# How is chemical and dynamic equilibrium related?

I have been told that dynamic equilibrium is attained when the reactants and products of a system has achieved the same concentration or when their rates of formation are the same. And I also know that even though nothing seems to go on in a dynamic equilibrium there's still so much activity. Now assuming I've got that right. I want to know how is chemical equilibrium any different from what I mentioned abt dynamic equilibrium. Also can chemical and dynamic equilibrium coexist??

• Duplicate of chemistry.stackexchange.com/questions/88509/… ? – Mithoron Jan 8 '18 at 14:28
• @Mith Well, it's a dupe as far as the title goes ;-) Then again, as pointed out in the comments there, the title and the actual question aren't matching. So I suppose this post is safe O:) – paracetamol Jan 8 '18 at 14:35
• @paracetamol sigh – Mithoron Jan 8 '18 at 14:39

Suppose now that there is a reaction such that $\ce{ A \to B}$ and initially only A is present so that the rate of forward reaction is large, i.e. lots of A and almost no B make the forwards reaction faster than the reverse reaction. As soon as some B is present it can react back to make A, but $\ce{ B \to A}$ will be slow compared to $\ce{ A \to B}$ because there is less B (smaller concentration) and the activation barrier is larger so the 'rate constant' is smaller.
Eventually the amount of A is reduced and B increased until both rates are the same and equilibrium is achieved. Note that the concentrations need not be the same, in fact they usually are not. Also note that molecules of A continuously convert to B; $\ce{ A \to B}$ and similarly B convert to A; $\ce{ B \to A}$ which is to say that the normal state of affairs is the 'dynamic equilibrium' and simply using the word 'equilibrium' implicitly assumes that this is the case.