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Chemical Kinetics Method of Initial Rates
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Determine the Rate Law (order of a reaction) experimentally
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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!
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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.
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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.
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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
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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
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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
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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.
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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.
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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
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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
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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
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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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