Given the reaction SO3 ---> SO2 + 1/2 O2 (triangle H degrees= 98.9 kJ , triangle S degrees = 93.9 J/ mol K):
A) is the reaction spontaneous at 25 degrees celsius?
B) at what temperature will the reaction reverse direction?
In order to find out if a reaction is spontaneous we need to find the Gibbs free energy value using the equation below:
where deltaG is the Gibbs free energy change, deltaH is the enthalpy change, T is the temperature in Kelvin, and deltaS is the entropy change. If the delta G value is negative, then the reaction is spontaneous. Remember that all of the units must match up (25 C is 298 K).
deltaG=98.9 kJ-(298 K)(0.0939 kJ/K)=70.92 kJ
Since the deltaG value is positive the reaction is not spontaneous.
The reaction will reverse direction when the deltaG value is equal to 0. This is when the reaction is at equlibrium and the forward and reverse reactions are both happening. We can use the same equation to solve for T:
T=deltaH/deltaS=98.9 kJ/0.0939 kJ/K=1053 K=780 C.
The temperature where the reaction begins to reverse itself is 780 C.