Kinetic Model for Matter
- The kinetic model of matter states that:
Matter is composed of a large number of small particles that are in constant motion
- This can be used to explain:
- The structure of solids, liquids and gases
- Why melting and boiling take place without a change in temperature
- Why the specific latent heat of vaporisation is higher than specific latent heat of fusion for the same substance
- Why a cooling effect accompanies evaporation
The Structure of Solids, Liquids & Gases
- In a solid:
- The molecules are very close together and held in place by strong intermolecular bonds
- These bonds prevent the molecules from moving, giving solid their rigid shape
- The atoms are arranged in a regular pattern
- The molecules vibrate about fixed positions
- In a liquid:
- The molecules are still close together (no gaps) but are no longer arranged in a regular pattern
- The molecules have enough energy that they are able to break the intermolecular bonds between them
- As a result, the molecules can move around and able to slide past each other, allowing the liquid to flow
- In a gas:
- The molecules are widely separated – about 10 times further apart in each direction than in liquids
- The molecules move about randomly at high speeds
- The molecules have no intermolecular forces between them
- As a result, the molecules are able to move freely
- The large spaces between molecules mean gases can easily be compressed and expanded
Why Melting & Boiling Take Place Without a Change in Temperature
- Melting is when a solid turns into a liquid eg. ice melting into water
- Boiling is when a liquid turns into a gas eg. water boiling to water vapour
- In both cases, the water must absorb energy in order to melt or boil
- This energy is not used to change the temperature of the water, only to change its state
- This is because the energy is needed to break the bonds between molecules
Why Latent Heat of Vaporisation is Higher than Latent Heat of Fusion
- Vaporisation is when a substance changes state from liquid to gas
- Fusion is when a substance changes state from solid to liquid
- In the vaporisation process:
- Energy is required to completely separate molecules to the point where intermolecular forces are no longer acting
- This requires much more energy than the separating molecules from a solid to a liquid
- In the fusion process:
- Energy is required to separate molecules from a rigid arrangement into a freely-flowing arrangement
- This requires less energy as the intermolecular bonds become weaker, but do not completely break
Why Cooling Accompanies Evaporation
- Evaporation requires heat energy to convert a liquid into gas since the intermolecular bonds in a liquid must be broken completely
- This energy is removed from the molecules as they change state, and hence taken away from the surface of the liquid as the gas moves away, hence causing a cooling effect
- This is why, for example, a person may feel instantly cooler when getting out of a hot shower or swimming pool