Question

Nitric acid hydrates have been connected to heterogeneous reactions related to the formation of the ozone hole above Antarctica:

HNO3 ˖ nH2O(s) ⇄ HNO3 (g) + nH2O (g) where n=1, 2 and 3

Cl radicals react with ozone (O3) to form simple oxygen in the upper atmosphere – creating an ozone hole. HNO3 (g) reacts with Cl radicals and is therefore essential to keeping ozone levels high. Worsnop et al, in the journal Science (1993), compiled thermodynamic data for these reactions at 220K

For n=3, the values obtained for the Gibbs energy and the enthalpy are: ΔG = 93 kJ/mol and ΔH = 237 kJ/mol:

(i) Calculate ΔS at 220 K and comment on whether the sign of ΔH and ΔS make sense.

(ii) Assuming ΔS and ΔH are constant over the temperature range, estimate ΔG at 180K

(iii) What is the point of this question – as in why calculate the Gibbs energy at 180K?

Answer 1

(i) dG = dH - TdS

with,

dG = 93 kJ/mol

dH = 237 kJ/mol

T = 220 K

we get,

dS = 237 - 93/220 = 0.654 kJ/mol

This makes sense, as with +ve values for dG and dH the value for dS would be +ve at low temperature.

(ii) dG at 180 K

dG = 237 - 180 x 0.654 = 119.28 kJ/mol

(iii) Calculation of gribbs free energy of a reaction tells us when and If the reaction is spontaneous. Looking at the data above we can say that as the sign of gibbs free energy in both cases calculated were +ve, the reaction would be non-spontaneous under the condiitons given.