Explain the trend in boiling points between HF, HCl and HBr.

There are two themes in this answer. The first being the hydrogen bonding potential of HF and the second being the difference in size of the of the halide ions. Fluorine is one of three elements that can form hydogen bonds (as well as N and O). HF can both donate and recieve a hydrogen bond and the network this creates requires energy to break before the molecules themselves can be given enough kinetic energy to transfer into the gas phase. Hence this has the highest boiling point of the three. Bromine is a larger ion than chlorine and thus has stronger Van der Waals forces. Larger amounts of energy (a higher temperature) are required to break these interactions and so the boiling point of HBr is higher than HCl.