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Ch.14 - Chemical Kinetics
Brown - Chemistry: The Central Science 15th Edition
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Tutti i libri di testoBrown 15th EditionCh.14 - Chemical KineticsProblema 39b
Capitolo 14, Problema 39b

(b) How can you calculate the rate constant for a first-order reaction from the graph you made in part (a)?

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Identify the type of graph you have created in part (a). For a first-order reaction, you should have plotted the natural logarithm of the concentration of the reactant, \( \ln[A] \), versus time.
Understand that the slope of the line in this graph represents the negative of the rate constant, \( k \), for the reaction. This is derived from the integrated rate law for a first-order reaction: \( \ln[A] = -kt + \ln[A_0] \), where \( [A_0] \) is the initial concentration.
Determine the slope of the line from your graph. This can be done by selecting two points on the line and using the formula for the slope: \( \text{slope} = \frac{\Delta y}{\Delta x} \), where \( \Delta y \) is the change in \( \ln[A] \) and \( \Delta x \) is the change in time.
Recognize that the slope you calculated is equal to \( -k \). Therefore, the rate constant \( k \) is the negative of the slope value you obtained.
Ensure that the units of the rate constant \( k \) are consistent with the units of time used in your graph. For a first-order reaction, \( k \) typically has units of \( \text{time}^{-1} \), such as \( \text{s}^{-1} \) or \( \text{min}^{-1} \).

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First-Order Reactions

First-order reactions are chemical reactions where the rate is directly proportional to the concentration of one reactant. This means that if the concentration of the reactant doubles, the reaction rate also doubles. The rate law for a first-order reaction can be expressed as rate = k[A], where k is the rate constant and [A] is the concentration of the reactant.
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First-Order Reactions

Rate Constant (k)

The rate constant (k) is a proportionality factor in the rate law that is specific to a given reaction at a specific temperature. For first-order reactions, the rate constant can be determined from the slope of a plot of the natural logarithm of the concentration of the reactant versus time. The units of k for a first-order reaction are typically s⁻¹.
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Equilibrium Constant K

Graphical Analysis of Reaction Kinetics

Graphical analysis involves plotting data to visualize relationships between variables. For first-order reactions, plotting ln[A] versus time yields a straight line, where the slope of the line is equal to -k. This method allows for the determination of the rate constant from experimental data, making it a powerful tool in kinetics studies.
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Dimensional Analysis
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Consider the reaction of peroxydisulfate ion (S2O82-) with iodide ion (I-) in aqueous solution:

S2O82-(aq) + 3 I-(aq) → 2 SO42-(aq) + I3-(aq)

 At a particular temperature, the initial rate of disappearance of S2O82- varies with reactant concentrations in the following manner:

Experiment [S2O82-] (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.

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(a) The gas-phase decomposition of SO2Cl2, SO2Cl2(g) → SO2(g) + Cl2(g), is first order in SO2Cl2. At 600 K the half-life for this process is 2.3 × 105 s. What is the rate constant at this temperature?

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(b) At 320°C the rate constant is 2.2 × 10-5 s-1. What is the half-life at this temperature?

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(a) For the generic reaction A → B what quantity, when graphed versus time, will yield a straight line for a first-order reaction?

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Consider the gas-phase reaction between nitric oxide and bromine at 273°C: 2 NO(g) + Br2(g) → 2 NOBr(g). The following data for the initial rate of appearance of NOBr were obtained:

Experiment [NO] (M) [Br2] (M) Initial Rate (M/s)

1 0.10 0.20 24

2 0.25 0.20 150

3 0.10 0.50 60

4 0.35 0.50 735 

(b) Calculate the average value of the rate constant for the appearance of NOBr from the four data sets.

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