Consider the following reaction and its corresponding thermodynamic data:
NH₃ (g) + HCl (s) →NH₄Cl (s) D H ° = - 175.9 kJ/mol and D S ° = - 284.6 J/mol × K At what approximate temperature does this reaction cross over from being considered spontaneous to being non-spontaneous?

Question

Quizplus · Accepted Answer

The spontaneity of a reaction is determined by the sign of ΔG, which is related to ΔH and ΔS through the equation: 
ΔG = ΔH - TΔS
When ΔG is negative, the reaction is spontaneous; when ΔG is positive, the reaction is non-spontaneous. If ΔG is zero, the reaction is at equilibrium. 
To determine the approximate temperature at which the reaction crosses over from being spontaneous to being non-spontaneous, we need to calculate the temperature at which ΔG would be zero. This corresponds to the equation: 
ΔH = TΔS
Solving for T: 
T = ΔH/ΔS 
Substituting the given values: 
T = (-175.9 kJ/mol)/(-284.6 J/mol × K) = 618 K 
Therefore, the answer is D. At a temperature above 618 K, the reaction would be non-spontaneous.

Consider the Following Reaction and Its Corresponding Thermodynamic Data