Why doesn't sodium carbonate decompose? I was asked to find the hypothetical $\Delta H$ and $\Delta S$ of the decomposition of sodium carbonate and then asked to explain why it doesn't decompose.

I calculated the enthalpy and entropy change and from that I calculated the Gibbs free energy change. I was hoping to get a positive answer, but actually I got a negative answer, indicating that the process is actually spontaneous. 

So is the reason that sodium carbonate doesn't decompose due to its kinetic properties, i.e. the reaction is too slow?
From the values on the Wikipedia page for sodium carbonate, the Gibbs free energy of formation is negative and thus, formation is thermodynamically preferred over decomposition at $\pu{298K}$. Your given values seem to be wrong after all. 

By the way, $\Delta H$ and $\Delta S$ should be the negative of those given on that page for your case, making $\Delta G$ a positive number.