Van der Waals forces

[aminzairi]

1. When going down the group 17 why does the boiling point increase ? what does it mean by polarizability

2. When water  evaporates is the Van Der Waals forces being broken down or just the hydrogen bonds ?

[SC2Player]

1. Polarizability refers to the ease with which the electron cloud is distorted, with larger clouds being more polarizeable.  As one goes down Group 17, the atomic radius increases, meaning that the electron cloud is larger, which results in the electron cloud being more polarizeable.  This makes the temporary dipoles associated with Van der Waals forces stronger, resulting in higher boiling point.  

2. In this case, its more accurate to say that all intermolecular bonds are being broken down (which includes both Van der Waals and the hydrogen bonds).  Air molecules with higher kinetic energy collide with water molecules, providing some of them with enough energy to break the intermolecular bonds.  

[Msj Chem]

With regards to question 2, when water evaporates, the water molecules gain enough energy to overcome the intermolecular forces (hydrogen bonds, dipole-dipole attractions and London dispersion forces) between the individual water molecules. 

 

Edited November 17, 2016 by Msj Chem

[aminzairi]

but if the Van der Waals forces are being broken down what forces cause attraction between evaporated water molecules ?

[Msj Chem]

21 minutes ago, aminzairi said:

but if the Van der Waals forces are being broken down what forces cause attraction between evaporated water molecules ?

You can think of evaporated water molecules (as in steam) as existing as individual water molecules. There are still intermolecular forces between the molecules but their kinetic energy is great enough to overcome these forces.  When they lose energy, they will condense back to liquid water. This is why condensation is exothermic.