Average Kinetic Energy (DP IB Chemistry): Revision Note

Temperature & average kinetic energy

• Temperature is a measure of the average kinetic energy (E_k) of the particles in a substance.

  • The Kelvin (K) is the SI unit of temperature

  • Temperature in Kelvin is directly proportional to the average kinetic energy

  • The higher the temperature, the faster the particles move (on average)

Interpreting state change graphs

• The relationship between temperature and energy during changes of state can be shown in a heating curve:

Explanation of graph regions:

• 1 → 2

  • The substance is heating up in the solid state

  • Particles vibrate more as they gain kinetic energy

  • So, the temperature increases

• 2 → 3

  • Melting occurs (solid → liquid)

  • Energy is used to overcome intermolecular forces, not increase kinetic energy

  • The temperature remains constant during this state change

• 3 → 4

  • The substance is heating in the liquid state

  • Particles move more freely and gain kinetic energy

  • Temperature rises again

• 4 → 5

  • Boiling occurs (liquid → gas)

  • Energy goes into overcoming intermolecular forces

  • So, the temperature remains constant during this state change

• 5 → 6

  • The substance is now a gas

  • The particles move rapidly and have the highest average kinetic energy

  • The temperature increases

Observations during state changes

• State changes are accompanied by physical changes that can be observed without altering the chemical identity of the substance:

  • Melting: solid becomes liquid, shape lost but volume remains constant

  • Boiling: bubbles form throughout liquid as gas escapes

  • Condensation: gas forms droplets on cooler surfaces

  • Freezing: liquid becomes rigid, shape is regained

  • Sublimation: solid disappears directly into vapour

Converting between Celsius and Kelvin

• The Kelvin scale is also known as the absolute temperature scale.

  • It uses the same size increments as the Celsius scale (1 K = 1 °C)

• 0 K (absolute zero) is the temperature at which particles have no kinetic energy

Conversions:

Temperature in K = Temperature in  °C + 273.15

Temperature in °C = Temperature in  K - 273.15

Conversion chart between temperature scales in Kelvin and Celsius