxygen)

The "+" symbol is used to show that the two substances are going to react. 

Balanced chemical reaction

A chemical reaction is said to be balanced if the number of atoms of reactants on the left-hand side of the reaction is equal to the number of atoms after the reaction.

Example2

Mg + O₂ ➡ MgO

In this reaction, count the number of atoms on the right and the left side:

On the left-hand side 

Mg = 1

O atom = 1

Right-hand side

Mg = 1

O = 1

So, it can be easily told that the number of oxygen atoms on the two sides is not equal. 

Rewrite the equation by multiplying by 2 for the magnesium atom.

2Mg + O₂ ➡ 2MgO

Again, count the number of atoms on both sides

LHS

Mg = 2

O = 2

RHS

Mg = 2

O = 2

Hence, this reaction is balanced on both sides.

 

Example 2:

Zinc + Sulphuric acid ➡ Zinc sulphate + Hydrogen gas

In the form of symbols, we can write

Zn + H₂SO₄ ➡ ZnSO₄ + H₂ 

Comparing the atoms on both sides

L.H.S. 

Zn = 1

H = 2

S = 1

O = 4

R.H.S.

Zn = 1

S = 1

O = 4

H = 2

On both sides, the number of atoms is the same. Hence, this reaction is balanced. 

It is also a heat-releasing (exothermic) process. When a burning matchstick is brought near the test tube, a 'pop' sound can be heard due to the hydrogen gas.

ZnSO₄ = Zinc sulphate

Example 3:

Iron + water ➡ Oxide of Iron + Hydrogen gas

Symbol of Iron = Fe

Symbol of water: H₂O 

Symbol of Iron Oxide = Fe₂O₃

Reaction

Fe + H₂O ➡ Fe₃O₄ + H₂

This reaction occurs when the water is in the gaseous form (steam)

Hot iron reacts with the steam to form ferric oxide and hydrogen gas.

This is not a balanced reaction.

LHS

Fe = 1

H = 2

O = 1

RHS
Fe =3

O = 4

H =2 

In the RHS, there are 4 iron atoms, and in the LHS, there is only 1; similarly, the count of oxygen atoms also does not match.

In LHS, multiply Fe by 3 and 4 by H₂O.

3Fe + 4H₂O ➡ Fe₃O₄ + H₂

Now, the hydrogen atom count is not balanced; to balance, multiply by 2

 

3Fe + 4H₂O ➡ Fe₃O₄ + H₂

 Now, it is a balanced reaction.

To make the reaction more informative, we write the state of the substances participating in the reaction. 

3Fe (s)  + 4H₂O (g) ➡ Fe₃O₄ (s) + H₂ (g)

s = solid

g = gas

This example is a kind of redox reaction. Here, oxygen is added to iron, and hydrogen is freed. 

In some reactions, temperature, pressure, and catalysts are used; temperature is represented by the symbol △, pressure is written in atm (atmospheric pressure), and the catalyst is represented by its symbolic name. 

Example 4

                        340 atm

CO (g) + H₂ (g) ⇾ CH₃OH (l)

                    Exercise Question on page 5

1. Why should magnesium ribbon be cleaned before burning in the air?

Ans: Because a tough layer of oxide covered the magnesium and did not allow it to react with the oxygen present in the air.

2. Write the balanced chemical reaction for the following:

(i) Hydrogen + Chlorine → Hydrogen chloride

(ii) Barium Chloride + Aluminium sulphate →  Barium sulphate + Aluminium chloride

Ans:

 (i) We use the molecular form of hydrogen, H₂, and of chlorine as Cl₂. 

H₂ (g) + Cl₂ (g) → 2HCl (g) (Hydrochloric Acid)

(ii) BaCl₂ (aq) + Al₂ (SO₄)₃ (aq) → BaSO₄ (s) + AlCl₃ (aq)

This is not a balanced reaction. Here (aq) represents the solution of the compound with water. In this reaction, barium sulfate settles down at the bottom of the container, so this reaction is also known as a precipitation reaction.

To balance it, we count the number of elements on both sides of the reaction.

