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1.1.4Additional lessons

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

5 min read 5 parts Lesson 3 of 98

📚 IB Chemistry SL 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 Kinetic Energy

Temperature measures the average kinetic energy of the particles in a substance.

Average, not total
  • Average, not total Temperature depends on the mean kinetic energy per particle, so a bath of warm water holds more total energy than a hot cup.
Direct proportion
  • Direct proportion The average kinetic energy of particles is directly proportional to the absolute temperature in kelvin.
Doubling T
  • Doubling T If the Kelvin temperature doubles, the average kinetic energy of the particles also doubles.
Heat flow
  • Heat flow Thermal energy transfers from the sample with higher average kinetic energy to the one with lower.

Part 2 of 5The Kelvin Scale

The kelvin is the SI unit of temperature and begins at absolute zero.

1
Absolute zero, 0 K, is the temperature at which particles would have the minimum possible kinetic energy.
2
A change of 1 K is the same size as a change of 1 °C, so differences are identical on both scales.
3
To convert, T in kelvin equals the Celsius temperature plus 273 (273.15 for more precise work).
4
A temperature in °C must be changed to kelvin before using it in any equation relating energy or gas behaviour.
Celsius (°C)−273025100
Kelvin (K)0273298373

Part 3 of 5Distribution of Energies

In a sample, the particles have a range of kinetic energies, not one value.

Distribution of Energies
Energy spread
  • Energy spread At a given temperature some particles have very low energy and a few have very high energy, with most near the average.
Raising temperature
  • Raising temperature Heating shifts the distribution to higher energies, and it spreads out and the peak becomes lower.
Evaporation
  • Evaporation Surface particles with enough energy to overcome attractions escape, so the remaining liquid cools.
Same temperature
  • Same temperature Different gases at the same temperature have the same average kinetic energy, though lighter particles move faster.

Part 4 of 5Worked Example

Question Convert 25 °C to kelvin, and find the factor by which the average kinetic energy changes when the sample is heated from 25 °C to 323 °C.

  1. T₁ = 25 + 273 = 298 K.
  2. T₂ = 323 + 273 = 596 K.
  3. Ratio T₂ ÷ T₁ = 596 ÷ 298 = 2.0, and average kinetic energy is proportional to T.

Answer

  • 298 K;
    • the average kinetic energy doubles

Part 5 of 5Exam Checklist & Tips

1.1.4 – Key Points for the Exam

  • Kinetic energy The energy a particle has because of its motion.
  • Absolute zero The lowest possible temperature, 0 K, equal to −273.15 °C.
  • Kelvin The SI unit of absolute temperature.
  • Average kinetic energy The mean value of the kinetic energy of all particles in a sample, proportional to T in kelvin.

Common Mistakes

  • Using °C instead of kelvin when applying proportionality; 20 °C is not twice 10 °C in energy terms.
  • Saying that all particles in a sample have the same kinetic energy at a given temperature.
  • Stating that temperature and heat are the same quantity.
  • Writing the kelvin unit with a degree sign, which is incorrect.

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