Question

1. 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: G = 93 kJ/mol and H = 237 kJ/mol:

(i) Calculate delta S at 220 K and comment on whether the sign of delta H and delta S make sense.

Answer 1

G = 93 kJ /mol

H = 237 kJ /mol

T = 220 K

G = H - T S

93 = 237 - 220 x S

220 x S = 144

S = 0.654 kJ / mol K

S = 654 kJ/mol

G = H - T S

S = +ve , H = +ve . G also positive .so the reaction is not spontaneoues.

for spontaneous reaction G should be negative and H = +ve S = +ve   : T S > H