This applet models the "Peroxy Mechanism" for the kinetics of the production and destruction of ozone in the stratosphere. The rate constant for the dissociation of O2 from UV-C is k1: O2 ⇒ 2O. The rate constant for the collisional recombintion of O3 is k2: O2 + O ⇒ O3. The rate constant for the dissociation of ozone by UV-B radiation is k3: O3 ⇒ O2 + O. The rate constant for step 4 is k4: O3 + O ⇒ 2O2. The rate constant for step 5 (step 1 of the mechanism) is k5: O3 + OH ⇒ HOO + O2. The rate constant for step 6 (step 2 of the mechanism) is k6: HOO + O3 ⇒ OH + 2O2. The mechanism presumes that the rate constant for the recombination of oxygen atoms (O) to form dioxygen (O2) is so slow that it can be ignored. To solve for the over-all rate law, the steady-state approximation is made; i.e., d[O]/dt = 0.
The mechanism is depicted visually on the left-hand side of the applet. No inference is to be drawn from the rates of reaction depicted in the graphic - it is intended only to demonstrate the steps involved in the mechanism.
This applet models the Peroxy Mechanism stochastically, by comparing a randomly generated number with the rate constant each small time interval. When the random number is less than the rate constant multiplied by the time interval, the event occurs. Because of the stochastic nature of the simulation, there is no need to postulate the steady-state approximation; in fact, the concentration of intermediate O can be observed over the course of time, to verify how well the approximation holds.
Click on the appropriate button below to increase/decrease the concentration of halogen atom X ([X]), or any of the rate constants k1, k2, k3, k4, k5, or k6.
© 2003-2011 by Lawrence T. Sein. All rights reserved.
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