Question

Experiment 21

Advance Study Assignment: Rates of Chemical Reactions, II. A Clock Reaction

1. A student studied the clock reaction described in this experiment. She set up Reaction Mixture 4 by mixing 10 mL 0.010 M KI, 10 mL 0.001 M Na2S2O3, 10 mL 0.040 M KBrO3, and 20 mL 0.10 M HCl using the procedure given. It took about 21 seconds for the color to turn blue.

a. She found the concentrations of each reactant in the reacting mixture by realizing that the number of moles of each reactant did not change when that reactant was mixed with the others, but that its concentration did. For any reactant A,

no. moles A = MA stock × Vstock = MA mixture × Vmixture
The volume of the mixture was 50 mL. Revising the above equation, she obtained

M =M × Vstock(mL)

A mixture A stock
Find the concentrations of each reactant using the equation above.

(I−) = ____________ M; (BrO3−) = ____________ M; (H+) = ____________ M

What was the relative rate of the reaction (1000/t)? ____________

Knowing the relative rate of reaction for Mixture 4 and the concentrations of I−, BrO3−, and H+ in that mixture, she was able to set up Equation 5 for the relative rate of the reaction. The only quantities that remained unknown were k′, m, n, and p. Set up Equation 5 as she did, presuming she did it properly. Equation 5 is on page 163.

2. For Reaction Mixture 1 the student found that 85 seconds were required. On dividing Equation 5 for Reaction Mixture 1 by Equation 5 for Reaction Mixture 4, and after canceling out the common terms [k′ and terms in (I−) and (BrO3−)], she got the following equation:

11.8=⎛0.020⎞p =⎛1⎞p 48 ⎝0.040⎠ ⎝2⎠

Recognizing that 11.8/48 is about equal to 14 , she obtained an approximate value for m. What was that value?

m = ____________ By taking logarithms of both sides of the equation, she got an exact value for m. What was that value?

m = ____________ Since orders of reactions are often integers, she rounded her value of m to the nearest integer, and reported

that value as the order of the reaction with respect to H+.

Answer 1

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