Organic chemistryHighersimple moleculesintermolecular forcesboiling point
AQA GCSE · Question 08.1 · Organic chemistry
This question is about propane (C₃H₈).
Figure 7 shows the displayed structural formula of propane.
Explain why propane has a low boiling point.
How to approach this question
1. Identify the type of structure propane has (e.g., giant ionic, giant covalent, metallic, simple molecular).
2. What kind of forces exist *between* the molecules in this type of structure? Are they strong or weak?
3. When a substance boils, what forces are being broken?
4. Relate the strength of the forces to the amount of energy needed to break them, and therefore to the boiling point.
Full Answer
Propane is a simple molecular substance. There are weak intermolecular forces between the propane molecules. Only a small amount of energy is needed to overcome these forces, resulting in a low boiling point.
Propane (C₃H₈) is a simple molecular substance.
1. **Structure:** It consists of individual C₃H₈ molecules. The covalent bonds within each molecule (C-C and C-H bonds) are very strong.
2. **Intermolecular Forces:** However, the forces of attraction *between* the propane molecules are weak intermolecular forces (van der Waals forces).
3. **Boiling:** When propane boils, it is these weak intermolecular forces that are overcome, not the strong covalent bonds. Because the forces are weak, only a small amount of thermal energy is required to separate the molecules and turn the liquid into a gas. This results in a low boiling point (-42 °C).
Common mistakes
✗ Confusing intermolecular forces with covalent bonds. It is incorrect to say that "covalent bonds are broken".
✗ Just saying "it is a small molecule" without mentioning the weak forces between molecules.
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