Liquids, Solids and Gases

States of Matter

  • States of matter: gas, liquid, solid.
  • Molecular liquids and solids are held together by intermolecular forces.
  • The stronger the intermolecular forces, the higher the boiling point.

Intermolecular Interactions vs Thermal Energy

  • Competition between intermolecular forces and thermal energy exists in all states of matter.
    • Gas: intermolecular interactions weak, thermal energy wins.
    • Liquid: intermolecular interactions strong enough to keep particles together, but thermal energy allows movement.
    • Solid: intermolecular interactions strong compared to thermal energy, molecules remain in fixed positions.

Energy & Changes of State

  • Heating a solid:
    1. Solid absorbs energy, temperature increases.
    2. Reaches a temperature where the solid starts to melt; temperature stops rising as the solid melts.
    3. Once the solid has all melted, the temperature of the liquid starts to rise.
    4. The liquid starts to convert to a gas (boils); the temperature stops rising as the liquid converts to gas.
    5. As heating of the gas continues, its temperature increases.

Molecular Level Explanation

  • Solid:
    • Energy is absorbed by molecules causing them to 'jiggle'.
    • ↓Temp = low Energy = low jiggling; ↑Temp = high Energy = High jiggling.
  • Solid + Liquid:
    • Molecules gain sufficient energy to overcome intermolecular forces and move into the liquid state.
    • Temperature stops rising as energy is used to overcome intermolecular forces.
  • Liquid:
    • Thermal energy increases the energy of molecules in the liquid, increasing their movement.
  • Liquid + Gas:
    • Molecules gain enough energy to escape the liquid and become a gas.
    • Energy is used to overcome intermolecular forces as the liquid turns into a gas.

Cooling a Gas

  • Process:
    • Gas cools to liquid + gas, then liquid, then solid + liquid, then solid as thermal energy is removed.
    • Releases the same amount of energy as you put in gas

Endothermic vs Exothermic

  • Melting/boiling (s) → (l) → (g): intermolecular 'bonds' weaken, molecular motion increases, energy is absorbed = ENDOTHERMIC process
  • Freezing/condensation (g) → (l) → (s): intermolecular 'bonds' strengthen, molecular motion decreases, energy is released = EXOTHERMIC process

Heat Capacity

  • Thermal Energy is absorbed & Temperature Changes.
  • Specific Heat Capacity (C_s): The quantity of heat (q) required to raise the temperature (T) of 1 g of a substance by 1 K.
  • q = m × C_s × ΔT

Latent Heat

  • Thermal Energy is absorbed & Temperature remains Constant
  • Latent Heat of Vapourisation (ΔH_{vap}): Energy required to turn 1 mole of liquid into a gas (kJ mol-1).
    • q = n × ΔH_{vap}
  • Latent Heat of Fusion: Energy required to melt 1 mole of a solid (ΔH_{fus}) (kJ mol-1).
    • q = n × ΔH_{fus}

Summary

  • Competition between intermolecular forces & thermal energy.
  • Temp changes when (s) ⇌ (l) ⇌ (g)
  • When thermal energy is absorbed & temperature changes = HEAT CAPACITY
  • When thermal energy is absorbed & temperature remains constant = LATENT HEAT (fusion or vapourisation)
  • Heat Capacity depends on mass, Latent Heat depends on #moles.
  • ENDOTHERMIC: add energy to get a process to happen
  • EXOTHERMIC: Processes that release energy