[Saionji]

If Ca is the concentration of A and B concentration Cb in function of time(in hours), can I say that A->B is an exothermic reaction and that at T = 398K the reaction reached the equilibrium since the concentrations remained constant?
http://img576.imageshack.us/img576/7986/imagemyv.png

[MrTeo]

I agree with you that the reaction is exothermic (K_{398 K}<K_{298 K}) but I don't understand why you (apparently) say the equilibrium is reached only at 398 K: both the graphs show an equilibrium, in fact it's from the comparison of the final concentrations of A and B that we can find out if ∆H is positive or not.

[Saionji]

The equilibrium at 298K is reached after 2 hours or when they meet together ?

[MrTeo]

The equilibrium at 298K is reached after 2 hours or when they meet together ?

The lines describing the reactants' trends will intersect (or meet) or not depending on the K value.
When the equilibrium is reached the reaction is (at least at our eyes) in a static state and does not proceed anymore (though the whole things continues working, as the chemical equilibrium is dynamic). This means that the concentrations don't vary after they've reached their "final" values and their values follow the well known law of mass action: we can see that in both graphs the reactions reach a static stage.

This happens after 3h (approximately) at 298 K and after less than 1h at 398 K (consider that we've completely negleted the reaction kinetics: with the raising of T the reaction reaches in a shorter time the equilibrium as we can see).