Free Alkanes MCQs with Answers

1. Alkanes are relatively unreactive towards most reagents because

• A. their carbon to hydrogen bonds are strong and almost non polar, offering nothing for a nucleophile or electrophile to attack
• B. they contain a double bond
• C. they are ionic compounds
• D. they have very high molar masses

Explanation: Carbon and hydrogen have similar electronegativities, so the bonds carry almost no dipole and there is no region of high or low electron density to attract a reagent. This is why alkanes are called paraffins, meaning little affinity, and why their main reactions are combustion and free radical substitution, both of which need harsh conditions. Alkenes react far more readily because the pi electrons are exposed.

2. The hybridisation of each carbon atom in an alkane such as ethane is

• A. sp
• B. sp2
• C. sp3
• D. unhybridised p

Explanation: Four sigma bonds require four equivalent orbitals, so one s and three p orbitals mix to give four sp3 hybrids pointing to the corners of a tetrahedron with bond angles of 109.5 degrees. Alkenes use sp2 with 120 degree angles and alkynes sp with a linear 180 degree arrangement. Counting the sigma bonds and lone pairs on an atom is the quickest way to assign hybridisation.

3. The general formula of the alkane homologous series is

• A. CnH2n
• B. CnH2n-2
• C. CnH2n+2
• D. CnH2n+1

Explanation: Every carbon in a saturated open chain carries the maximum hydrogen, giving two per carbon plus two extra for the ends of the chain, so hexane is C6H14. CnH2n belongs to alkenes and cycloalkanes and CnH2n-2 to alkynes. CnH2n+1 is not a molecule at all but an alkyl group, such as methyl or ethyl.

4. The boiling points of the alkanes increase with increasing chain length because

• A. the covalent bonds within the molecules become stronger
• B. the molecules become more polar
• C. hydrogen bonding develops between longer chains
• D. the larger surface area increases the dispersion forces between molecules

Explanation: Longer chains touch each other over a greater area, so the temporary induced dipole attractions between molecules add up to more, and more energy is needed to separate them. This is why methane is a gas, hexane a liquid and long chain alkanes waxy solids. Branching lowers the boiling point of an isomer, because a compact molecule presents less contact area.

5. Cracking of long chain alkanes in the petroleum industry is carried out mainly to

• A. produce shorter, more useful alkanes and alkenes for fuel and plastics
• B. remove sulphur from crude oil
• C. convert alkenes into alkanes
• D. increase the molar mass of the products

Explanation: Fractional distillation yields more of the heavy fractions than the market wants and too little petrol, so cracking breaks the long chains into shorter alkanes plus alkenes such as ethene, which are the feedstock for polymers. Catalytic cracking uses zeolites at about 500 degrees Celsius. Removing sulphur is a separate process called desulphurisation.