Question

Dispersion forces are the only intermolecular forces present in motor oil, yet it has a high boiling point. Explain.

Short answer

Motor oil has a high boiling point due to the cumulative effect of dispersion forces acting on its large hydrocarbon molecules.

Step-by-step solution

- Understanding Dispersion Forces

Dispersion forces, also known as London dispersion forces, are weak intermolecular forces caused by temporary fluctuations in the electron distribution within molecules. These forces are present in all molecules but are the only type of intermolecular force in nonpolar substances like motor oil.

- Nature of Motor Oil Molecules

Motor oil is composed predominantly of long chain hydrocarbons. These molecules are large and nonpolar, meaning they rely entirely on dispersion forces for intermolecular attraction.

- Impact of Molecular Size

Although dispersion forces are weak, their strength increases with the size of the molecules. The long hydrocarbon chains in motor oil result in significant surface area where these forces can act, thereby aggregating a stronger overall intermolecular attraction.

- Boiling Point Correlation

A high boiling point indicates that a greater amount of energy is required to overcome the intermolecular forces holding the molecules together in the liquid phase. In motor oil, the cumulative effect of many weak dispersion forces across the large surface areas of long hydrocarbon chains leads to a high boiling point.

Conclusion

Even though dispersion forces are individually weak, the large number of interactions in long chain hydrocarbons creates significant intermolecular attraction, resulting in a high boiling point for motor oil.

Key concepts

Dispersion Forces

Dispersion forces, also known as London dispersion forces, are weak intermolecular forces that arise due to temporary fluctuations in the electron distribution within a molecule. These fluctuations create small, temporary dipoles that attract adjacent molecules.

• They are present in all molecules, regardless of whether the molecules are polar or nonpolar.
• In nonpolar substances, dispersion forces are the only type of intermolecular force.
• The strength of dispersion forces increases with molecular size and surface area.

These forces are essential for understanding why and how nonpolar molecules interact. Although each individual dispersion force is weak, collectively they can create a significant overall attraction.

Nonpolar Molecules

Nonpolar molecules do not have a permanent dipole moment, meaning they do not possess an uneven distribution of electric charge. Their electron distribution is symmetrical.
Here are some core properties:

• They do not dissolve well in polar solvents like water but dissolve in nonpolar solvents.
• Common examples include oxygen, nitrogen, and the hydrocarbons found in motor oil.
• Since nonpolar molecules only have dispersion forces, these forces are critical for understanding their physical properties.

Motor oil consists predominantly of long chain hydrocarbons, which are nonpolar molecules. Despite relying solely on dispersion forces, the large surface areas of these long chains enable significant intermolecular attractions.

Boiling Point

The boiling point of a substance is the temperature at which its liquid phase transforms into a gas. This characteristic is directly related to the strength of the intermolecular forces present in the substance.

• Strong intermolecular forces mean a higher boiling point since more energy is required to separate the molecules.
• In the case of motor oil, although individual dispersion forces are weak, the sheer number of these interactions across the large surface areas of its long hydrocarbon chains results in a high boiling point.

So, even though motor oil relies solely on dispersion forces, these combined effects demand a lot of energy to overcome, giving it a high boiling point.