Name:
Matter starting to ‘come apart’
The energy per particle in matter at temperature T is typically of the order of magnitude kT, where k is the
Boltzmann constant 1.38 x 10–23 J K–1. A few particles will have energies quite a lot greater than average.
In these questions you will see examples of processes which happen at an appreciable rate, when the
energy required is in the range roughly 10 to 30 times kT. Electron charge = 1.6 x 10-19C, NA = 6.03 x 1023.
1. Energy 40 kJ mol–1 is required to evaporate water molecules from the liquid. Convert this to joules
per particle.
2. Water evaporates at a noticeable rate at 300 K, even though it only boils (at atmospheric pressure)
at 373 K. Calculate the value of kT at 300 K. What is the ratio /kT, where is the energy needed to
evaporate one molecule?
3. Electrons ‘evaporate’ from heated metals. This ‘thermionic emission’ is used to obtain the electron
beam in a television set. The energy required to remove an electron from platinum is 5.4 eV. Express this
energy in joules per electron.
