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Flashcards · 9203 · Topic 4

Particle model of matter flashcards

16 cards on Topic 4, subtopics 3.4.1 to 3.4.2. Pick a subtopic or study them all.

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3.4.1 Kinetic theory

Particle arrangement, motion and forces in solids, liquids and gases
Solid: lattice, vibrating, strong forces. Liquid: disordered, sliding past each other, weaker forces. Gas: large gaps, fast and free, almost no forces.
Why does temperature stay constant during a change of state?
The energy changes the separation (potential energy) of the particles, not their speed (kinetic energy).
Specific heat capacity
The energy that warms 1 kg of a substance by 1 °C. E = m c Δθ (J, kg, J/kg °C, °C).
Energy to raise 0.30 kg of copper (c = 390 J/kg °C) from 25 °C to 85 °C?
E = mcΔθ = 0.30 × 390 × 60 = 7020 J (7.0 × 10³ J).
Specific latent heat of fusion and of vaporisation
Fusion (LF): energy to melt 1 kg of solid at constant temperature. Vaporisation (LV): energy to vaporise 1 kg of liquid at constant temperature. E = m L.
A 1.8 kW heater for 90 s evaporates 0.075 kg of liquid at its boiling point. Find LV.
E = Pt = 1800 × 90 = 162 000 J. LV = E ÷ m = 162 000 ÷ 0.075 = 2.16 × 10⁶ J/kg.
Reading a temperature–time heating graph
Sloping section: one state, temperature rising (kinetic energy increases). Flat section: change of state at constant temperature (potential energy changes). The boiling flat is usually longer, as LV > LF.
Stearic acid cooling practical: method and expected result
Melt the acid in hot water, then record its temperature at regular intervals (e.g. every 30 s) as it cools. The graph falls, has a plateau at the freezing point (close to 69 °C), then falls again.
How do impurities affect melting and boiling?
They lower the melting point, raise the boiling point, and make the change happen over a range rather than a sharp step.

3.4.2 Energy transfers and particle motion

Why are metals good thermal conductors?
Vibrating particles pass energy on, and free electrons move through the lattice carrying energy. Insulators have no free electrons; gases have widely spaced particles.
Convection
Warmed fluid expands, its particles move apart and its density falls, so it rises; cooler, denser fluid sinks to replace it. Only in liquids and gases.
Why does evaporation cool a liquid, and what speeds it up?
The fastest particles leave the surface, so the average kinetic energy of those left falls. Faster with higher temperature, larger surface area, moving air and drier air.
Factors that increase the rate of energy transfer by heating
A larger surface area to volume ratio, a better conducting material in contact, a dull dark surface, and a bigger temperature difference.
Surface area to volume ratio of cubes of side 1 cm and 3 cm
Ratio = 6/s: 6 per cm and 2 per cm. The larger cube has a lower ratio, so it cools more slowly.
Thermal expansion: cause, a hazard and a use
Heated particles vibrate with larger amplitude and move slightly further apart. Hazard: bridges need expansion joints. Use: a bimetallic strip bends (the metal that expands more is on the outside) to work a thermostat.
Condensation in terms of particles
Vapour particles lose energy and become liquid, transferring energy to the surroundings.

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