Intermolecular Forces

Question 1

How does the size of the electron cloud and the molar mass of a molecule affect the polarizability of the molecule?

Answer 1

The larger the molecule, it more likely it is to have London dispersion forces. This is because the larger electron cloud makes it more likely to get temporary imbalances in charge. As such, more “positive” region of the electron could will be attracted to region on another molecule with a high density of charge in the electron cloud at that moment.

Question 2

What is a London dispersion force?

Answer 2

London dispersion forces result from coulombic interactions between instantaneous dipoles. These forces are present between all molecules and atoms and are typically greater for heavier, more polarizable molecules with larger surface area.

Question 3

How does the shape of a molecule affect its ability to form London dispersion forces?

Answer 3

Molecules with a larger surface area can interact more with neighbors, which increases the possibility of forming instantaneous dipoles. For example, n-butane has a higher boiling point than iso-butane due to its elongated shape.

Question 4

Reaching the boiling point can be thought of as overcoming ______ between molecules.

Answer 4

Intermolecular forces

Question 5

Are London dispersion forces permanent or temporary?

Answer 5

Temporary

Question 6

What are dipole-dipole forces between molecules?

Answer 6

Dipole-dipole forces are permanent attractive forces between the positive end of one polar molecule and the negative end of another polar molecule.

Question 7

What direction is the dipole force?

Answer 7

The “arrow” in the dipole force will point toward the more electronegative atom (more negative). See example of N-H bond dipole below.

Question 8

Hydrogen bonding. is a special type of ________ interaction that occurs between the lone pair of a highly electroneative atom and the hydrogen atom in a N-H, O-H, or F-H bond.

Answer 8

Dipole-dipole

Question 9

What is the strongest form of dipole-dipole interaction?

Answer 9

Hydrogen bonding.

Question 10

What is an ion-dipole force?

Answer 10

An ion-dipole force occurs between ions and polar molecules. For example, an Na+ ion in solution would be attracted to the partial negative end of a dipole on a water molecule.

Question 11

How can we increase the strength of an ion-dipole force?

Answer 11

Ion-dipole forces can be understood in terms of Coulomb’s law, where the electric force, F, is equal to (kq1q2)/(r²). Therefore, an ion-dipole force will be stronger if we increase the magnitude of the charge on the ion and decrease the distance between the ion and the dipole.