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1.1.4Additional lessons

Average Kinetic Energy — IB Chemistry HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 3 of 172

📚 IB Chemistry HL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 5Temperature and Energy

The temperature of a substance measures the average kinetic energy of its particles.

Kinetic energy
  • Kinetic energy Particles in a sample have a range of speeds, so they have different kinetic energies at any moment.
Direct link
  • Direct link The average kinetic energy is directly proportional to the absolute temperature in kelvin.
Kelvin scale
  • Kelvin scale T(K) = temperature in °C + 273, and 0 K is the lowest possible temperature.
Doubling
  • Doubling Doubling the temperature in kelvin doubles the average kinetic energy, but this does not hold for °C.

Part 2 of 5Distribution of Energies

Distribution of Energies
Curve shape
  • Curve shape A plot of number of particles against kinetic energy has a peak and a long tail towards high energy.
Heating
  • Heating On heating, the peak moves to a higher energy and flattens, with more particles in the high-energy tail.
Same area
  • Same area The area under each curve stays the same because the number of particles is unchanged.
Consequence
  • Consequence More particles exceed a given energy at higher temperature, which explains faster evaporation and reaction.
Temperature / °CTemperature / K
0273
25298
100373

Part 3 of 5Using the Idea

Average kinetic energy explains what happens when a liquid evaporates.

1
Particles in a liquid have a spread of energies, with some moving much faster than the average.
2
A fast particle at the surface can overcome attractions and escape into the gas phase.
3
The particles that remain have a lower average kinetic energy, so the liquid cools.
4
Heating the liquid shifts the energy distribution upwards so that more particles can escape per second.
5
At the boiling point enough particles throughout the liquid can escape that vapour bubbles form.

Part 4 of 5Worked Example

Question By what factor does the average kinetic energy change when a gas is heated from 27 °C to 327 °C?

  1. Convert to kelvin27 + 273 = 300 K and 327 + 273 = 600 K.
  2. Average kinetic energy is proportional to T, so the factor is 600 ÷ 300 = 2.

Answer The average kinetic energy doubles.

Part 5 of 5Exam Checklist & Tips

1.1.4 – Key Points for the Exam

  • Kelvin The SI unit of absolute temperature, with 0 K as absolute zero.
  • Kinetic energy The energy of a particle due to its motion.
  • Absolute zero The lowest possible temperature, 0 K or −273 °C.
  • Energy distribution A plot showing how many particles have each value of kinetic energy.

Common Mistakes

  • Using degrees Celsius in a proportionality, which gives the wrong ratio.
  • Claiming that all particles at a given temperature have the same speed.
  • Saying the peak of the energy distribution rises as temperature increases, when it moves right and becomes lower.
  • Stating that the area under the curve increases on heating.

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