PARAMOUNT SCHOOL SYSTEM
Subject: Chemistry – I
Unit 14: Hydrocarbons
CONCEPT ASSESSMENT EXERCISE 14.1
Complete the following reactions.
CONCEPT ASSESSMENT EXERCISE 14.2
Complete the following reactions.
a. 
b. 
REVIEW QUESTIONS
1. Encircle the correct answer.
- Which molecule contains a carbon-carbon single bond?
(a) Ethane (b) Ethene (c) Ethyne (d) Methanol
- Which product is obtained when chloromethane (or methyl chloride) is reduced?
(a) Ethane (b) Ethene (c) Methane (d) Ethyne
- By hydrogenation we mean, the addition of
(a) Hydrogen (b) Water (c) Halogen (d) Hydrogen halide
- Combustion of methane produces
(a) Carbon dioxide (b) Water (c) Heat (d) All of these
- Reduction of chloromethane gives.
(a) Hydrogen (b) Chlorine (c) Methane (d) All of these
2. Give short answer.
- Give three examples of saturated hydrocarbons.
Hydrocarbons containing carbon-carbon multiple bonds are called unsaturated.
| Ethene | 1-Pentene | 1-Butyne |
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- Draw structure for ethane.
Ethane consists of two carbon atoms (C) connected by a single bond.
- Draw structural formulas of an alkane containing five carbon atoms.
| Alkane (Pentane) | Alkene (1 – Pentene) | Alkyne (2 – Pentyne) |
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 |
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- What do you mean by dehydration reaction? Give one example
Dehydration means a chemical reaction that involves the loss of a water molecule from the reacting molecule.Example:
Alcohols dehydrate when their vapours are passed over heated alumina.
- What is meant by cracking?
A large hydrocarbon (alkane) molecule breaks into smaller hydrocarbons when heated at high temperatures such as 450 – 7500C and high pressure. This process is called thermal cracking. This process produces a mixture of alkanes and alkenes. For example, when decane is heated at high temperature and high pressure, it breaks down into octane and ethene.
3. Discuss methods for the preparation ethane.
Methods for the preparation of ethane:
By the Hydrogenation of Alkenes and Alkynes:
Ethane can be prepared by hydrogenating alkenes or alkynes. In the presence of finely divided nickel at 200-300°C and high pressure, or in the presence of platinum (Pt) or palladium (Pd) at room temperature, the hydrogen molecules add across the carbon-carbon multiple bonds of alkenes or alkynes, producing alkanes.
By the Reduction of Alkyl Halides:
When an alkyl halide is treated with Zn in presence of an aqueous acid, an alkane is produced. Usually aqueous solution of HCL or CH, COOH is used.
Zn reacts with aqueous acid to liberate atomic hydrogen called nascent hydrogen. Nascent hydrogen reduces alkyl halide. Addition of nascent hydrogen is called reduction.
By the Cracking of Larger Hydrocarbons:
A large hydrocarbon (alkane) molecule breaks into smaller hydrocarbons when heated at high temperatures such as 450 – 7500C and high pressure. This process is called thermal cracking. This process produces a mixture of alkanes and alkenes. For example, when decane is heated at high temperature and high pressure, it breaks down into octane and ethene.
4. Describe properties of alkanes.
Alkane molecules are essentially non-polar. They are less dense than water and do not dissolve in it. Chemically, alkanes do not react with most ionic compounds. The lack of reactivity makes them useful solvents. For example, hexane is used to extract vegetable oils from corn, soybeans, cotton seeds, etc. Alkanes containing up to four carbon atoms are colourless and odourless gases. Alkanes, containing five to seventeen atoms, are colourless and odourless liquids. Higher alkanes are solids that are also colourless and odourless.
5. Write a chemical equation to show the preparation of an alkane from an alkene and an alkyne.
Ethane can be prepared by hydrogenating alkenes or alkynes. In the presence of finely divided nickel at 200-300°C and high pressure, or in the presence of platinum (Pt) or palladium (Pd) at room temperature, the hydrogen molecules add across the carbon-carbon multiple bonds of alkenes or alkynes, producing alkanes.
THINK TANK
6. Write chemical equations for the preparation of propane.
