Question

a. What trend occurs in the boiling points of alkanes? b. How would you explain this trend? c. How is the trend in alkane boiling points used in petroleum fractional distillation?

Short answer

a. Boiling points of alkanes increase with molecular weight. b. Due to stronger van der Waals forces. c. Used in distillation to separate hydrocarbons by their boiling points.

Step-by-step solution

- Identify the Trend in Boiling Points of Alkanes

Observe the physical properties of alkanes, particularly their boiling points. As the molecular weight (or the length of the carbon chain) of alkanes increases, the boiling points also increase. This is due to the greater van der Waals forces in larger molecules.

- Explain the Trend

When the length of the carbon chain in alkanes increases, the surface area of the molecules increases. This results in stronger van der Waals forces (London dispersion forces) which require more energy (heat) to break, thus leading to higher boiling points.

- Relate the Trend to Petroleum Fractional Distillation

Petroleum fractional distillation takes advantage of the trend in boiling points of alkanes. During distillation, crude oil is heated, and its components evaporate at different temperatures based on their boiling points. Smaller alkanes evaporate at lower temperatures, while larger alkanes need higher temperatures to evaporate, allowing the separation of different hydrocarbons.

Key concepts

Molecular Weight

Molecular weight or molecular mass refers to the sum of the atomic masses of all the atoms in a molecule. In the case of alkanes, this includes the total number of carbon and hydrogen atoms.
As the molecular weight of alkanes increases, so does the length of their carbon chains.
This increase in carbon chain length directly impacts the boiling points of alkanes.
Here's why: As the chain length grows, larger molecules have a greater surface area, leading to stronger van der Waals forces.
Simply put, these are the forces of attraction between molecules.
They become more significant as the size and shape of the molecules increase, resulting in higher boiling points for larger alkanes.

Van der Waals Forces

Van der Waals forces are a type of weak intermolecular force that includes London dispersion forces.
These forces come into play due to the temporary dipoles that form when electrons move around within a molecule.
When discussing alkanes, van der Waals forces become more effective as the size of the molecule increases.
Larger molecules have more surface area, allowing these weak forces to accumulate and create a stronger overall attraction.
This means that more energy (in the form of heat) is required to separate the molecules, leading to higher boiling points.
In summary, the molecular size and shape significantly influence these forces, contributing to the boiling point trend observed in alkanes.

Petroleum Fractional Distillation

Petroleum fractional distillation is an essential process in separating the different components of crude oil.
This process relies on the different boiling points of various hydrocarbons present in the crude oil.
During fractional distillation, crude oil is heated in a distillation column.
As the temperature rises, different hydrocarbons evaporate at different temperatures based on their molecular sizes (and hence their boiling points).
Smaller alkanes with lower boiling points evaporate first and are collected at lower levels of the column.
Larger alkanes, needing higher temperatures to vaporize, evaporate later and are collected at higher sections.
This process effectively separates crude oil into useful fractions such as gasoline, kerosene, and diesel.
Understanding the boiling point trends of alkanes is essential for optimizing this separation technique.