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Chemical Kinetics Method of Initial Rates

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Presentation on theme: "Chemical Kinetics Method of Initial Rates"— Presentation transcript:

1 Chemical Kinetics Method of Initial Rates

2 Determine the Rate Law (order of a reaction) experimentally

3 Method of Initial Rates
The rate law for a reaction can be determined by studying what happens to the initial instantaneous rate of reaction when we start with different initial concentrations of the reactants. Strategy!

4 Same reaction as before.
Using the Method of Initial Rates to determine the Rate Law of the decomposition reaction of hydrogen iodide. 2 HI (g)  H2 (g) + I2 (g) Same reaction as before. Determine m.

5 We will look at the data from two experiments at a time.
Using the Method of Initial Rates to determine the Rate Law of the decomposition reaction of hydrogen iodide. 2 HI (g)  H2 (g) + I2 (g)   Initial [HI] (M)          Initial Instantaneous                                                  Rate of Reaction (M s-1)           Expt 1:     1.0 x 10-2                   x 10-6 Expt 2:     2.0 x 10-2                   x 10-5 Expt 3:     3.0 x 10-2                   x 10-5 We will look at the data from two experiments at a time.

6 2 HI (g)  H2 (g) + I2 (g) 2m = 4 m = 2 Compare Expt 1 and 2
  Initial [HI] (M)          Initial Instantaneous                                                  Rate of Reaction (M s-1)           Expt 1:     1.0 x 10-2                   x 10-6 Expt 2:     2.0 x 10-2                   x 10-5 Expt 3:     3.0 x 10-2                   x 10-5 Compare Expt 1 and 2 Double the initial Concentration of HI The initial rate of reaction increases 4x 2m = 4 m = 2

7 2 HI (g)  H2 (g) + I2 (g) 3m = 9 m = 2 Compare Expt 1 and 3
  Initial [HI] (M)          Initial Instantaneous                                                  Rate of Reaction (M s-1)           Expt 1:     1.0 x 10-2                   x 10-6 Expt 2:     2.0 x 10-2                   x 10-5 Expt 3:     3.0 x 10-2                   x 10-5 Compare Expt 1 and 3 Triple the initial concentration of HI The initial rate of reaction increases 9x 3m = 9 m = 2

8 2 HI (g)  H2 (g) + I2 (g) 1.5m = 2.25 m log (1.5) = log (2.25) m = 2
  Initial [HI] (M)          Initial Instantaneous                                                  Rate of Reaction (M s-1)           Expt 1:     1.0 x 10-2                   x 10-6 Expt 2:     2.0 x 10-2                   x 10-5 Expt 3:     3.0 x 10-2                   x 10-5 Compare Expt 2 and 3 Increase the initial concentration of HI by 1.5 x The initial rate of reaction increases 2.25x 1.5m = 2.25 m log (1.5) = log (2.25) m = 2

9 Using the Method of Initial Rates to determine the Rate Law of
S2O82-(aq) + 2 I- (aq)  2 SO42- (aq) + I2 (aq) Determine m and n.

10 Using the Method of Initial Rates to determine the Rate Law of
S2O82-(aq) + 2 I- (aq)  2 SO42- (aq) + I2 (aq)   Initial Concentration (M)        Initial Instantaneous                    [I- ]             [S2O82- ] Rate of Reaction (M s-1)           Expt 1:                       x 10-5 Expt 2:     0.060                   x 10-5 Expt 3:     0.030                 x 10-5 Determine m and n.

11 S2O82-(aq) + 2 I- (aq)  2 SO42- (aq) + I2 (aq)
  Initial Concentration (M)        Initial Instantaneous                    [I- ]             [S2O82- ] Rate of Reaction (M s-1)           Expt 1:                       x 10-5 Expt 2:     0.060                   x 10-5 Expt 3:     0.030                 x 10-5 n = 1

12 S2O82-(aq) + 2 I- (aq)  2 SO42- (aq) + I2 (aq)
  Initial Concentration (M)        Initial Instantaneous                    [I- ]             [S2O82- ] Rate of Reaction (M s-1)           Expt 1:                       x 10-5 Expt 2:     0.060                   x 10-5 Expt 3:     0.030                 x 10-5 m = 1

13 Using the Method of Initial Rates to determine the Rate Law of
S2O82-(aq) + 2 I- (aq)  2 SO42- (aq) + I2 (aq) First order with respect to S2O82- . First order with respect to I-. Overall Reaction Order: 2nd Order

14 Determine the rate constant, k, for
S2O82-(aq) + 2 I- (aq)  2 SO42- (aq) + I2 (aq)   Initial Concentration (M)        Initial Instantaneous                    [I- ]             [S2O82- ] Rate of Reaction (M s-1)           Expt 1:                       x 10-5 Expt 2:     0.060                   x 10-5 Expt 3:     0.030                 x 10-5


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