Intermolecular Forces and State Changes
This topic explains the difference between the strong bonds inside molecules and the weak forces between them. Understanding this is key to explaining why molecular substances melt and boil at specific temperatures without decomposing.
Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- Covalent bonds are strong forces that hold atoms together *within* a molecule (intramolecular forces).
- Intermolecular forces are weak attractive forces that exist *between* neighbouring molecules.
- During melting and boiling, energy is used to overcome the weak intermolecular forces, allowing molecules to move past each other or separate.
- The strong covalent bonds inside the molecules do not break during a physical state change.
- Because the covalent bonds remain intact, the chemical identity of the substance is preserved (e.g., H₂O remains H₂O when it turns to steam).
Definitions
- Intermolecular forces
- The weak attractive forces that exist between separate molecules in a solid, liquid, or gas.
- Intramolecular forces
- The strong forces that hold atoms together within a single molecule, such as covalent bonds.
Worked example
When solid methane (CH₄) melts at -182°C, which statement correctly describes the process on a molecular level?
- 1
Step 1:
Identify the bonding within methane.
Methane (CH₄) consists of discrete molecules, with strong covalent bonds between the carbon and hydrogen atoms.
- 2
Step 2:
Identify the forces between methane molecules.
In solid methane, these individual CH₄ molecules are held in a fixed lattice by weak intermolecular forces.
- 3
Step 3:
Define melting.
Melting is a phase change from solid to liquid, which requires energy to disrupt the fixed lattice and allow particles to move.
- 4
Step 4:
Determine which forces are overcome.
The energy supplied during melting is sufficient to overcome the weak intermolecular forces between the CH₄ molecules, but not the strong C-H covalent bonds within them.
- 5
Step 5:
Conclude the composition of the resulting liquid.
The liquid consists of intact CH₄ molecules that are now free to move past one another.
Answer: The weak intermolecular forces between CH₄ molecules are overcome, but the covalent C-H bonds within the molecules remain intact.
Common mistakes
- ×Thinking that covalent bonds break during boiling. Boiling is a physical change, not a chemical reaction. Water boils to form gaseous H₂O molecules, not hydrogen and oxygen atoms.
- ×Confusing the strength of covalent bonds with the melting point of a molecular substance. The melting point is determined by the strength of the intermolecular forces, which are always much weaker than the covalent bonds themselves.
No-calculator tips
- ✓Remember the 'water rule': Ice, liquid water, and steam are all made of H₂O molecules. If the formula doesn't change during a state change, the strong bonds inside the molecule cannot have been broken.
- ✓If a substance is described as having discrete molecules and a low melting/boiling point, immediately recognise that you are dealing with weak intermolecular forces that are being overcome, not strong covalent bonds.