Both the pure hydrogen and hydrogen bromide is held by weak intermolecular forces of attaction between molecules. Bromide has larger size of electron cloud than Hydrogen, leading to a greater extent of polarisation of the electron cloud. This results in stronger van der Waals' forces of attraction in hydrogen bromide compared to the pure hydrogen. More energy is required to break the stronger bonds in hydrogen bromide than pure hydrogen, leading to a higher boiling point in hydrogen bromide. Thus, pure hydrogen has a lower boiling point.
Explanation:
the strength of intermolecular forces of attraction rely on the size of the electron cloud. the greater the atomic number, the greater the size of the electron cloud and thus, the stronger the forces of attraction.
When asnwering a qn, you want to first state the structure and bonding of the compound, then compare the size of their electron clouds.
Answer structure:
greater size of electron clouds -> greater extent of polarisation of electron cloud -> stronger van der Waals' forces of attraction -> more energy required to overcome the bonds -> higher boiling point
