Time step help
 

If I am modeling a reaction in laminar flow whose first order rate constant is very high i.e. 100000 sec^-1 do I need to limit my time step the timescale of the chemistry? Or can I use a bigger time step. I have experiemnted a little, and It seems when I use a bigger time step the chemistry changes. Any thoughts? I am using the unsteady state pressure based solver with stiff chemistry enabled. My reaction network is 7 species with 6 reactions. I am solving the problem by initiating a steady state cold flow, turning on the reactions and solving. I am currently using a time step of 1e-5.
Aaron