Rate Law and Order of Reactions

Question

The reaction:

2I− (aq) + S2O82− (aq) →I2 (aq) + 2SO42− (aq)

was studied at 25 Celsius. The following results were obtained where

Rate=(−)∆[S2O82−]∆t

[I−]0(mol/L) [S2O82−]0(mol/L) Initial Rate(mol/L s)

0.080 0.040 12.5 ×10−6

0.040 0.040 6.25 ×10−6

0.080 0.020 6.25 ×10−6

0.032 0.040 5.00×10−6

0.060 0.030 7.00×10−6

a.) Determine the rate law.

b.) What is the overall order of the reaction?

Answer 1

a). To determine the rate law, you need to find the order with respect to each reactant. To do this, find 2 experiments where the concentration of the reactant of interest changes while the other remains constant, and see what happens to the rate.

For I^-, look at expt. # 2 vs. #1. The concentration doubles and the rate doubles.-> 1st order in I^-

For S₂O₈^2-, look at expt. # 3 vs. #1. The concentration doubles and the rate doubles.-> 1st order in S₂O₈^2-

RATE LAW: rate = k[I^-][S₂O₈^2-]

b). Overall order of the reaction is the sum of the orders for each reactant = 1 + 1 = second order overall