1.4.1Physical Chemistry
Changes in State: Energy Changes — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 4 parts Lesson 17 of 102
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 4State Changes as Physical Changes
Reversible
- Reversible A change of state is physical and reversible, so the chemical identity of the substance is the same before and after the change.
Melting
- Melting On melting a solid becomes a liquid;
- the particles gain enough energy to leave fixed positions but the substance is still held together in the liquid.
Boiling
- Boiling Boiling happens at one temperature, when the vapour pressure equals the external pressure, and bubbles form throughout the liquid.
Evaporation
- Evaporation Evaporation occurs only at the surface and at temperatures below the boiling point, as the fastest particles escape from the liquid.
Energy direction
- Energy direction Melting, boiling and sublimation absorb energy and are endothermic,
- whereas freezing, condensing and deposition release energy and are exothermic.
Part 2 of 4What Gets Overcome
What Gets Overcome
Ionic solid
- Ionic solid Strong electrostatic attractions between oppositely charged ions in the lattice must be overcome, so a lot of energy is needed.
Simple molecular
- Simple molecular Only weak intermolecular forces between molecules are broken;
- the covalent bonds inside each molecule stay intact.
Giant covalent
- Giant covalent Strong covalent bonds throughout the lattice must be broken, which needs very large amounts of energy.
Metallic
- Metallic The attraction between positive metal ions and delocalised electrons must be overcome, and its strength varies with the metal.
| Structure | Bonds or forces broken on melting | Typical melting point |
|---|---|---|
| Giant ionic | Ionic bonds | High |
| Simple molecular | Intermolecular forces | Low |
| Giant covalent | Covalent bonds | Very high |
| Giant metallic | Metallic bonds | Mostly high |
Part 3 of 4Heating Curve Reasoning
1
As a solid is heated, its particles vibrate more strongly, which shows as a steady rise in temperature.
2
At the melting point the temperature stays constant because the energy supplied is used to weaken the attractions between particles, not to speed them up.
3
Once fully liquid, added energy again raises the average kinetic energy of the particles and the temperature rises.
4
At the boiling point the temperature plateaus again as energy is used to separate the particles completely into a gas.
5
On cooling, the same energy is released to the surroundings as the gas condenses and the liquid freezes.
Part 4 of 4Exam Checklist & Tips
1.4.1 – Key Points for the Exam
- Physical change A change that does not form a new substance and can be reversed, such as melting.
- Vaporisation The change from liquid to gas, including both evaporation and boiling.
- Sublimation The direct change from solid to gas without passing through the liquid state.
- Intermolecular forces Weak attractions between separate molecules, overcome when simple molecular substances change state.
Common Mistakes
- Saying covalent bonds break when a simple molecular substance such as water boils;
- only intermolecular forces are overcome.
- Stating that temperature rises while a substance is melting; it stays constant during a change of state.
- Treating evaporation and boiling as identical; boiling occurs throughout the liquid at one temperature.
- Describing energy as released when bonds or forces are broken; breaking them is always endothermic.
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