Why is the Boiling Point of HBr Lower than Cl2 Despite Higher Molecular Mass?

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The boiling point of HBr is lower than that of Cl2 despite HBr having a higher molecular mass. This counterintuitive outcome is primarily due to the nature of intermolecular forces at play. While larger molecular mass typically suggests stronger London dispersion forces, this is not the sole factor influencing boiling points. The significant electronegativity difference between hydrogen and bromine leads to a polar bond in HBr, which affects its intermolecular interactions. Additionally, the absence of hydrogen bonding in HBr compared to other compounds must be considered. Therefore, boiling points cannot be accurately predicted by molecular mass alone; a comprehensive analysis of all intermolecular forces is essential.
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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 .
 
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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.
 
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