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A-Level Chemistry October/November 2024 Q3(e)(iii): SiO2 is a white solid that melts above 1700°C. SiCl4 is a colourless liquid at room tem…
A-Level Chemistry · Paper 9701/23 · October/November 2024 · Question 3(e)(iii) · [2 marks]
SiO2 is a white solid that melts above 1700°C. SiCl4 is a colourless liquid at room temperature. Explain the difference in the melting points of these two compounds with reference to their structure and bonding.
A full-marks model answer with a mark-by-mark examiner breakdown is below.
1 answer
- accepted ✓
Silicon dioxide, SiO₂, has a giant covalent structure. Silicon tetrachloride, SiCl₄, has a simple molecular structure.
A large amount of energy is required to break the many strong covalent bonds throughout the giant lattice of SiO₂, resulting in a very high melting point. In contrast, only a small amount of energy is needed to overcome the weak intermolecular forces between the discrete SiCl₄ molecules, resulting in a low melting point.
How the marks are awarded
- M1 — The first mark is awarded for correctly identifying that SiO₂ has a giant covalent structure AND that SiCl₄ has a simple molecular structure.
- M2 — The second mark is for correctly linking the energy requirement to the forces being overcome, by stating that less energy is needed to overcome the weak intermolecular forces in SiCl₄ compared to the energy needed to break the strong covalent bonds in SiO₂.
Common mistakes
- Stating that covalent bonds are broken in SiCl₄ when it melts, instead of correctly identifying that weak intermolecular forces are overcome.
- Describing SiO₂ as having an ionic lattice because it is a solid with a high melting point.
- Failing to mention both substances and make a direct comparison, for example, only explaining why SiO₂ has a high melting point.
- Using vague terms like 'stronger bonds' without specifying whether they are covalent bonds being broken or intermolecular forces being overcome.
Examiner tip: When comparing physical properties, always explicitly state the structure and bonding for both substances and clearly identify the specific forces (covalent bonds or intermolecular forces) that are overcome during the change of state.
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