Boiling points of Saturated hydrocarbons [Alkanes] vs. water:

Boiling Points:

The temperature at which the vapor pressure of any liquid becomes equal to the atmospheric pressure or external pressure.

Hydrocarbons:

Hydrocarbons are those compounds which are made up of only carbon and hydrogen.

For example: CH4 [Methane], Ethane [C2H6] e.t.c.

We will discuss the boiling points of first ten alkanes in comparison of water:

Some main factors which effects the Boiling Points:
  • Following Intermolecular forces which effect the boiling points:

Ionic bonds > Ion dipole Interactions > H-Bonding > Dipole-dipole interactions > Vander Walls London dispersion forces.

  • Increasing the numbers of carbon atoms increase the boiling points (BP) of that compound.
  • Increase the branching with carbon atoms decrease the boiling points.

Boiling point of first ten Alkanes vs. Water H2O:

First four members of alkanes Methane CH4, Ethane (C2H6), Propane (C3H8) and n.Butane (C4H10) are gases at room temperature. Other members of alkane from n-pentane to n-decane are liquid at room temperature. With the increase of number of carbon atoms melting and boiling points of hydrocarbons are also increased.

Compounds

Melting Points °C

Boiling Points °C

Water

0

100

Methane

-183

-164

Ethane

-172

-89

Propane

-188

-42

n-Butane

-138

0

n-Pentane

-130

36

n-Hexane

-95

69

n-Heptane

-91

98

n-Octane

-57

125

n-Nonane

-51

151

n-Decane

-30

174

Methane CH4 is simplest aliphatic hydrocarbon. It has low melting (M.P) and boiling points (B.P) because of very weak intermolecular forces.

Methane is lighter gas than air. Its specific gravity is 0.054. Methane is faintly soluble in water. It burns freely in air forming CO2 and water vapors.

The boiling point of methane (CH4) is -164°C and its melting point is −183 °C.

Ethane (C2H6) is gas at room temperature. Its melting and boiling points high as compare to methane but very low as compare to water. Ethane (C2H6) has -172°C melting point and -89°C boiling point.

Propane (C3H8) is also gas at room temperature. Due to increase in number of carbon atoms it’s melting and boiling points are high as compare to methane and ethane but low as compare to water because water has strong hydrogen bonding and highly electronegative atom of oxygen. Its melting point is -188°C and boiling point is -42°C.

n.Butane (C4H10) is fourth member of alkane family is gas at room temperature having 0°C boiling point and melting point is -138°C. As we can say butane boils at that temperature where water freeze.

n-Pentane (C5H12) to n-Decane (C10H22) all members are liquid at room temperature. Due to increase in number of carbon atoms their boiling points also increased.

The small size molecules can easily boil and evaporate as compared to big sized molecules. e.g. hexane C6H14 is a small sized molecule as compare to that of decane C1oH22 . Therefore hexane evaporates rapidly than decane (C1oH22). So it is proving that increase the number of carbon atoms increase the boiling points of compounds.

Decreasing order of boiling points of hydrocarbons and water:

Decane> Nonane > Octane > Water > Heptane>Hexane >Pentane > Butane > Propane > Ethane > Methane