Year 9 Science · Unit 2 · Lesson 9
Challenge Worksheet
Learning Goals
1. Name what is overcome when (a) ice melts and (b) quartz (SiO₂) melts.
2. Use dot-and-cross diagrams to show the triple bond in N₂ and the two double bonds in CO₂. Check that every atom ends up with 8 outer electrons.
Someone claims…
“Silicon dioxide (SiO₂) and carbon dioxide (CO₂) are both covalent compounds of a non-metal with oxygen, with the same type of formula. So sand should be a gas at room temperature, just like CO₂.”
3. What error is the person making? Explain why CO₂ and SiO₂ have such different properties even though both are covalent.
4. In SiO₂ each silicon atom bonds to four oxygen atoms, and each oxygen to two silicon atoms. Use this to explain why quartz melts at about 1700 °C and why the formula is SiO₂.
Read the data
Bond energy is the energy needed to break one mole of a covalent bond. A student predicts: “The molecule with the strongest bond will have the highest boiling point.”
| Substance | Bond | Bond energy (kJ/mol) | Boiling point (°C) |
|---|---|---|---|
| Nitrogen, N₂ | N≡N | 945 | −196 |
| Water, H₂O | O–H | 463 | 100 |
| Hydrogen, H₂ | H–H | 436 | −253 |
| Hydrogen chloride, HCl | H–Cl | 431 | −85 |
| Methane, CH₄ | C–H | 414 | −162 |
| Oxygen, O₂ | O=O | 498 | −183 |
Source: data book values. For H₂O and CH₄ the bond energy is the average for one bond.
5. Rank the six substances by bond energy and by boiling point. Does the data support the student’s prediction? Quote two substances as evidence.
6. Explain what the boiling point of a molecular substance actually depends on. What does water’s boiling point suggest about the forces between water molecules?
7. Evaluate the statement: “Covalent bonds are weak, which is why covalent substances melt easily.” Use Table 1 and at least one example of each kind of covalent structure.