Practice Problem 7

The rate constants for the forward and reverse reactions in the following equilibrium have been measured.   At 25C, kf is 7.3 x 103 liters per mole-second and kr is 0.55 liters per mole-second.  Calculate the equilibrium constant for this reaction:

ClNO2(g) + NO(g) <----> NO2(g) + ClNO(g)


Solution

We start by assuming that the rates of the forward and reverse reactions at equilibrium are the same:

At equilibrium:     rateforward = ratereverse

We then substitute the rate laws for these reactions into this equality:

At equilibrium:     kf[ClNO2][NO] = kr[NO2][ClNO]

We then rearrange this equation to get the equilibrium constant expression for the reaction :

ex22_7a.gif (1424 bytes)

The equilibrium constant for the reaction is therefore equal to the rate constant for the forward reaction divided by the rate constant for the reverse reaction:

ex22_7b.gif (1495 bytes)

 

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