Reactions of Alkanes Halogenation Combustion - UNSOLVED PRACTICE SET

Q7. Write the mechanism of chlorination of methane showing the initiation, propagation, and termination steps.

Q8. Why is fluorination of alkanes more vigorous and difficult to control than chlorination or bromination?

Q9. Explain why the combustion of alkanes is exothermic. Write the balanced equation for the complete combustion of propane.

Q10. What is pyrolysis or cracking of alkanes? Why is this process industrially important?

Q11. Your school chemistry lab demonstrates the chlorination of methane. Why is the reaction mixture kept away from direct sunlight until the experiment begins? What would happen if exposed to bright light prematurely?

Q12. Why does isobutane (2-methylpropane) give a higher yield of tert-butyl chloride than n-butane gives of sec-butyl chloride during chlorination?

Q13. Discuss the free radical mechanism of halogenation of alkanes:

(a) Initiation step — homolytic fission of halogen molecule

(b) Propagation steps — abstraction of hydrogen and formation of alkyl halide

(c) Termination steps — combination of radicals

(d) Why this is a chain reaction

Write the complete mechanism for chlorination of ethane.

Q14. Explain the factors affecting the reactivity and selectivity in halogenation of alkanes:

(a) Bond dissociation energies of C–H bonds (1°, 2°, 3°)

(b) Stability of free radicals formed (3° > 2° > 1°)

(c) Reactivity of halogens (F₂ > Cl₂ > Br₂ > I₂)

(d) Selectivity of bromination vs. chlorination

Illustrate with the halogenation of propane and 2-methylpropane.

Q15. The reactions of alkanes have profound impacts on energy and environment in India. Discuss:

(a) Why LPG (mainly propane and butane) is the preferred cooking fuel in Indian households and how complete combustion is ensured

(b) The problem of incomplete combustion of fuels in Delhi's winters and formation of carbon monoxide

(b) How catalytic cracking of heavy alkanes in Indian refineries produces petrol and diesel

(d) The environmental impact of methane emissions from paddy fields in India and its contribution to global warming

Q16. The chlorination of 2-methylbutane can give four monochlorinated products.

(a) Identify all four possible products with their IUPAC names.

(b) Using the relative reactivity ratios (3° H : 2° H : 1° H = 5 : 4 : 1), calculate the percentage yield of each product.

(c) Identify the major product and explain why it is formed in the highest yield.

Q17. The heat of combustion values for some alkanes are:

Methane: -890 kJ/mol

Ethane: -1560 kJ/mol

Propane: -2220 kJ/mol

Butane: -2877 kJ/mol

(a) Calculate the heat of combustion per CH₂ unit for each alkane.

(b) Determine if the heat of combustion increases linearly with the number of carbon atoms.

(c) Calculate the mass of CO₂ produced when 1 kg of butane is completely combusted.

[Given: Atomic masses: C = 12, O = 16, H = 1]

Q18. A city bus in Bangalore runs on CNG (compressed natural gas, mainly methane). The bus has a 100 kg CNG tank.

(a) Write the balanced equation for complete combustion of methane.

(b) Calculate the total energy released when all the methane is combusted.

(c) If the bus consumes 20 kg of CNG per 100 km, calculate how far the bus can travel on a full tank and compare this with a diesel bus that gives 5 km per litre of diesel (density of diesel = 0.85 kg/L, calorific value = 45 kJ/g).

[Given: Calorific value of methane = 55 kJ/g]