Question

Is it possible for the dispersion forces in a particular substance to be stronger than the hydrogen bonding forces in another substance? Explain your answer.

Short answer

Yes, it is possible for the dispersion forces in a particular substance to be stronger than the hydrogen bonding forces in another substance. Although hydrogen bonding forces are generally stronger than dispersion forces, the strength of dispersion forces increases with the size and molecular mass of the molecule. If a large molecule with only dispersion forces is compared to a smaller molecule with hydrogen bonding forces, the larger molecule may exhibit stronger dispersion forces. An example of this is comparing octane (C8H18) and water (H2O); the dispersion forces between octane molecules can be stronger than the hydrogen bonding forces between water molecules, as evidenced by their boiling points (octane: 126°C, water: 100°C).

Step-by-step solution

1. Understanding Dispersion Forces

Dispersion forces, also known as London dispersion forces or van der Waals forces, are weak intermolecular forces that occur between all molecules. These forces arise due to the instantaneous polarization of the electron cloud in a molecule, resulting in the formation of temporary dipoles that can attract each other. Dispersion forces are generally weaker than other intermolecular forces like hydrogen bonding or dipole-dipole interactions. However, their strength increases with the size and molecular mass of the molecule, as there are more electrons that can contribute to the temporary dipoles.

2. Understanding Hydrogen Bonding Forces

Hydrogen bonding is a strong intermolecular force that occurs between molecules containing strongly electronegative atoms (like nitrogen, oxygen, or fluorine) bonded to hydrogen atoms. These atoms are capable of attracting the bonding electrons from the hydrogen atom, creating an uneven distribution of electron density. This leads to the formation of partial positive and negative charges, enabling the formation of hydrogen bonds between the charged regions of adjacent molecules.

3. Comparing the Strengths of Dispersion Forces and Hydrogen Bonding Forces

Dispersion forces are generally weaker than hydrogen bonding forces. However, this does not mean that dispersion forces cannot be stronger than hydrogen bonding forces in certain cases. As mentioned earlier, the strength of dispersion forces increases with the size and molecular mass of the molecule. If we compare a small or medium-sized molecule with hydrogen bonding forces to a much larger molecule with only dispersion forces, it is possible for the larger molecule to exhibit stronger dispersion forces than the hydrogen bonding forces in the smaller molecule.

4. Providing an Example

One example illustrating this possibility is comparing the intermolecular forces between water (H2O) and octane (C8H18). Water molecules exhibit strong hydrogen bonding forces due to the presence of oxygen bonded to hydrogen atoms. On the other hand, octane is a hydrocarbon that consists only of carbon and hydrogen atoms, and its intermolecular forces are entirely due to dispersion forces. However, octane is a much larger molecule compared to water, with a greater number of electrons contributing to the dispersion forces. In this case, the dispersion forces between octane molecules can be stronger than the hydrogen bonding forces between water molecules, despite hydrogen bonding being generally stronger than dispersion forces. This can be observed in the difference in boiling points: water has a boiling point of 100°C, while octane has a higher boiling point of 126°C. This is an indication that the intermolecular forces in octane are stronger than those in water, highlighting that it is indeed possible for dispersion forces in one substance to be stronger than hydrogen bonding forces in another substance.