MnO 4 - + Cr 3+ -> Mn 2+ + CrO 4 2- The solution starts out a purple color due t
ID: 487905 • Letter: M
Question
MnO4- + Cr3+ -> Mn2+ + CrO42-
The solution starts out a purple color due to the permanganate ion, and then turns yellow as the chromate ion is produced. The rate of this reaction can be studied by measuring the time it takes for the CrO42- to reach a concentration of 0.020M. The results for several reaction mixtures are given below:
a. Explain how the times are related to the reaction rate.
b. Determine the reaction order with respect to each reactant.
c. Determine the rate law, including the rate constant (with the correct units).
Expt [MnO4-], M [Cr3+], M Time for [CrO42-] to reach 0.020 M Rate of the Reaction 1 1.00 1.00 23 min 2 2.00 1.00 11 min 3 1.00 0.50 45 minExplanation / Answer
For the given reaction data,
1 mole of MnO4- gives 1 mole of CrO4^2-
a. The rate of formation of CrO4^2- is equal to the rate of disappearence of MnO4- which is used to find the rate of the reaction.
rate = -d[MnO4-]/dt = d[C2O4^2-]/dt
b. when the concentration of MnO4- was doubled, the rate of formation of C2O4^2- also doubled. So reaction is first order with respect to MnO4-
when the concentration of Cr3+ was halved, the rate of formation of CrO4^2- was doubled. So the rate is inversely proportional to Cr3+
c. rate law,
rate = k[MnO4-]/[Cr3+] = 0.0833 M/min
rate constant k = 0.0833 x 1/1 = 0.0833 M/min