This set of Class 11 Chemistry Chapter 13 Multiple Choice Questions & Answers (MCQs) focuses on “Hydrocarbons – Alkynes”.
1. What is a general formula of alkynes?
Explanation: Alkynes are unsaturated hydrocarbons with the general formula of CnH2n-2. Few examples are ethyne (put n = 2 in the general formula) C2H2, propyne (put n = 3 in the general formula) C3H4, butyne (put n = 4 in the general formula) C4H6 etc.
2. Are alkynes more reactive than alkenes?
c) Cannot say
Explanation: The alkynes exhibit electrophilic addition reaction, they are less reactive when compared with alkenes and this is due to the dissociation of π-electron cloud requires more energy comparatively. So we can say that they are not more reactive when compared to alkenes.
3. Alkynes can be prepared from calcium carbide.
Explanation: Calcium carbonate is heated and results in the formation of calcium oxide and carbon dioxide and then calcium oxide combines with carbon in order to form calcium carbide and carbon monoxide, when calcium carbide combines with water it forms calcium hydroxide as well as an alkyne that is Ethyne C2H2.
4. All alkynes are odourless gases.
Explanation: The first two members are gases, the next eight members are liquids and higher members are solids. Alkynes are colourless and odourless with the exception of acetylene, which has slightly garlic or due to the presence of phosphine gas and hydrogen sulphide gas.
5. Alkynes are ________________ in water and the melting point ________________ with increase in molar mass.
a) soluble, decrease
b) insoluble, increase
c) insoluble, decrease
d) soluble, increase
Explanation: Alkynes are insoluble in water but soluble in organic solvents like ethers, carbon tetrachloride and Benzene. The melting point, boiling point and density of alkynes increase with the increase in the molar mass.
6. Alkynes show ________________ reactions.
a) neither electrophilic nor nucleophilic addition
b) nucleophilic addition only
c) electrophilic only
d) both electrophilic and nucleophilic addition
Explanation: Alkynes show electrophilic as well as nucleophilic addition reactions. Electrophilic addition reactions follow the markownikoff rule, various examples for these reactions are the addition of dihydrogen, halogens, water and hydrogen halides.
7. Do alkynes react with arsenic trichloride?
c) Cannot say
Explanation: Yes, when ethyne reacts with arsenic trichloride in presence of anhydrous aluminium trichloride, it forms 2 chloro vinyl arsonous dichloride, it is also known as Lewisite, which is a poisonous gas. So we can say that alkynes do react with arsenic trichloride.
8. Ethyne on oxidation with alcoholic KMnO4 forms ________________
a) hydrochloric acid
b) oxalic acid
c) acetic acid
d) formic acid
Explanation: When one mole of ethane reacts with alcoholic KMnO4 1 mole of oxalic acid is formed, when it reacts with acidic KMnO4 two moles of formic acid are formed and when it reacts with K2Cr2O7 one mole of Acetic acid is formed.
9. At which temperature, does cyclic polymerization of an alkyne occur?
a) 871 Kelvin
b) 872 Kelvin
c) 873 Kelvin
d) 874 Kelvin
Explanation: An example of cyclic polymerization of an alkyne is given by; when 3 moles of ethane is at 873 Kelvin in presence of red hot iron tube they polymerise in a cyclic form in order to form Benzene a six-membered ring with alternate double bonds.
10. When ethyne is subjected to ozonolysis of, what is the end product?
a) formic acid
b) acetic acid
c) oxalic acid
Explanation: When a mole of ethane is subjected to ozonolysis, it reacts with a mole of ozone in presence of carbon tetrachloride and zinc or water, in order to give one mole glyoxal and then it forms two moles formic acid.
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