Physical Properties of Covalent Substances (HL IB Chemistry)

• The physical properties of molecular covalent compounds are largely influenced by their intermolecular forces
• If you know the type of intermolecular forces present you can predict the physical properties like melting and boiling point, solubility, and conductivity

Melting and boiling point

• When covalent molecular substances change state you are overcoming the intermolecular forces
• The stronger the forces the more energy need to break the attraction
• Intermolecular forces are much weaker than covalent bonds, so many covalent substances are liquid or gases at room temperature
• Substance with a low melting and boiling point are said to be very volatile
• The strength of the intermolecular forces increases with
  • the size of the molecule
  • the increase in the polarity of the molecule
  • Drawing the structure of the molecule helps identify and rank molecules according to boiling point as the following example shows

Solubility

• The general principle is that 'like dissolves like' so non-polar substances mostly dissolve in non-polar solvents, like hydrocarbons and they form dispersion forces between the solvent and the solute
• Polar covalent substances generally dissolve in polar solvents as a result of dipole-dipole interactions or the formation of hydrogen bonds between the solute and the solvent
• A good example of this is seen in organic molecules such as alcohols and water:

Diagram to show the hydrogen bonding between ethanol and water

A hydrogen bond forms between oxygen atom on the ethanol and the hydrogen atom of the water

• As covalent molecules become larger their solubility can decrease as the polar part of the molecule is only a smaller part of the overall structure
  • This effect is seen in alcohols for example where ethanol, C₂H₅OH, is readily soluble but hexanol, C₆H₁₃OH, is not

• Polar covalent substances are unable to dissolve well in non-polar solvents as their dipole-dipole attractions are unable to interact well with the solvent
• Giant covalent substances generally don't dissolve in any solvents as the energy needed to overcome the strong covalent bonds in the lattice structures is too great

Conductivity

• As covalent substances do not contain any freely moving charged particles they are unable to conduct electricity in either the solid or liquid state
• However, under certain conditions some polar covalent molecules can ionise and will conduct electricity
• Some giant covalent structures are capable of conducting electricity due to delocalised electrons but they are exceptions to the general rule

Comparing the Properties of Covalent Compounds Table