The table below gives the Lennard-Jones parameters for some different substances.

                      He   Ne      H2      O2    N2  CH4   CO2   Cl2

σ [nm]           0.26  0.28  0.30  0.43  0.37  0.38  0.40   0.41

ε [×10−22 J] 1.41  4.93  4.60  15.6  12.6  18.9  26.2  49.3

(a) In Lennard-Jones theory, the molar enthalpy of vaporisation is estimated as

∆Hvap = 1/2 nLε,

where L = 6.02 × 1023 mol−1 is Avogadro’s number.

(i) State the meaning of n and also two fundamental assumptions necessary to find the expression for ∆Hvap . 

(ii) How much energy is carried away by the evaporation of 200 cm3 of liquid nitrogen (molar volume 35 cm3/mol)?

(b) Find an estimate for the surface tension of liquid oxygen.