I read on the textbook a sentence like this:
"The melting and boiling points of alkanes increase as their molecular sizes increase. This is because as alkane molecules become bigger, the attractive forces between the alkane molecules become stronger."
Please tell me if my understanding of that is wrong!
So, are they trying to say that as the molecular size of alkanes increase, the molecular mass also increases. When the size of alkane molecules become bigger, the attractive forces, or the intermolecular forces (van der Waals force) between alkane molecules increase, and hence a greater amount of thermal energy is needed to overcome this stronger van der waals forces. Hence an increased melting and boiling point. Is my understanding about the intermolecular force parts correct?
Please help, thanks!
Your understanding of the material is correct!
As the molecular size of alkanes increases, their molecular mass also increases. This means that the atoms in the molecule are farther apart from each other, resulting in a larger volume. The increased volume leads to a larger surface area for the intermolecular forces, such as van der Waals forces, to act upon.
Van der Waals forces are weak attractive forces between molecules that arise from temporary fluctuations in electron distribution. These forces become stronger as the surface area for interaction increases. In the case of alkanes, as the size of the molecules increases, there is a greater number of electrons available for temporary shifts, causing the van der Waals forces to become stronger.
The increased strength of van der Waals forces between larger alkane molecules requires more thermal energy to break these intermolecular attractions. As a result, a higher amount of energy is needed to overcome these forces, leading to higher melting and boiling points.
Overall, your understanding of the relationship between molecular size, intermolecular forces, and melting and boiling points of alkanes is accurate. Well done!