LHS 

 Ba = 1

CL = 2

Al = 2

SO₄ = 3

In the RHS

Ba = 1

SO₄ = 1

Al = 1

Cl =3

To balance this reaction on the right-hand side, multiply BaSO₄ by 3, as in the LHS, SO₄ is 3, and multiply BaCl₂ by 3 to match the count of Ba. To make the count similar in the RHS of Aluminium, multiply AlCl₃ by 2

3BaCl₂ (aq) + Al₂SO₄ (aq) → 3BaSO₄ (s) + 2AlCl₃ (aq)

Now, checking the atoms 

LHS

Ba = 3

Cl = 3*2 = 6

Al = 2

SO₄ = 1

 RHS

Ba = 3

SO₄ = 1

Al = 2

Cl = 3*2 = 6 

Now, the reaction is balanced.

Q3: (i) Solutions of barium chloride and sodium sulfate in water react to give insoluble barium sulfate and a solution of sodium chloride.

Ans:

BaCl₂ (aq) + Na₂SO₄ → BaSO₄ (s) + 2NaCl (aq)

(ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.

Answer: NaOH(aq) + 2HCl(aq) ➡ 2NaCl(aq) + H₂O(l)

Types of Chemical Reaction 

(1) Combination Reaction 

When two or more reactants in the reaction form a single or more complex material, it is known as a combination reaction. 

In symbols, we can write this as

A + B → C + D

Example:

Calcium oxide reacts very fast with the water and produces (makes) slaked lime (calcium hydroxide) and a large amount of heat. 

CaO (s) + H₂O (l) → Ca(OH)₂ + Heat

Quick lime, slaked lime

Calcium hydroxide and water react and form calcium hydroxide and a large amount of heat. 

This slaked lime solution is used for whitewashing in the home. It reacts with the CO₂ present in the atmosphere slowly and forms CaCO₃. CaCO₃ is formed in 2 or 3 days. 

Its formula is 

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

Endothermic and Exothermic Reactions

  If heat is evolved in a reaction, it is called an exothermic reaction, and if heat is required to initiate the reaction, it is called an endothermic reaction. 

Example 

Endothermic Reaction

Photosynthesis:  Food preparation of plants is an endothermic reaction, as it requires heat from the Sun.

Evaporation: To start evaporation, heat is required.

Exothermic reaction

Respiration: Living organisms release heat energy during the respiration process, which is utilized in the burning of food to produce energy.

C6H12O6 (aq) + 6O2 (aq) → 6CO2 (aq) + 6H2O (l) + energy (Glucose)

Decomposition reaction

When a single compound breaks into two or more substances, it is said to be a decomposition reaction. 

                  heat

ex: CaCO₃ → CaO(s) + CO₂(g)

Here, burning of calcium carbonate produces calcium oxide and carbon dioxide gas. 

Example 2:

 Pb(NO₃)₂ + Heat → 4NO₂ (g) + O₂ (g) + PbO

Example 3:

 2AgCl(s)  + sunlight → Ag (s) + Cl₂ (g) 

In-text question, page 9 

Question 1(i): The substance 'X' is CaO.

(ii) Reaction

 CaO (s) + H₂O (l) → Ca(OH)₂ (aq)

Aq- Aqus (solution of substance and water)

Question (2) H₂O consists of 2 hydrogen atoms and one oxygen atom. The ratio of hydrogen to combination is 2:1.

Therefore, the amount of hydrogen gas collected over the container is double the amount of oxygen.

Example 3

When ferrous sulfate is burnt, it forms ferric oxide, sulfur dioxide, and sulfur trioxide

                 heat

2FeSO₄(s) → Fe₂O₃(s) + SO₂(g) + SO₃(g)

It is a decomposition reaction; here, ferrous sulfate breaks down while heating and forms 3 components. 

In this reaction, the oxidation of iron and the reduction of sulfur happen simultaneously.

Example 4

 When Silver chloride or bromide is exposed to sunlight, it breaks down into silver and chlorine or bromine gas

                sunlight

2AgCl (s) → 2Ag (s) + Cl₂ (g) 

2AgBr(s) ➡ 2Ag (s) + Cl₂ (g) 

This is the decomposition of the silver chloride/bromide used in black and white photography.

In-text question 

1. A solution of a substance ‘X’ is used for whitewashing. (i) Name the substance ‘X’ and write its formula. (ii) Write the reaction of the substance ‘X’ named in (i) above with water

[This question is important for any board or competitive exam.]

Answer: This substance "X" is calcium oxide, or quicklime; its formula is CaO. 

