The boiling points of the alkanes increase with increasing chain length because
Correct answer: D. the larger surface area increases the dispersion forces between molecules
- 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.
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About Alkanes
Alkanes are saturated hydrocarbons containing only carbon-carbon and carbon-hydrogen single bonds, with the general formula CnH2n+2 for open chains. Their structure, isomerism, IUPAC nomenclature, physical properties, combustion, free-radical halogenation and conformations are covered, including the difference between substitution in alkanes and addition in unsaturated hydrocarbons.
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