Case Study:

Rohit noticed that his motorcycle was suddenly giving poor mileage and releasing thick black smoke from the exhaust while climbing a flyover in heavy traffic. Even though the petrol tank was full, the bike engine was making unusual sounds and the pickup had reduced. When the mechanic checked the bike, he found that the air filter was clogged with dust. Because of this, enough air was not entering the engine chamber for proper burning of petrol.

Inside a bike engine, petrol reacts with oxygen from air to produce carbon dioxide, water vapour, and energy. This process is an oxidation reaction because oxygen combines with fuel during combustion. It is also an example of a combination reaction, as substances combine to form new products.

If you want to see how other processes like digestion or photosynthesis fit into these categories, check out our guide on real-life examples of chemical reactions to broaden your understanding

For complete combustion, a balanced amount of oxygen is necessary. When airflow becomes insufficient, petrol burns incompletely, producing carbon particles (soot) and carbon monoxide along with less energy.

Under normal conditions, the simplified balanced chemical equation for complete combustion of petrol (octane) is:

2C₈H₁₈ + 25O₂ → 16CO₂ + 18H₂O + Energy

Balancing complex equations like the one below can be tricky for many students. Before you dive in, make sure you aren't making these 5 common balancing mistakes that often lead to incorrect answers.

However, when oxygen supply decreases, incomplete combustion occurs, leading to black smoke and reduced engine efficiency. During testing, the mechanic observed that the bike’s mileage dropped from 55 km/L to 38 km/L after the air filter blockage. The engine temperature also increased unusually during long rides.

This situation shows how balanced chemical reactions directly affect fuel efficiency, pollution, and engine performance in everyday life.

Case Based Questions

 MCQ 

Q1. The black smoke released from the bike engine mainly indicates:
A. Complete combustion of petrol
B. Presence of excess oxygen
C. Incomplete combustion due to insufficient air
D. Cooling of the engine

Q2. Which type of reaction mainly takes place when petrol burns properly inside the engine?
A. Displacement reaction
B. Oxidation reaction
C. Decomposition reaction
D. Neutralisation reaction

 Assertion–Reason 

Q3. Assertion (A): A blocked air filter can reduce the efficiency of a bike engine.
Reason (R): Proper combustion of petrol requires sufficient oxygen supply.
A. Both A and R are true, and R is the correct explanation of A.
B. Both A and R are true, but R is not the correct explanation of A.
C. A is true, but R is false.
D. A is false, but R is true.

 Application-Based 

Q4. The mechanic cleaned the bike’s air filter, after which the mileage improved significantly. Explain scientifically why cleaning the filter increased fuel efficiency.

Q5. A student says that petrol is still entering the engine, so energy production should remain the same even if the airflow decreases. Is the statement correct? Give a reason based on chemical reactions.

 Data/Logic-Based 

Q6. A bike covered 550 km using 10 litres of petrol before the air filter blockage. After blockage, it covered only 380 km using the same amount of fuel.
Calculate: 1. Initial mileage
2. Final mileage
3. Percentage decrease in mileage

 Application + Reasoning 

Q7. Why does incomplete combustion inside engines increase air pollution more than complete combustion?
A. It produces only water vapour
B. It releases unburnt carbon particles and harmful gases
C. Oxygen completely disappears from air
D. Petrol changes into solid fuel

ANSWER KEY WITH EXPLANATION

A1. C. Incomplete combustion due to insufficient air
Explanation: Black smoke contains soot particles formed when petrol burns without enough oxygen. Incomplete combustion releases less energy and produces pollutants.

A2. B. Oxidation reaction
Explanation: During combustion, oxygen combines with petrol to form new substances like carbon dioxide and water. This matches the NCERT definition of oxidation.

Oxidation isn't just about burning fuel; it's also why metal decays over time. You can learn more about this by exploring why iron rusts and how it affects the world around us.

A3. A. Both A and R are true, and R is the correct explanation of A.
Explanation: A blocked air filter reduces oxygen supply. Without sufficient oxygen, petrol cannot burn completely, lowering engine efficiency.

A4. Cleaning the filter allowed more oxygen to enter the engine chamber. This improved complete combustion of petrol, producing more energy and reducing fuel wastage.
Explanation: Balanced combustion reactions require proper reactant proportions for maximum efficiency.

A5. No, the statement is incorrect.
Explanation: Petrol alone cannot release full energy without sufficient oxygen. Incomplete combustion produces less energy and harmful substances like carbon monoxide and soot.

A6. 1. Initial mileage = 550 ÷ 10 = 55 km/L
2. Final mileage = 380 ÷ 10 = 38 km/L
3. Percentage decrease = ((55 - 38) ÷ 55) × 100 = 30.9% (approx.)
Explanation: Reduced oxygen supply caused incomplete combustion, lowering energy output and mileage.

A7. B. It releases unburnt carbon particles and harmful gases
Explanation: Incomplete combustion forms soot and carbon monoxide, both of which increase pollution and harm health.

HOTS EXTENSION QUESTIONS

Q1. Suppose a bike engine receives excess oxygen compared to petrol. Predict how combustion efficiency and pollution levels may change. Explain scientifically.

Q2. Modern vehicles use sensors to maintain the correct air-fuel ratio automatically. How can this technology help in reducing fuel wastage and environmental pollution?

Think you've mastered the science of combustion? Put your knowledge to the test with our interactive Chemistry quizzes!

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