CaO (s) + H₂O (l) ➡ Ca(OH)₂ (aq) + Heat 

This is an exothermic reaction, and heat is evolved during the reaction. CaO is calcium oxide, or quicklime; when kept in water, it reacts very quickly, releases heat, and makes a milky substance called calcium hydroxide. Calcium hydroxide reacts with carbon dioxide present in the air and forms CaCO₃, calcium carbonate.

2. Why is the amount of gas collected in one of the test tubes in Activity 1.7 twice the amount collected in the other? Name this gas.

Answer: Due to the electrolysis of the water, the amount of hydrogen gas is just double the amount of oxygen produced. 

Displacement Reaction

In a chemical reaction, a highly reactive element displaces the less reactive element from its compound. After this reaction, the low-reactive element separated out and became pure and remained in an uncombined state. 

Example 

Why does the iron nail become brownish in colour, and the blue colour of the copper sulphate solution fades?

Answer

 The chemical reaction for the above is:

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

           Copper sulphate  Iron sulphate

In this reaction, the solution of the copper sulphate is of a blue color due to the presence of the Cu++ ions. As the reaction started, the iron nail, which is more reactive in comparison to the Copper replaces it, and the whole solution color gets changed to a brownish color due to the Cu++ ions. So, this is an example of a displacement reaction. 

Example2: 

Zinc (s)+ copper sulphate (aq) →Zin suphate (aq) +Copper(s)

This can be written as 

Zn(s) + CuSO4(aq) → ZnSO4 (aq) + Cu(s)

Here, Zn is more reactive in comparison to the copper sulphate, so it replaces the copper from its compound. 

Double Displacement reaction

When two elements displace each other's place in a reaction, it is called a double displacement reaction.  

Example

Na2SO4 (aq) +BaCl2 (aq) →BaSO4 (s) + 2NaCl (aq) 

In this reaction, sodium sulphate and barium chloride, sodium reacts with the chlorine, and Barium reacts with SO4  to form Barium sulphate and Sodium chloride. 

2KI(aq) + Pb(NO3)2 (aq)  →2KNO3 (aq)+ 2PbI(s)

Here,  Potassium (K ) displaces the Barium from the compound, and Lead(Pb) does the same and reacts with Iodine (I)

Oxidation and Reduction

It is also known as the redox reaction. In this reaction, one element transfers one or more electrons to the other element and become positively charged and other element who recieves the electron becomes negatively charged, in this way the element who gained positive charged is said to be oxidised and and the element which become negatively charged is said to become reduced. 

A- 1e- →A+ (oxidised)

B+1e- → B- (reduction)

So, in a reaction, oxidation and reduction take place simultaneously. 

Old Concept

An element that loses oxygen is said to be reduced, and another element that receives the oxygen is said to be oxidised. 

Example 

Fe(s) + O2 (g)→Fe2O3 (g) 

It is an example of oxidation 

2CuO +H2 (g) → Cu2O +Heat (Oxidation)

CuO +H2 →Cu(s) +H2O (l) Reduction

Corrosion 

In this process, metal reacts with the oxygen and the moisture present in the atmosphere and gets oxidised. In this process, a more stable compound is formed. The metal deteriorates slowly with the passage of time due to the formation of an oxide layer over it. 

Example: 

Iron forms a reddish brown surface over its top surface when it is exposed to the air and moisture for a longer duration. This oxidation process deteriorates the next layer of iron. This process is known as rusting. 

To prevent rusting, paint or oil is used, which prevents the top layer from coming into direct contact with the air and moisture.

Rancidity 

When the food is prepared from the oil and the ghee, and they are exposed to the air, they give a foul smell.  This happens due to the oxidation of the fatty substances. This food becomes toxic and inedible. 

In-text question page 13

Question 1: Why does the colour of the copper sulphate solution change when an iron nail is dipped in it?

Answer: When an iron nail is kept in a copper sulphate solution, it displaces the less reactive element copper from this solution and shows a prominent reddish brown color due to the presence of the Ferrus ion.

Important for Board

Question2: 

Identify the substances that are oxidised and the substances that are reduced in the following reactions. (i) 4Na(s) + O2 (g) → 2Na2O(s) 

(ii) CuO(s) + H2 (g) → Cu(s) + H2O(l)

Answer (i) Sodium gets oxidised and oxygen gets reduced. 

Ans2: Copper(Cu) gets reduced and Hydrogen gets oxidised. 

