Boiling points of HBr, ethanol, oxygen, intermolecular forces

Different compounds have various boiling points. It can be a negative temperature or very high temperature. This depends on several factors of compounds.

Boiling Points of HBr, Ethanol and O₂

• HBr - -66⁰
• Ethanol - 78⁰C
• O₂ - -183⁰C

Question

Explain, why oxygen has a boiling point of -183⁰C, hydrogen bromide (-66⁰C) and ethanol (C₂H₅OH) 78⁰C? Please explain this behavior.

asked by: Sorcha Eckhardt, 2019-06-02

Why oxygen, hydrogen bromide and ethanol have different boiling points?

Oxygen (O₂), hydrogen bromide (HBr) and ethanol (C₂H₅OH) have boiling points of -183⁰C, -66⁰C and 78.37⁰C respectively.

From these three compounds, ethanol has the highest boiling point. Now we try to understand what are the reasons for those different boling points.

Reason to boiling point differences

Boiling point depends on following factors.

• Intermolecular forces between molecules - When intermolecular forces between molecules increases or strength of intermolecular forces increses, boiling point increases.
• Molecular mass - When molecular mass increases, boiling point increases.

Intermolecular forces of O₂, HBr and C₂H₅OH

Oxygen gas- Intermolecular Forces

• O₂ is a non-polar molecule. Therefore there are only weak Van der Waals force between O₂ molecules.

Intermolecular forces of HBr

• Electronegativity of bromine is higher than hydrogen. Therefore H-Br molecule is polarized as H^δ+-Br^-δ. Therefore between HBr molecules, there are Dipole-Dipole forces attractions.

Intermolecular forces of Ethanol

• In C₂H₅OH, there are hydrogen bonds which are the most strong intermolecular forces.
• Now we know intermolecular forces are increased like this.
  O₂ < HBr < C₂H₅OH

Strength of intermolecular forces and effect to the boiling point.

Van der Waals force < Dipole-Dipole forces < Hydrogen bonds

When intermolecular forces increases, the ability of increasing boiling point is high.

Molecular weight of O₂, HBr and C₂H₅OH

• Molecular weight of HBr is higher than both O₂ and C₂H₅OH.

Lowest boiling point

Due to weakest intermolecular forces and lowest molecular weight, O₂ has the lowest boiling point.

Highest boiling point

Hydrogen bonds effect is much stronger than molecular weight. Therefore, C₂H₅OH has the highest boiling point.

More examples

• Molecular weight of H₂S is higher than H₂O. But due to very strong hydrogen bonds in water, its boiling point is 100⁰C at 1 atm. H₂S boiling point is -60⁰C.

Intermolecular forces of ethanol in the aqueous solutions

Both ethanol and water molcules have the capability of form hydrogen bonds. So, in aqueous ethanol solution, there are hydrogen bonds between ethanol and water molecules. If we simplify this furthermore, we can say, there are hydrogen bonds between ethanol molecues and water molecules.

Questions

molecular weight and boiling point, what is the relationship?

Usually, we say, when molecular weight increases, boiling point increases. As an example, boiling point of ethanol is higher than methanol.

But, this is not true at sometimes. As an example, boiling point of water is much higher than H₂S. This is caused due to strength of intermolecular forces.

Related Tutorials

Boiling point trends of periodic table and compounds Redox reaction characteristics(oxidation - reduction) Metal Characteristics Oxidation states of elements