Some initial rate data for the iodide-ion catalyzed decomposition of hydrogen pe

Question

Some initial rate data for the iodide-ion catalyzed decomposition of hydrogen peroxide at 25.0 degree C in a total volume of 25 mL of solution are shown below: 2 H_2O_2 aq) rightarrow 2 H_2O(l) + O_2(g) Expt. No. mL 3.0% H_2O_2 mL 0.10 M KI mL H_2O initial rate, mLO_2/s 1 5.0 10.0 10.0 0.031 2 10.0 10.0 5.0 0.065 3 5.0 20.0 - 0060 Use the data to determine the rate law for the reaction, and; If the total volume of O_2 collected in expt. 1 was 55 mL at 1 atm pressure, what was the molarity of the H_2O_2. The vapor pressure of water at 25.0 degree C is 23.1 mm.

Explanation / Answer

a)

determine rate law:

assume H2O2 and KI are important

so

Rate = k*[H2O2]^a[KI]^b

r1/ r2 = k [A]1a [B]1b / k [A]2a [B]2b

choose 1 and 2:

r1 / r2 = k [A]1a [B]1b / k [A]2a [B]2b

0.013 / 0.065 = 5^a * 10^b / (10^a * 10^ b)

0.2 = (1/2)^a

a = ln(0.2) / ln(0.5) =2.3

for b:

choose 1 and 3

r1 / r2 = k [A]1a [B]1b / k [A]2a [B]2b

0.013 / 0.06= 5^a * 10^b / (5^a * 20^ b)

0.216 = (1/2)^b

ln(0.216 ) / ln(0.5) = b

b =2.210

so

rate = K * [H2O2]^2.2 * [KI]^2

b)

V = 55 mL P = 1 atm

P° = 23.1 mm Hg

find molarity

PV = nRT

n/V = P/(RT)

n/V = 1/(0.082*298)

M = 0.041 mol / L

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