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.

The stronger the intermolecular forces, the higher the boiling point.

The higher the molecular mass, the higher the boiling point.


Intermolecular forces of O2, HBr and C2H5OH


Oxygen gas0- 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 Hδ+-Br. 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.



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