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S4 Chemistry Bonding and Structure, Paper 1: ionic, covalent and melting points
S4 Chemistry Bonding and Structure, Paper 1: ionic, covalent and melting points
Ten graded items from formulae and shared electron pairs through melting point comparisons to lattice energy and the conductivity of graphite.
Focus Classify the solid before comparing melting points, count valence electrons for shared pairs, and compare charge products for lattice energy.
01 Paper details (check answers afterwards)
| Item | Detail |
|---|---|
| Questions | 10 |
| Suggested time | 25 |
| Difficulty mix | Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1 |
| Chapter | Chapter: S4 Chemistry: Bonding and Structure |
| Level | Four basic, three advanced, two challenge and one error-repair item |
02 Questions (10 items)
- FoundationWhat is the formula of the compound formed from Mg^2+ and Cl^- ions?
- FoundationHow many pairs of electrons are shared between the two nitrogen atoms in N2?
- FoundationIn carbon dioxide, is each carbon to oxygen link a single or a double bond?
- FoundationDoes sodium chloride conduct electricity as a solid or when molten?
- IntermediateSodium chloride melts at 801 degrees and magnesium oxide at 2852 degrees. Why is the difference so large for two ionic solids?
- IntermediateBoth diamond and graphite are carbon. Why is diamond so hard?
- IntermediateIodine melts near 114 degrees, far below sodium chloride. Explain the difference.
- ChallengeFind the molar mass of magnesium oxide, with Mg as 24.3 and O as 16.0.
- ChallengeComparing only the ionic charge products, roughly how many times larger is the lattice energy of magnesium oxide than that of sodium chloride?
- Error redoA student says no covalent compound conducts electricity. What is wrong with that claim?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | MgCl2 | One magnesium ion carries two positive charges, so two chloride ions are needed and the formula is MgCl2. |
| 2 | Three pairs, a triple bond | Each nitrogen has five valence electrons, so three shared pairs complete the octet and give a triple bond. |
| 3 | A double bond, O=C=O | Carbon needs eight electrons, so each oxygen is joined by a double bond in O double bond C double bond O. |
| 4 | When molten, and also in solution | In the solid the ions are locked in the lattice, and only when molten or dissolved can they move and carry current. |
| 5 | Magnesium oxide has 2+ and 2- ions, so the charge product is four times bigger and the lattice far stronger. | The charge product is two times two against one times one, so the lattice energy is about four times larger. |
| 6 | Each carbon joins four others in a giant three-dimensional covalent network. | Every carbon is bonded to four others in a giant three-dimensional covalent network, so scratching means breaking many strong bonds. |
| 7 | Iodine is simple molecular and only weak forces between molecules must be overcome. | Iodine is a simple molecular solid held by weak van der Waals forces, so melting only overcomes those forces. |
| 8 | 40.3 g mol^-1 | Adding 24.3 and 16.0 gives 40.3 grams per mole for the ionic formula unit. |
| 9 | About four times | The charge products are four and one, so the lattice energy ratio is about four. |
| 10 | Graphite conducts with delocalised electrons and hydrogen chloride conducts in water, so only simple molecular solids such as sugar fail to conduct. | Graphite conducts through delocalised electrons, so the word all is wrong; conduction depends on mobile charged particles. |
04 What to do after marking
- After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
- Repeat the same item two days later; move to the next paper only when both attempts are correct.
- Parents only need the answer and note columns; there is no need to explain every item.
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