ANSWERS TO THE SHORT QUESTIONS.
- No.1. Differentiate between saturated and unsaturated hydrocarbons.
Answer. The compounds in which all the four valences of carbon atoms are fully satisfied (saturated) by single bonds with other carbon and hydrogen atoms are called saturated hydrocarbons. Saturated hydrocarbons are also called alkanes.
The compounds in which two carbon atoms are linked by a multiple bond i.e. double or a triple bond are called unsaturated hydrocarbons. The compounds in which two carbon atoms are linked by a double bond are called alkenes. For example, ethene and propene
While the compounds in which two carbon atoms are linked by a triple bond are called alkynes, For example,
CH≡CH H3C-C ≡ CH
- No.2. A compound consisting of four carbon atoms has a triple bond in it. How many hydrogen atoms are present in it?
Answer. A compound consisting of triple bond is called alkyne. The general formula for alkynes is CnH2n-2. Where ‘n’ is number of carbon atoms. For a compound having 4 carbon atoms number of hydrogen atoms are calculated as:
2(n) ― 2= 2(4) ―2 = 6
Therefore, a compound consisting of four carbon atoms and a triple bond will have six hydrogen atoms.
- No.3. Why the alkanes are called ‘paraffin’s’?
Answer. Alkanes are the least reactive compounds so they are called paraffin’s. Paraffin is derived from Para means less, and affine means affinity or reactivity.
- No.4. What do you know about hydrogenation of alkenes?
Answer. Hydrogenation means addition of hydrogen in alkenes. This reaction is carried out in the presence of nickel catalyst at 250°C to 300°C. However, in the presence of catalyst platinum or palladium, the reaction takes place at room temperature.
h2c=ch2 + h2 Ni H3C ― CH3
- No.5. How alkyl halides are reduced?
Answer. Nascent hydrogen changes alkyl radicals into alkanes. This reaction takes place in the presence of Zn metal and HC1.
CH3Br + 2[H] zn/dil.HCl CH4 + HBr
CH3CH2Br + 2[H] zn/dil.HCl CH3-CH3 + HBr
- No.6. Why the alkanes are used as fuel?
Answer. The main constituents of fuels (coal, petroleum and natural gas) are hydrocarbons. When hydrocarbons are burnt in air CO2 and H2O along with heat are produced. This reaction is called combustion. Alkanes burn in the presence of excess of air or oxygen to produce a lot of heat, carbon dioxide and water. It is highly exothermic reaction, i.e. it produces a lot of heat. The basic combustion reaction is
CH4 + 2O2 ——— > CO2 + 2H2O + heat
The heat energy produced is used for various processes, in homes, transportations, as well as in industries