Internal Energy and Energy Transfers

• Internal energy is the total energy that the particles of a system have.
• It is made up of the kinetic energy from the particles' movement and potential energy from their interactions.
• An increase in temperature causes an increase in the internal energy of a system as the particles move faster.
• However, temperature and internal energy do not always increase together. When a substance changes state, its temperature stays the same until all the substance has changed state.

Changes of State

• A change of state happens when a substance changes from one state (solid, liquid, or gas) to another.
• This is different from a change in temperature, which doesn't always result in a change of state.
• The energy needed for a change of state comes from the internal energy of the substance. This energy is used to change the arrangement of the particles.
• For example, when ice melts into water, the energy is used to break the bonds holding the ice particles together.
• Similarly, when water evaporates, energy from the surroundings is absorbed and used to break the intermolecular forces holding the water molecules together.
• During a change of state, the temperature of a substance remains constant. The transition from one phase to another happens at a specific temperature for a given substance under a fixed pressure.
• The terms melting (solid to liquid), freezing (liquid to solid), boiling (liquid to gas), evaporating (liquid to gas at any temperature), condensing (gas to liquid), and sublimation (solid to gas without becoming a liquid) are used to describe changes of state.

Specific Latent Heat

• Specific latent heat is the amount of energy needed to change the state of 1 kg of a substance without changing its temperature.
• It is measured in joules per kilogram (J/kg) and is different for each substance and for each change of state.
• There are two types of specific latent heat: latent heat of fusion (for melting and freezing) and latent heat of vaporisation (for boiling and condensing).
• The amount of energy needed to change a substance's state from a solid to a liquid or a liquid to a gas is equal to the mass of the substance multiplied by its specific latent heat.
• The formula for calculating energy during a change of state is: energy (J) = mass (kg) × specific latent heat (J/kg).
• Note that during a change of state, no energy goes into changing the temperature – all the energy goes into breaking or forming bonds between particles.
• Therefore, changes in state do not involve changes in kinetic energy, but rather involve changes in potential energy.

The Particle Model and Changes of State

• The particle model helps us understand what's happening on a microscopic level during a change of state.
• When a substance melts, for example, the particles gain energy from the surrounding environment. This energy goes into breaking the bonds that are holding the particles in place.
• As a substance evaporates, the particles at the surface gain enough energy to overcome the attractive forces holding them in the liquid, and they escape into the air.
• Understanding changes of state in terms of the particle model helps to explain why the temperature stays constant during a change of state, why different substances need different amounts of energy to change state (their specific latent heats), and why some substances can sublimate.