At what temperature does the following reaction become nonspontaneous?
2 NO (g) + O₂ (g) → 2 NO₂ (g)

Determine the conditions for nonspontaneity: A reaction becomes nonspontaneous when ΔG > 0. Therefore, you need to find the temperature at which ΔG changes from negative to positive.

Analyze the signs of ΔH and ΔS: If ΔH is positive and ΔS is negative, the reaction is nonspontaneous at all temperatures. If ΔH is negative and ΔS is positive, the reaction is spontaneous at all temperatures. If both ΔH and ΔS are positive or both are negative, the temperature will affect spontaneity.