Question

question 9 (1 point)
why does the boiling point of an alkane increase as its chain length increases?
○ a) there is more hydrogen bonding.
○ b) there are hydrogen atoms available for hydrogen bonds to form.
○ c) the molecules are highly branched.
○ d) the strength of the dispersion forces increases with increased molecular size.
○ e) none of the above

Explanation:

Alkanes are non - polar molecules. The only intermolecular forces acting between alkane molecules are London dispersion forces. London dispersion forces are weak intermolecular forces that arise due to the temporary dipoles formed in molecules. As the chain length (molecular size) of an alkane increases, the number of electrons in the molecule increases. A larger number of electrons means that it is easier to form temporary dipoles, and the strength of the London dispersion forces increases.

• Option a: Alkanes do not have hydrogen bonding. Hydrogen bonding occurs when a hydrogen atom is bonded to a highly electronegative atom (such as N, O, or F). In alkanes (C - H bonds), the electronegativity difference between C and H is not large enough for hydrogen bonding.
• Option b: Similar to option a, alkanes do not have the necessary conditions (highly electronegative atoms like N, O, F) for hydrogen bonds to form, even though they have hydrogen atoms.
• Option c: Highly branched molecules have a lower boiling point compared to their straight - chain isomers. This is because branched molecules have a more compact shape, which reduces the surface area available for intermolecular interactions (London dispersion forces).
• Option d: As explained above, as the molecular size (chain length) of alkanes increases, the strength of London dispersion forces (dispersion forces) increases. Since boiling point is related to the strength of intermolecular forces (higher intermolecular forces require more energy to convert the liquid to gas), the boiling point of alkanes increases with increasing chain length.

Answer:

d) The strength of the dispersion forces increases with increased molecular size.