1.3 latent heat

energy & change of state §

• if water is heated -> temperatures will rise.
• if enough energy is transferred to the water, the water will boil.
• latent energy is the energy released or absorbed during a change of state (e.g. when water is boiled, when water is frozen etc etc)
  • latent means hidden/unseen. latent heat is present, but we just do not see it.
• when a substance changes state, temperature remains constant
• image for anki flashcards :grin:
• temperature is added at a constant rate, but there are sections where temperature remains unchanged while the material change states.

latent heat §

• the energy needed to change the state of a substance is called latent heat
• latent heat is calculated using the equation: $Q=mL$
• where Q is the heat energy transferred in joules (J)
• m is the mass in kilograms (kg)
• L is the latent heat (Jkg^-1)

latent heat of fusion (melting) §

• as thermal energy is transferred to a solid, temperature of solid increases.
  • particles within solid gain internal energy (as mostly kinetic energy and some potential energy) and speed of vibration increases
  • internal energy: sum of potential and kinetic energy in substance.
• at the point solid begins to melt, the particles move further apart, reducing the strength of the bonds holding them in place.
  • instead of increasing temperature, the extra energy increases the potential energy of the particles, reducing the interparticle or intermolecular forces.
• tldr: no change in temeprature occurs, as all the extra energy supplied is used in reducing forces between particles.
• amount required to melt solid = potential energy released when liquid reforms into a solid. it is termed latent heat of fusion
• for a given substance:
  • heat energy transferred = mass of substance x specific latent heat of fusion
  • $Q=m{L}_{\text{fusion}}$
  • where Q is the heat energy transferred in joules (J)
  • m is the mass in kilograms (kg)
  • $L_{\text{fusion}}$ is the latent heat of fusion in $Jk{g}^{-1}$
  • 80 times as much energy to turn 1kg of ice into water (no temp change), as it does to raise temperature of 1kg of water by 1degC.
  • it takes a lot more energy to overcome large intermolecular forces within ice than it does to simply add kinetic energy in raising the temperature.
  • 
• latent heat vaporisation $Q=m{L}_{vapour}$ (boiling)
• average kinetic energy decrease as higher kinetic energy particles will have equipped the liquid.
• dependent on the volatility of the liquid, the surface area, the temperature, the humidity, and the air movement.