# 11: Chemical Kinetics I

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- 84364

Chemical kinetics is the study of how fast chemical reactions proceed from reactants to products. This is an important topic because while thermodynamics will tell us about the direction of spontaneous change, it is silent as to how fast processes will occur. But additionally, the power of studying reaction rates is that it gives us insight into the actual pathways chemical processes follow to proceed from reactants to products.

- 11.1: Reaction Rate
- The rate of a chemical reaction (or the reaction rate) can be defined by the time needed for a change in concentration to occur. But there is a problem in that this allows for the definition to be made based on concentration changes for either the reactants or the products. Plus, due to stoichiometric concerns, the rates at which the concentrations are generally different!

- 11.2: Measuring Reaction Rates
- There are several methods that can be used to measure chemical reactions rates. A common method is to use spectrophotometry to monitor the concentration of a species that will absorb light. If it is possible, it is preferable to measure the appearance of a product rather than the disappearance of a reactant, due to the low background interference of the measurement.

- 11.3: Rate Laws
- A rate law is any mathematical relationship that relates the concentration of a reactant or product in a chemical reaction to time. Rate laws can be expressed in either derivative (or ratio, for finite time intervals) or integrated form.

- 11.4: 0th order Rate Law
- If the reaction follows a zeroth order rate law, it can be expressed in terms of the time-rate of change of [A]. The solution of the differential equation suggests that a plot of concentration as a function of time will produce a straight line.

- 11.5: 1st order rate law
- If the reaction follows a first order rate law, it can be expressed in terms of the time-rate of change of [A]. The solution of the differential equation suggests that a plot of log concentration as a function of time will produce a straight line.

- 11.6: 2nd order Rate Laws
- If the reaction follows a second order rate law, it can be expressed in terms of the time-rate of change of [A]. The solution of the differential equation suggests that a plot of 1/concentration as a function of time will produce a straight line.

- 11.7: The Method of Initial Rates
- The method of initial rates is a commonly used technique for deriving rate laws. As the name implies, the method involves measuring the initial rate of a reaction. The measurement is repeated for several sets of initial concentration conditions to see how the reaction rate varies. This might be accomplished by determining the time needed to exhaust a particular amount of a reactant (preferably one on which the reaction rate does not depend!)

- 11.8: The Method of Half-Lives
- Another method for determining the order of a reaction is to examine the behavior of the half-life as the reaction progresses. The half-life can be defined as the time it takes for the concentration of a reactant to fall to half of its original value. The method of half-lives involved measuring the half-life’s dependence on concentration.

- 11.9: Temperature Dependence
- In general, increases in temperature increase the rates of chemical reactions. It is easy to see why, since most chemical reactions depend on molecular collisions. And as we discussed in Chapter 2, the frequency with which molecules collide increases with increased temperature. But also, the kinetic energy of the molecules increases, which should increase the probability that a collision event will lead to a reaction. An empirical model was proposed by Arrhenius to account for this phenomenon.

- 11.10: Collision Theory
- Collision Theory was first introduced in the 1910s by Max Trautz (Trautz, 1916) and William Lewis (Lewis, 1918) to try to account for the magnitudes of rate constants in terms of the frequency of molecular collisions, the collisional energy, and the relative orientations of the molecules involved in the collision.

- 11.11: Transition State Theory
- Transition state theory was proposed in 1935 by Henry Erying, and further developed by Merrideth G. Evans and Michael Polanyi (Laidler & King, 1983), as another means of accounting for chemical reaction rates. It is based on the idea that a molecular collision that leads to reaction must pass through an intermediate state known as the transition state.

- 11.E: Chemical Kinetics I (Exercises)
- Exercises for Chapter 11 "Chemical Kinetics I" in Fleming's Physical Chemistry Textmap.

- 11.S: Chemical Kinetics I (Summary)
- Summary for Chapter 11 "Chemical Kinetics I" in Fleming's Physical Chemistry Textmap.

## Contributors and Attributions

Patrick E. Fleming (Department of Chemistry and Biochemistry; California State University, East Bay)