1.1.4Additional lessons
Average Kinetic Energy — IB Chemistry SL Revision Notes & Topical Questions
5 min read 5 parts Lesson 3 of 98
Part 1 of 5Temperature and Kinetic Energy
Temperature measures the average kinetic energy of the particles in a substance.
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
- The average kinetic energy of particles is directly proportional to the absolute temperature in kelvin.
Doubling T
- If the Kelvin temperature doubles, the average kinetic energy of the particles also doubles.
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) | −273 | 0 | 25 | 100 |
|---|---|---|---|---|
| Kelvin (K) | 0 | 273 | 298 | 373 |
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
- At a given temperature some particles have very low energy and a few have very high energy, with most near the average.
Raising temperature
- Heating shifts the distribution to higher energies, and it spreads out and the peak becomes lower.
Evaporation
- Surface particles with enough energy to overcome attractions escape, so the remaining liquid cools.
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.
- T₁ = 25 + 273 = 298 K.
- T₂ = 323 + 273 = 596 K.
- 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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