Intermolecular forces problems

[ngkamsengpeter]

Why the boiling point of HBr will smaller than the Cl2 ?
Since the relative molecular mass of HBr is bigger than Cl2 , so the temporary dipole induced dipole forces should be bigger . Thus , the boiling point of HBr should be bigger .

 

[Artermis]

This is not true... the molecular mass that relates to London forces (which is a relatively weak intermolecular force) does not have such an impact in this case. Think about the electronegativity difference between H and Br. Also, think about whether or not there is hydrogen bonding (a relatively strong intermolecular force) between H and Br. You cannot deduce relative boiling points simply looking at one aspect of the problem.

 

[ravenprp]

You are correct in saying that the relative molecular mass of HBr is greater than Cl2, which would suggest that HBr would have a higher boiling point. However, there are other factors at play when it comes to determining the boiling point of a substance. One important factor is the strength of the intermolecular forces between molecules.

In the case of HBr and Cl2, HBr molecules have stronger intermolecular forces compared to Cl2. This is due to the presence of a polar covalent bond between the hydrogen and bromine atoms, creating a permanent dipole moment. This permanent dipole moment contributes to the strength of the intermolecular forces in HBr, making it more difficult for the molecules to break apart and transition from a liquid to a gas.

On the other hand, Cl2 molecules only have temporary dipole moments, which are induced by the movement of electrons in the bond. These temporary dipoles are weaker compared to permanent dipoles, resulting in weaker intermolecular forces between Cl2 molecules. As a result, it is easier for Cl2 molecules to overcome these forces and transition from a liquid to a gas, resulting in a lower boiling point compared to HBr.

In summary, while the relative molecular mass may suggest that HBr should have a higher boiling point than Cl2, the strength of the intermolecular forces between the molecules plays a significant role in determining the boiling point. In this case, the stronger intermolecular forces in HBr outweigh the effect of its higher molecular mass, resulting in a lower boiling point compared to Cl2.