Changes in State: Energy Changes (AQA A-Level Chemistry): Revision Notes

1.4.1 Changes in State: Energy Changes

Changes in the state of matter—such as melting, boiling, freezing, and condensing—are accompanied by energy changes due to the breaking or forming of intermolecular forces or bonds. These processes are critical to understanding the energy dynamics in physical and chemical systems.

Melting (Solid to Liquid)

• Melting point (m.p.): The temperature at which a solid turns into a liquid.

• Energy change: Energy is absorbed when a substance melts. This energy is used to overcome the intermolecular forces or bonds holding the particles together in the solid state. The stronger the forces or bonds, the more energy required to melt the substance. For example:

• Ionic compounds like sodium chloride have high melting points due to the strong electrostatic forces between ions.

• Covalent molecular substances like ice have lower melting points due to weaker intermolecular forces (hydrogen bonds in the case of water).

Freezing (Liquid to Solid)

• Freezing point: The temperature at which a liquid turns into a solid, which is the same as the melting point.
• Energy change: Energy is released when a substance freezes, as particles form new bonds or intermolecular forces. The stronger the forces or bonds that form, the more energy is released during freezing.

Boiling (Liquid to Gas)

• Boiling point (b.p.): The temperature at which a liquid becomes a gas.
• Energy change: Energy is absorbed when a substance boils. This energy is required to overcome the intermolecular forces between particles in the liquid state to allow them to move freely as gas particles. Liquids with strong intermolecular forces, like hydrogen bonding or van der Waals forces, require more energy (and hence have higher boiling points) compared to those with weaker forces.

Condensing (Gas to Liquid)

• Condensation point: The temperature at which a gas becomes a liquid, which is the same as the boiling point.
• Energy change: Energy is released when a substance condenses, as the particles come closer together and intermolecular forces or bonds form. The more energy is released, the stronger the forces that are formed during condensation.

Sublimation (Solid to Gas and Gas to Solid)

• Sublimation: This is the process where a solid changes directly into a gas upon heating, or a gas changes directly into a solid upon cooling. Substances that can sublime include:
  • Solid iodine
  • Solid carbon dioxide (dry ice) In sublimation, energy is absorbed to overcome all intermolecular forces holding the solid particles together, or released when the gas particles form a solid.

Energy and Bond Strength

The amount of energy absorbed or released during a change of state depends on the strength of the bonds or intermolecular forces:

• Ionic and covalent bonds require significant amounts of energy to break, leading to higher melting and boiling points.
• Van der Waals forces and hydrogen bonds are weaker than ionic or covalent bonds and require less energy to overcome.

Summary of Changes in State

Process	State Change	Energy Change	Example
Melting	Solid → Liquid	Energy absorbed	Ice to water
Freezing	Liquid → Solid	Energy released	Water to ice
Boiling	Liquid → Gas	Energy absorbed	Water to steam
Condensing	Gas → Liquid	Energy released	Steam to water
Sublimation	Solid → Gas (and vice versa)	Energy absorbed or released	Iodine crystals or dry ice