1.7.3Principles of Chemistry
Simple Molecular Structures — Edexcel International GCSE Science Double Award 4SD0 - Chemistry Revision Notes & Topical Questions
5 min read 11 parts Lesson 44 of 50
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5Structure of simple molecular substances
Small molecules
- Small molecules These substances are made of molecules containing only a few atoms joined by strong covalent bonds, such as H2O, CO2, CH4 and Cl2.
Two kinds of force
- Two kinds of force Within each molecule the atoms are held by strong covalent bonds.
- Between molecules there are only weak intermolecular forces.
Discrete particles
- Discrete particles Each molecule is separate; there is no giant lattice of bonded atoms.
Part 2 of 5Properties and explanations
| Property | Observation | Explanation |
|---|---|---|
| Melting and boiling point | Low | Only the weak forces between molecules need to be overcome; the covalent bonds do not break. |
| State at room temperature | Often gases or liquids, sometimes soft solids | Low boiling points mean little energy is needed to separate the molecules. |
| Electrical conductivity | Does not conduct in any state | Molecules are uncharged and there are no free ions or electrons to carry current. |
| Solubility | Usually insoluble in water, but soluble in some other solvents | They are not ionic, so water does not pull them apart. |
Part 3 of 5What happens when a simple molecular substance melts or boils
1Heat supplied
- Heat supplied Particles gain kinetic energy and vibrate more.
2Weak forces overcome
- Weak forces overcome The intermolecular forces between molecules are broken.
3Molecules move apart
- Molecules move apart The substance becomes liquid, then gas.
4Bonds unchanged
- Bonds unchanged Each molecule stays intact: boiling water still gives H2O molecules, not hydrogen and oxygen.
Part 4 of 5Effect of molecular size
Bigger molecules, stronger forces
- Bigger molecules, stronger forces As the relative molecular mass increases, the intermolecular forces become stronger.
Higher boiling points
- Higher boiling points More energy is needed to separate larger molecules,
- so the melting and boiling points rise down a series such as the halogens: chlorine is a gas, bromine a liquid and iodine a solid.
Use in separation
- Use in separation This difference in boiling points between substances is what fractional distillation of crude oil depends upon.
Part 5 of 5Answering exam questions
1
State that the substance is made of simple molecules.
2
Say that the forces between the molecules are weak, so little energy is needed to overcome them.
3
Make it clear that covalent bonds are not broken when it melts or boils.
4
For conductivity, say that there are no charged particles free to move because molecules have no overall charge.
Key terms
| Term | Meaning |
|---|---|
| Simple molecular substance | A substance made of small molecules with strong covalent bonds inside and weak forces between them. |
| Intermolecular force | A weak force of attraction between separate molecules. |
| Covalent bond | A strong bond formed by a shared pair of electrons within a molecule. |
| Boiling point | The temperature at which a liquid changes to a gas throughout its volume. |
| Relative molecular mass | The sum of the relative atomic masses of atoms in a molecule. |
Avoid these mistakes
1
Saying that covalent bonds are weak. They are strong; it is the intermolecular forces that are weak.
2
Writing that covalent bonds break when a simple molecular substance boils.
3
Explaining the poor conduction as 'no electrons'. The point is that there are no free charged particles.
4
Forgetting to link larger molecules with stronger intermolecular forces and higher boiling points.
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