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Kinematics: Rate Law and Rate Constant

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Consider the reaction of peroxydisulfate ion S2O8 ^2 - with iodide ion (I-) in aqueous solution:
S2O8^2-(aq) + 3I-(aq)  2SO4 ^2-(aq) + I3-(aq)
At a particular temperature the initial rate of disappearance of S2O8^2- varies with reactant concentrations in the following manner:
Experiment [S2O8 2-] (M) [I-] (M) Initial Rate (M/s)
1 0.018 0.036 2.6 × 10-6
2 0.027 0.036 3.9 × 10-6
3 0.036 0.054 7.8 × 10-6
4 0.050 0.072 1.4 × 10-5

(a) Determine the rate law for the reaction and state the units of the rate constant.
(b) What is the average value of the rate constant for the disappearance of S2O8^2- based on the four sets of data?
(c) How is the rate of disappearance of S2O8^2- related to the rate of disappearance of I-?
(d) What is the rate of disappearance of I- when S2O8^2- =0.025M and [I-] = 0.050 M?

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Solution Summary

This solution offers guidelines on how to solve a numerical problem on kinetics, by showing steps and explanation.

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1. The isomerization of cyclopropane to form propene

CH3 - CH CH2CH2

is a first-order reaction. At 760 K, 15 percent of a sample of cyclopropane changes to propene in 6.8 minutes. What is the half-life of cyclopropane at 760 K?

2. Consider the two gaseous equilibria (K1 and K2):

SO2(g) + ½O2(g) SO3(g) K1

2SO3(g) 2SO2(g) + O2(g) K2

The values of the equilibrium constants K1 and K2 are related by ________.

3. The following reactions occur at 500 K. Arrange them in order of increasing tendency to proceed to completion (i.e., least completion greatest completion).
1. 2NOCl 2NO + Cl2 KP = 1.7 x 10-2
2. N2O4 2NO2 KP = 1.5 x 103
3. 2SO3 2SO2 + O2 KP = 1.3 x 10-5
4. 2NO2 2NO + O2 KP = 5.9 x 10-5

4. On analysis, an equilibrium mixture for the reaction

2H2S(g) 2H2(g) + S2(g)

was found to contain 1.0 mol H2S, 4.0 mol H2, and 0.80 mol S2 in a 4.0 L vessel. Calculate the equilibrium constant for this reaction.

5. At 35oC, the equilibrium constant for the following reaction is Kc = 1.6 x 10-5.

2NOCl(g) 2NO(g) + Cl2(g)

An equilibrium mixture was found to have the following concentrations of Cl2 and NOCl:

[Cl2] = 1.2 x 10-2 M; [NOCl] = 2.8 x 10-1 M. Calculate the concentration of NO(g) at equilibrium.

6. For the following reactions the equilibrium constants are defined.

A + 2B C K1
C D + E K2

Then for the reaction

A + 2B D + E Kc

the equilibrium constant must be equal to ________.

7. At 700 K, the reaction

2SO2(g) + O2(g) 2SO3(g)

has an equilibrium constant Kc = 4.3 x 106, and the following concentrations are present:

[SO2] = 0.10 M
[SO3] = 10 M
[O2] = 0.10 M

Is the mixture at equilibrium? If not at equilibrium, in which direction-left to right or right to left- will the reaction occur to reach equilibrium?

8. For the following reaction at equilibrium, which choice gives a change that will shift the position of equilibrium to favor formation of more products?

2NOBr(g) 2NO(g) + Br2(g) Horxn = 30 kJ

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