Wednesday, January 30, 2019

kinetics - At equilibrium are the rates of the forward and backward reactions equal to zero?


At equilibrium, the concentrations do not change with time. So, is it true that the rates of the chemical reactions are zero at equilibrium? Wikipedia says that they are not zero. Why is this?



Answer



In equilibrium, the forward and backward rates are equal to each other. The net is zero, but the individual rates are not zero.


Consider something as simple as water — we know that the water molecule can dissociate in to an $\ce{OH-}$ and $\ce{H+}$, and the pH is a direct measure of the amount of $\ce{H+}$. Once equilibrium is achieved, it isn't like $\ce{H2O}$ stops splitting, or $\ce{H+}$ and $\ce{OH-}$ get back together — it is just at a macro level nothing changes.



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