Which one among the following statements is not correct?
- A.The reaction of methane and chlorine in sunlight produces hydrogen gas.
- B.Palladium catalyzes the addition of hydrogen gas on an alkene.
- C.Ethanol burns in oxygen to give carbon dioxide, water and releases heat and light.
- D.Unsaturated hydrocarbons burn with a sooty flame.
Correct answer
Explanation
The correct answer is A. Methane and chlorine react in sunlight to give chloromethane and hydrogen chloride, not hydrogen gas.
This is a substitution reaction: CH4 + Cl2 → CH3Cl + HCl in the presence of sunlight. One hydrogen atom of methane is replaced by a chlorine atom, and the hydrogen removed joins chlorine as HCl. With more chlorine the reaction goes on to give dichloromethane, chloroform and carbon tetrachloride. Saturated hydrocarbons such as methane are fairly unreactive, and substitution is their typical reaction, while unsaturated ones take part in addition reactions.
B is correct because nickel or palladium catalysts add hydrogen across the double bond of an alkene; this is how vegetable oils are hydrogenated into vanaspati ghee. C is correct because ethanol burns completely in oxygen, giving CO2 and water with heat and light. D is correct because unsaturated hydrocarbons have a higher share of carbon and burn incompletely in air, giving a yellow, sooty flame.
Exam tip: saturated hydrocarbons undergo substitution, unsaturated ones undergo addition.
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