Temperature & Average Kinetic Energy
- The relationship between temperature and kinetic energy during state changes can be represented graphically using the following sketch graph
Change of State Graph
The relationship between temperature and energy during state changes
How to explain the energy changes during a state change?
- Between 1 & 2, the particles are vibrating and gaining kinetic energy and the temperature rises
- Between 2 & 3, all the energy goes into breaking bonds – there is no increase in kinetic energy or temperature
- Between 3 & 4, the particles are moving around and gaining in kinetic energy
- Between 4 & 5, the substance is boiling, so bonds are breaking and there is no increase in kinetic energy or temperature
- From 5 & 6, the particles are moving around rapidly and increasing in kinetic energy
Worked example
Which of these gases has the highest average kinetic energy?
Answer
Argon, Ar, is the substance with the highest temperature so its particles have the highest average kinetic energy.
Converting between Celsius and Kelvin
- The kelvin (K) is the SI unit of temperature and has the same incremental value as the Celsius degree (°C)
- The kelvin scale is known as absolute temperature and the kinetic energy of the particles is directly proportional to their temperature in kelvin
- The lowest possible temperature is absolute zero, equal to 0 K or -273.15 °C
- This is the temperature at which the atoms and molecules in all substances have zero kinetic and potential energy
Temperature can be converted from Celsius to kelvin by adding 273:
Temperature in K = Temperature in °C + 273
Temperature in °C = Temperature in K - 273
Conversion chart between temperature scales in Kelvin and Celsius
A change in a temperature of 1 K is equal to a change in temperature of 1 °C
Worked example
In many ideal gas problems, room temperature is considered to be 300 K.
What is this temperature in degrees Celsius?
Answer:
Step 1: Recall the kelvin to Celsius conversion
θ / °C = T / K − 273
Step 2: Substitute in the value of 300 K
300 K − 273 = 27 °C
Exam Tip
Students often confuse kinetic energy with the temperature of particles and assume that all particles at the same temperature have the same velocity. Particles of a substance at the same temperature have the same average kinetic energy, but some particles will move faster and some slower than the average.
Particles of different substances at the same temperature will diffuse at different speeds because they have different masses. The kinetic energy is governed by the relationship:
Ek = ½mv2
where Ek is the kinetic energy, m is the mass and v is the velocity of the particle
