Chemistry Multiple Choice Questions on “Hydrocarbons – Alkanes”.
1. Alkanes are also known as __________
a) alkenes
b) paraffin
c) aromatic
d) alicyclic
Answer: b
Clarification: Alkanes are saturated aliphatic open chain hydrocarbons with carbon-carbon single bonds. They are inert under normal conditions they do not react with acids, bases and other reagents. They were earlier known as paraffin, in Latin Param = little and affine means affinity.
2. Is of hydrogenation is __________ on steric crowding.
a) may be related to
b) dependent
c) independent
d) not related to
Answer: b
Clarification: Ease of hydrogenation depends on the steric crowding across multiple Bond, more the steric crowding, the less is reactivity towards hydrogenation. This concept is used in one of the methods of preparation of alkanes from the hydrogenation of alkenes and alkynes.
3. How many carbons are there in the product of a decarboxylation reaction when compared with the reactant?
a) two carbons more
b) one carbon more
c) one carbon less
d) an equal number of carbons
Answer: c
Clarification: Decarboxylation of sodium or potassium salt of fatty acids is decarboxylation reaction. This reaction is used for descending of series as the alkane obtained has one carbon less than the parent compound. Here quicklime is used as it is more hygroscopic than sodium hydroxide and keeps Sodium Hydroxide in a dry state.
4. Which of the following reaction is used to increase the length of the carbon chain?
a) Wolff Kishnn’s reaction
b) Clemmensen reduction
c) Kolbe’s electrolysis
d) Wurtz reaction
Answer: d
Clarification: Wurtz reaction is used to increase the length of the carbon chain, Kolbe’s electrolysis is used when alkanes require even number of carbon atoms while clemmensen reduction and wolff-kishner are used for removing water molecule.
5. Corey-House synthesis is used for alkanes having __________ number of carbon atoms.
a) 6
b) 3
c) 2
d) 4
Answer: b
Clarification: Corey-House synthesis is one of the methods of preparation of alkanes and this method can be used to prepare alkanes having an odd number of carbon atoms. As 6, 2, and 4 are even numbers only the compound with three carbon atoms can be prepared.
6. Alkynes are __________ in nature and first four members are __________ gases.
a) polar, white
b) nonpolar, colourless
c) polar, colourless
d) nonpolar, white
Answer: b
Clarification: Alkanes being nonpolar in nature, soluble in nonpolar solvents but insoluble and polar solvent such as water. The first four members of alkanes are colourless gases, the next 13 members are colourless liquids and next higher members are colourless solids, this can be explained on the basis of the magnitude of attraction forces.
7. Which of the following is not a process of halogenation of alkanes?
a) acylation
b) chlorination
c) bromination
d) iodination
Answer: a
Clarification: Chlorination, bromination and iodination are the processes of halogenation of alkanes. Mechanism of halogenation of alkanes is free radical in nature that is the attacking reagent is a halogen-free radical, therefore it is a chain reaction.
8. In the combustion reaction of alkanes if Ethane is used how many moles of oxygen are required?
a) 3
b) 4
c) 7
d) 3.5
Answer: d
Clarification: The combustion reaction of alkanes has a standard reaction that is CnH2n+2 + (3n/2 + 1/2)O2 → nCO2 + (n + 1)H2O. In the case of combustion of ethane, n = 2. That means the number of moles of oxygen required is 3(2)/2 + 1/2 = 3.5
9. Methane cannot be prepared by the reduction of alkenes or alkynes.
a) true
b) false
Answer: a
Clarification: Methane cannot be prepared by reduction of alkenes or alkynes because alkanes or alkynes require a minimum of two carbon atoms in order to form, but Methane has only a single carbon in it Methane cannot be prepared by Kolbe’s electrolysis and woods reaction also.
10. Which of the following is true regarding the boiling point?
a) cannot say
b) n-Octane is greater than isooctane
c) n-Octane is less than isooctane
d) n-Octane is equal to isooctane
Answer: b
Clarification: The boiling point of alkanes decreases on branching and boiling point is directly proportional to the van der walls forces and the van der wall forces are directly proportional to the molecular mass as well as surface area. So we can say that the boiling point of n-Octane is greater than that of isooctane.