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

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SUMMARY

The boiling point of HBr is lower than that of Cl2 despite HBr having a higher molecular mass. This phenomenon is attributed to the nature of intermolecular forces at play, particularly the significance of hydrogen bonding and electronegativity differences. While HBr has a larger molecular mass, the weaker London dispersion forces do not dominate the boiling point comparison. Therefore, one cannot determine boiling points based solely on molecular mass; a comprehensive analysis of intermolecular forces is essential.

PREREQUISITES
  • Understanding of intermolecular forces, including London dispersion forces and hydrogen bonding.
  • Knowledge of electronegativity and its impact on molecular interactions.
  • Familiarity with molecular mass and its relevance to boiling points.
  • Basic principles of thermodynamics related to phase changes.
NEXT STEPS
  • Research the role of hydrogen bonding in molecular interactions.
  • Study the impact of electronegativity on boiling points in various compounds.
  • Explore the differences between London dispersion forces and other intermolecular forces.
  • Examine case studies of boiling points in halogen compounds for comparative analysis.
USEFUL FOR

Chemistry students, educators, and professionals interested in molecular chemistry and thermodynamics, particularly those studying intermolecular forces and their effects on physical properties.

ngkamsengpeter
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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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