Question

Arrange the following molecules in order of decreasing energy for IR Absorption H2,HI,HF,HBr,HCl. (red herring) my demonstrator said that there was a trick to this question :-/

Answer 1

i farted

Answer 2

poop

Answer 3

stain

Answer 4

In order for a compound to be IR active, it must a have a transition dipole (not necessary to have a permanent dipole).

Going down any main group in periodic table increase the mass and decrease the bond strength -> Lowering stretching Frequency

From left to right along a row the effect of increasing the mass is outweighed by the increase in bond strength -> Frequency increase

Answer 5

See :
\[f=1/2\pi \sqrt{K/m}\]

in this molecules that you have mentioned, the reduced mass in changing.
I like to let you, yourself discover the answer.