Question

For the reaction ,2 O3 —> 3 O2,the reaction rate , delta[O2]/delta t , = 3.0M/min. What is the reaction rate, delta[O3]/delta t, for the same time period?
A) 3.0 M/min B) –3.0M/min C) 2.0 M/min D) –2.0M/min E) .67M/min

Answer 1

2O2 --> 3O2

rate rxn = 1/3(delta O2/delta t)
rate rxn = 1/3(3.0) = 1

rate rxn = -1/2(delta O3/delta t)
(delta O3/delta t) = -rate rxn x 2
-(delta O3/delta t) = -1 x 2 = -2 M/min. Since the O3 is disappearing, that needs to be a -2 M/min.

I think an easier way to do it is the following (just use the coefficients).
(delta O2/delta t) = 3 M/min x (2 moles O3/3 moles O2) = 2.0 M/min and since O3 is disappearing, you must add a negative sign.