chem modelsofparticulatenatureofmatter
Structure 1.1.2 - the kinetic molecular theory is a model to explain physical properties of matter (solids, liquids and gases) and changes of state
names of phase changes: melting, freezing, vaporisation (evaporation, boiling), condensation, sublimation, deposition
sublimation:
- direct conversion of a solid to a gas without going through liquid state
- e.g.:
deposition:
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direct conversion of a gas to a solid without going through liquid state
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e.g.:
when snow and frost are formed
liquid to gas: evaporation and boiling -
evaporation only takes place at the surface of the liquid and at a range of temperatures below the boiling point
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boiling takes place at boiling point when bubbles of gas form throughout the liquid
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boiling point is defined as the temperature at which the vapour pressure is equal to the external pressure, so the vapour can escape into the external
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thus, when external pressure is reduced, the vapour pressure needed to boil is reduced, and boiling point is lower
kinetic molecular theory:
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states of matter are characterised by the total system energy
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more energy → more particle movement - note: particles are always moving, not possible to have no energy (absolute zero)
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the temperature of the system is directly related to the average kinetic energy of the particles
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the state of matter at a given temperature and pressure is determined by the strength of inter-particle forces that exist between the particles relative to this average kinetic energy
behaviour of particles in states:
| solid | liquid | gas |
|---|---|---|
| particles packed closely | particles more spaced out | particles fully spread out |
| inter-particle forces strong | weaker inter-particle forces | negligible inter-particle forces |
| particles vibrate in position | particles can slide over each other | free moving particles |
| fixed shape | no fixed shape | no fixed shape |
| fixed volume | fixed volume | no fixed volume |
liquids and gases are both fluids because they can flow
to explain diffusion, read 1.4.2 Gibbs energy (HL)
diffusion:
- the process by which particles of a substance spread out evenly due to random movement
kinetic energy, where m is mass and v is velocity
since the average kinetic energy of all particles at the same temperature is the same, there’s an inverse relationship between mass and velocity:
thus, particles with smaller mass diffuse more quickly than those with greater mass at the same temperature
state symbols:
solutions are a homogenous mixtures of two or more components
- solute: less abundant component
- solvent: more abundant component
solute can be solid, liquid, gas
solvent can be liquid, gas
challenge questions
- What evidence based on simple observations can you think of that supports the idea that water is made from discrete particles?
- evaporation of water - suggests that it consists of particles that can independently separate and move independently because:
- water gradually evaporates over time
- not all water evaporates at once
- water vapour mixes uniformly with the air without clumping or remaining in large droplets
- Brownian motion
- Propane (
) and butane ( ) are both commonly used in portable heating devices. Their boiling points are butane -1 C and propane -42 C. Suggest why butane is less suitable for use in very cold climates.
Butane may turn liquid even when released from pressure and be ineffective as a fuel.
interesting questions
- “During very cold weather, snow often gradually disappears without melting. Explain how this is possible.”
- the snow sublimates as the humidity of the air would be very low due to the low temperature, so the vapour pressure of the air may be very low, and as the snow gains energy from a source such as sunlight, may rise above the vapour pressure to directly transition to a gas phase