# Homework Help: Intermolecular forces

1. Oct 21, 2008

### magma_saber

1. The problem statement, all variables and given/known data
There are two compounds, butane and pentane. one is a liquid at at room temp and the other is gas. which is a liquid and why?

2. Relevant equations

3. The attempt at a solution
I'm guessing that its pentane. For a gas to liquify, it should have a have a higher melting point, meaning that it has a higher freeing point. They both only have london dispersion forces but pentane has a greater mass so it should have a higher melting point.

2. Oct 22, 2008

### Staff: Mentor

More or less correct, although I am not sure if mentioning just 'greater mass' will do. Why do molecules of greater mass interact stronger?

3. Oct 22, 2008

### magma_saber

im guessing as the mass of the molecules increases, so does the strength of the dispersion force acting between the molecules, so more energy is required to weaken the attraction between the molecules resulting in higher boiling points.

4. Oct 22, 2008

### Staff: Mentor

OK, but why higher mass means higher dispersion forces?

Hint: higher mass - more electrons, or less electrons?

5. Oct 22, 2008

### granpa

it also means that the molecules move more slowly at any given temp

6. Oct 22, 2008

### magma_saber

so since there are more electrons, higher dispersion forces?

7. Oct 23, 2008

### Staff: Mentor

Yes.

8. Oct 23, 2008

### magma_saber

thanks for all the help.

9. Oct 23, 2008

### granpa

larger molecules have the same kinetic energy as smaller molecules at a given temp but they have more surface area for intermolecular forces to act upon.