g Below which temperature does the following reaction change from being spontaneous to non-spontaneous? 2 HNO3 (aq) + NO (g) → 3 NO2 (g) + H2O (l) ΔHº = + 136.5 kJ; ΔSº = + 287.5 J/K Below which temperature does the following reaction change from being spontaneous to non-spontaneous? 2 HNO3 (aq) + NO (g) → 3 NO2 (g) + H2O (l) ΔHº = + 136.5 kJ; ΔSº = + 287.5 J/K 475 K 39.2 K 151 K This reaction is nonspontaneous at all temperatures. This reaction is spontaneous at all temperatures.

Below 475 K the reaction changes from being spontaneous to non-spontaneous.

Explanation:

Let's consider the following reaction.

2 HNO₃ (aq) + NO (g) → 3 NO₂ (g) + H₂O (l)

The spontaneity of the reaction is given by the standard Gibbs free energy (ΔG°). When ΔG° > 0 the reaction is non-spontaneous and when ΔG° < 0 the reaction is spontaneous. ΔG° is related to the standard enthalpy (ΔHº) and the standard entropy (ΔSº) using the following expression.

ΔG° = ΔHº - T.ΔSº

The reaction will change from spontaneous to non-spontaneous when ΔG° = 0. Then,

ΔHº - T.ΔSº = 0

ΔHº = T.ΔSº

T = ΔHº/ΔSº = (136.5 * 10³ J) / (287.5 J/K) = 475 K