Overview
* Covalent bonds are also present in simple molecular structures. However, they are not broken during physical reactions (e.g. boiling, melting)
For questions related to Chemical Bonding:
- Determine the type of substance (metal/ ionic/ covalent) and structure (giant metallic/ giant ionic/ giant covalent/ simple molecular) [learn how]
- Focus on that particular column to determine the bonding and factors that affect it.
Students tend to make mistakes when they try to pinpoint a factor without regards for the structure and bonding.
Simple Molecular Structure
To compare strength of intermolecular forces for two molecules:
- Check molar mass (Mr): larger Mr → larger e– cloud → more easily polarised → more extensive dispersion
- For molecules with similar Mr, check type of molecules (polar/ non polar) [learn how]
There are three possible cases:
Case 1 (both polar):
- Check for H-bonding: molecule with H-bonding will have higher bp/mp.
- If both have H-bonding, compare the extent of H-bonding i.e. no. of possible H-bond per molecule, presence of intra H-bonding that leads to less extensive inter H-bonding.
Case 2 (one polar, one non-polar):
- The polar molecule will always have higher bp/mp.
- pd-pd interactions stronger than dispersion.
Case 3 (both non-polar):
- Difference must be due to surface area.
- Increased branching → more spherical e– cloud → lesser area of interaction → less extensive dispersion
