Question

The molecules of alkene family are represented by a general formula CnH2n. Now answer the following :(a) What do 'n' and '2n' signify?(b) What is the molecular formula of alkene when n = 6?(c) What is the molecular formula of the alkene if there are six H atoms in it?(d) What is the molecular formula and structural formula of the first member of the alkene family?(e) Write the molecular formulae of lower and higher homologues of an alkene which contains four carbon atoms.Give two differences between soap and synthetic detergents? Give the name of the by product of soapindustry? How is it formed?

Answer 1

Answer:

The general formula for alkanes is CnH2n+2, where n stands for number of carbon atoms and 2n+2 for number of hydrogen atoms in the molecule.

hope this helps you

Answer 2

Answer:

(a) The 'n' and '2n' represents the number of carbon atoms and hydrogen atoms present in a given alkene. 'n' can have values starting from 0, 1, 2, ...

(b) The molecular formula of alkene when n = 6 is $C_{6}H_{12}$.

• When n = 6, 2n = 2×6 = 12.
• Therefore, there will be 6 carbon atoms and 12 hydrogen atoms.
• The general formula for an alkene is $C_{n} H_{2n}$.
• So, the molecular formula is $C_{6}H_{12}$ and it is named cyclohexane.

(c)The molecular formula of the alkene when there are six H atoms is $C_{3}H_{6}$.

• There are six hydrogen atoms, i.e., 2n = 6 ⇒ n = 3
• So there are 3 carbon atoms and 6 hydrogen atoms.
• The general formula for an alkene is $C_{n} H_{2n}$.
• Therefore, the molecular formula is $C_{3}H_{6}$ and it is named propene.

(d) The molecular formula of the first member of the alkene family is $C_{2}H_{4}$ and its structural formula is $H_{2}C=C H_{2}$.

• The first member of the alkene family, ethene, has 2 carbon atoms, i.e., n = 2. Therefore, the number of hydrogen atoms is 2n = 2×2 = 4.
• The carbon atoms of the alkene group are bonded with other carbon atoms via a double bond. This is why they are also called unsaturated hydrocarbons.
• To complete the valency of carbon in ethene, each carbon atom is attached to two hydrogen atoms and one carbon atom with a double bond.
• Therefore, its molecular formula is $C_{2}H_{4}$ and the structural formula is $H_{2}C=C H_{2}$.

(e)The lower and higher homologues of an alkene containing four carbon atoms are $C_{3}H_{6}$ and $C_{5}H_{10}$ respectively.

• An alkene containing four carbon atoms, i.e., n = 4 will have 2n = 2×4 = 8 hydrogen atoms.
• So, the molecular formula will be $C_{4}H_{8}$. This is a butene molecule.
• A lower homologue of butene will have one carbon atom less, i.e., n = 3 and 2n = 2×3 = 6.
• Therefore, the lower homologue will have 3 carbon atoms and 6 hydrogen atoms. Its molecular formula will be $C_{3}H_{6}$.
• A higher homologue of butene will have one carbon atom more, i.e., n = 5 and 2n = 2×5 = 10.
• Therefore, the higher homologue will have 5 carbon atoms and 10 hydrogen atoms. Its molecular formula will be $C_{5}H_{10}$.

(f) The two differences between soap and synthetic detergents are as follows:

1. Soaps are sodium or potassium salts of fatty acids whereas synthetic detergents are made of fats or kerosene.
2. Soaps can only be used with soft water. Synthetic detergents can be used with soft or hard water.

(g) The by-product of the soap industry is called glycerol.

• In the process of saponification, an ester is made to react with an inorganic base to give alcohol and soap.
• The fat reacts with NaOH or KOH to give sodium or potassium salts of fatty acids, i.e., soap and glycerol as the by-product.
• Glycerol is an alcohol that is used as a solvent, sweetening agent as well as a medicine.

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