KPK Board 10th class Chemistry Ch 4 Hydrocarbons short questions answers
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We know that addition reaction is the main property of unsaturated compounds like alkenes and alkynes. In order to test whether alkene undergo addition reactions or not, ist we will react hydroqen with alkene in the presence of a catalyst and then will analyse the products formed, if the products formed are alkanes, it confirms that addition of hydrogen has taken place to alkenes, so alkenes can undergo addition reaction.
We know that addiction is the main property of unsaturated compounds like alkene so we will canduct a chemical test for its unsaturation.
Test for unsaturation:
1) Make a solution of Bromine (red colour).
2) Pass alkene like ethane gas from this red colour solution.
3) If the reddish brown colour disappears, it canfirm the unsaturated nature of alkene and thus alkene will underqo addition reactions like hydrogenation addition of halogen acids, chlorination etc.
Alkenes are more reactive than alkanes.
It is because alkene contain pi( bonds. Which are weaker and easily brocken down. Imole pi( bonds require energy = 63 K calories. Imole sigma (ᴽ) bonds require energy = 83 K. calories. Similarly alkenes are unsaturated compounds & have the capacity to add hydrogen, halogen, and halogen acids so these are more reactive than alkanes.
On the other hand alkanes contain all carbon- carbon and carbon- hydrogen single covalent bonds [sigma bonds], which are shong bonds and require more / high energy to break so these are less reactive than alkenes.
Ans. If we look at the structural formulae of alkenes and alkynes, they contain at least one carbon – carbon double bond and carbon – carbon triple bond respectively. Existance of double and triple bonds (pi( bonds). Show their unsaturated nature. If we look at the chemical reactivity of alkenes and alkynes these can add hydrogen, halogen and halogen acids so these addition justify their unsaturated nature.
Alkanes are also called paraffines.
Paraffin means inert / less reactive. Alkanes are inert / less reactive because;
i) The carbon atoms in alkanes are fully saturated.
ii) All the fours bonds of alkanes carbon are single covalent (sigma) bonds which are strong bonds and require high energy for breaking.
iii) Carbon – hydrogen bonds are very less polar.
iv) Carbon – carbon bonds are non polar.
v) As a whole alkanes are non-popar.
When alkyl halides are reduced by the addition of atomic au nascent hydrogen, these are converted to alkanes.
Ans.When an alkene, let say ethane gas is passed through acified solution disappears. The products formed are ethylene glycol, Manganese dioxide and potassium hydroxide.
OH OH + 2KOH
When ethane gas is passed through Bromine water, the reddish brown colour of Bromine discharge / disappear, it is due to oxidation of ethane. As we know that addition of an electronegative element to a compound is called oxidation. During this reaction ethane is oxidized to 1 , 2 – dibromoethane.
Ans. Alkanes are saturated hydrocarbons so these are less reactive [inert while alkenes are unsaturated hydrocarbons so these are more reactive.
Due to unsaturated nature alkenes undergo addition reactions and due to saturated nature alkanes undergo substitution reactions.
Ans. Addition reactions take place in ethane and ethyne because these are unsaturated and contain weak pi – bonds. On the other hand addition reactions do not take place in ethane because its carbon atoms contain all single covalent bonds and there valences are fully satisfied. There is no capacity for further addition.
Ans. Chemical equations for the preparation of ethane from ethyl alcohol and ethyl chloride are as under: