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Chemistry in Automobiles: For an internal combustion engine to move a vehicle down the road, it must convert the energy stored in the fuel into mechanical energy to drive the wheels. In your car, the distributor and battery provide this starting energy by creating an electrical "spark", which helps in combustion of fuels like gasoline. Below is the reaction depicting complete combustion of gasoline in full supply of air:

Reaction depicting complete Combustion Question - Teachoo-01.jpg


3.1 Which of the following are the products obtained from the reaction mentioned in the above case? 

Product ‘X’           Product ‘Y’ 

  1. CO 2           H 2 O 2  
  2. H 2 O          CO  
  3. CH 3 OH    H 2 O  
  4. CO 2          H 2 O



In a Combustion reaction,

Fuel burns in oxygen to form Carbon Dioxide and Water

2C 8 H 18 (l) + 25O 2 (g) → 16CO 2 + 9H 2 O  


Reaction depicting complete Combustion of gasoline in full supply - Teachoo-01.jpg

So, the correct answer is (d) 


3.2 Identify the types of chemical reaction occurring during the combustion of fuel: 

a) Oxidation & Endothermic reaction 

b) Decomposition & Exothermic reaction 

c) Oxidation & Exothermic reaction 

d) Combination & Endothermic reaction





  • Decomposition Reaction:  The reaction in which complex compounds are broken down into simpler ones i.e. AB → A + B
  • Oxidation Reaction: The reaction in which an oxygen atom is gained i.e. A + O 2 → AO 2
  • Combination Reaction: When two or more substances combine to form a single product i.e. A + B → AB  



  • Endothermic Reaction: Endo means in and thermic means heat. In these reactions, heat is absorbed.
  • Exothermic Reaction: Exo means out and thermic means heat. In these reactions, heat is released.


Since in Combustion

  • Energy is released
  • Oxygen is gained 


Thus, it is an oxidation and exothermic reaction


So, the correct answer is (c)

3.3 On the basis of evolution/absorption of energy, which of the following processes are similar to combustion of fuel? 

a) Photosynthesis in plants  

b) Respiration in the human body 

c) Decomposition of vegetable matter 

d) Decomposition of ferrous sulphate. 


(a) (ii) & (iii) 

(b) (i) & (ii) 

(c) (iii) & (iv)

(d) (ii) & (i) 



In a Combustion reaction, heat is released (evolved)


Let’s check all the options:

  1. In photosynthesis , the plants make food by absorbing the energy of the sun.
    So, this is not similar to Combustion

  2. The breakdown of food to release energy is known as respiration . Hence, energy is released.
    So, this is similar to Combustion

  3. Decomposition of vegetable matter involves release of energy.
    So, this is similar to Combustion

  4. In order to carry out the decomposition of ferrous sulphate, we need to supply heat.
    So, this is not similar to Combustion


So, the correct answer is (a) 


3.4 ‘A student while walking on the road observed that a cloud of black smoke belched out from the exhaust stack of moving trucks on the road.’ Choose the correct reason for the production of black smoke: 

a) Limited supply of air leads to incomplete combustion of fuel 

b) Rich supply of air leads to complete combustion of fuel. 

c) Rich supply of air leads to a combination reaction.

d) Limited supply of air leads to complete combustion of fuel. 



In the presence of limited supply of air, there isn’t enough oxygen to carry out complete combustion of fuels. Therefore, incomplete combustion takes place and a black sooty flame is observed

Behind black smoke from car-Teachoo.jpg

3.5 ‘Although nitrogen is the most abundant gas in the atmosphere, it does not take part in combustion’. Identify the correct reason for this statement. 

a) Nitrogen is a reactive gas 

b) Nitrogen is an inert gas 

c) Nitrogen is an explosive gas 

d) Only hydrocarbons can take part in combustion


In nitrogen gas i.e. N 2 , there are three bonds between the nitrogen atoms.

Triple covalent bond between two atoms of nitrogen-Techoo-01.jpg



Therefore, it is v ery difficult to break these.  

Since a chemical reaction occurs by breaking and making of chemical bonds, nitrogen is relatively a very inert gas and therefore, doesn’t undergo combustion. 


So, the correct answer is (b)

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Maninder Singh

CA Maninder Singh is a Chartered Accountant for the past 13 years and a teacher from the past 17 years. He teaches Science, Economics, Accounting and English at Teachoo