Exercise Question Answer

1. Which of the statements about the reaction below are incorrect?

 2PbO(s) + C(s) → 2Pb(s) + CO2 (g)

 (a) Lead is getting reduced.

 (b) Carbon dioxide is getting oxidised. (c) Carbon is getting oxidised. (d) Lead oxide is getting reduced. (i) (a) and (b)

 (ii) (a) and (c)

 (iii) (a), (b)

 and (c) (iv) all

Answer- 1(a ) and (b) Because oxidation and reduction happen together in a reaction.

Question 2:  Fe2O3 + 2Al → Al2O3 + 2Fe The above reaction is an example of a (a) combination reaction. (b) double displacement reaction. (c) decomposition reaction. (d) displacement reaction.

Answer 2: It is a displacement reaction, as Al displaces the Oxygen from the Fe2O3 

Question 3

What happens when dilute hydrochloric acid is added to iron filings?

 Tick the correct answer. (a) Hydrogen gas and iron chloride are produced. (b) Chlorine gas and iron hydroxide are produced. (c) No reaction takes place. (d) Iron salt and water are produced

Answer: Metal +Acide→ Salt + hydrogen gas

(a) is correct

4. What is a balanced chemical equation? Why should chemical equations be balanced?

Already explained above

Question5. Translate the following statements into chemical equations and then balance them.

 (a) Hydrogen gas combines with nitrogen to form ammonia.

 (b) Hydrogen sulphide gas burns in air to give water and sulfur dioxide.

 (c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.

 (d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas

Answer (a) 

H2(g) +N2(g) →2NH3 (g) (Ammonia gas is formed; it is a combination reaction)

b) H2S (g) +O2(g) →H2O(l) +SO2 (g) 

c)  BaCl2 (s) + Al2(So4)3 →2AlCl3(aq) +BaSO4 (s)⬇

d)2K +H2O(l) →2KOH(aq) + H2 (g) 

Question 6: Balance the following chemical equations. 

(a) HNO3 +Ca(OH)2 → Ca(NO3 ) 2 + H2O 

(b) NaOH + H2SO4 → Na2SO4 + H2O 

(c) NaCl + AgNO3 → AgCl + NaNO3

 (d) BaCl2 + H2 SO4 → BaSO4 + HCl

Solution (a) HNO3 +Ca(OH)2 → Ca(NO3 ) 2 + H2O 

Compare the number of atoms on both sides

LHS

H = 1+2 = 3

N = 1

O = 3 +2 = 5

Ca = 1

RHS

Ca =1

N = 2

O = 2*3 +1 = 7

H = 2

 Multiply HNO3  and H2O in the RHS by 2 by 2 to balance the count of NO3

2HNO3 +Ca(OH)2 → Ca(NO3 )2 +2 H2O 

Now in LHS

H = 4

N =2 

O = 6 +2 =8

Ca =1

RHS

Ca = 1

N =2

O = 6+2 =8

H = 4

On both sides of the equation, the number of atoms is the same, so the equation is balanced now.

(b) NaOH + H2SO4 → Na2SO4 + H2O 

     

LHS 

Na =1, O = 1+4 = 5, H = 2+1 =3, S= 1

RHS

Na = 2, S =1 , O = 4+1 = 5, H = 2, 

To balance the number of sodium atoms, multiply NaOH on the LHS by 2 and 2 to the H2O on the RHS           

2NaOH + H2SO4     → Na2SO4 + 2 H2O      

LHS

Na =2, O = 2+4 =6, H = 4 S= 1

RHS

Na =2, S =1, O = 4+2 =6, H = 2*2 = 4

On both sides, the number of atoms is similar, hence the equation is balanced.    

(c)      NaCl + AgNO3 → AgCl + NaNO3

LHS

Na =1, Cl =1, Ag =1, O = 3

RHS 

Na = 1, N =1, O = 3, Cl =1

All the atoms count is the same, hence no need to balance. 

d) BaCl2 + H2 SO4 → BaSO4 + HCl

LHS

Ba = 1, Cl =2, H =2, S = 1, O = 4

RHS

Ba= 1, S = 1, O = 4, H =1, Cl =1

To balance the Cl and H in the RHS, multiply HCl by 2 inthe  RHS

d) BaCl2 + H2 SO4 → BaSO4 + 2HCl

Now, it is balanced