For a particular reaction, ΔH equals -111.4 kilojoules and ΔS equals -25 joules per Kelvin. To calculate ΔG for this reaction at 298 Kelvin, we consider the following equation:
Δ G = Δ H − T Δ SΔH is -111.4 kilojoules, and the temperature (T) is 298 Kelvin. We need to convert our Joules to kilojoules for consistency, so -25 Joules per Kelvin is equivalent to -0.025 kilojoules per Kelvin. Now substituting these values, we get:
Δ G = ( - 111.4 ) − ( 298 × - 0.025 ) k i l o j o u l e sThe calculation yields ΔG as -103.95 kilojoules. Since the ΔG value is less than 0, we can conclude that the reaction is spontaneous at 298 Kelvin. The system is spontaneous as written, i.e., it is not at equilibrium because for equilibrium ΔG would have to equal 0. It would be spontaneous in the reverse direction only if we had gotten a ΔG that is greater than 0, since it would be non-spontaneous in the forward direction and we would have to reverse the reaction to achieve spontaneity.
