# 28: Chemical Kinetics I - Rate Laws

• 28.1: The Time Dependence of a Chemical Reaction is Described by a Rate Law
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!
• 28.2: Rate Laws Must Be Determined Experimentally
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.
• 28.3: First-Order Reactions Show an Exponential Decay of Reactant Concentration with Time
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.
• 28.4: Different Rate Laws Predict Different Kinetics
It is possible to determine the reaction order using data from a single experiment by plotting the concentration of the reactant as a function of time. Because of the characteristic shapes of such lines for zero-order, first-order, and second-order reactions, the graphs can be used to determine the reaction order of an unknown reaction.
• 28.5: Reactions can also be Reversible
Many chemical reactions are reversible, in that the products formed during the process react to re-form the original reactants. These reversible reactions eventually reach a state of dynamic equilibrium, in which the rate of the overall forward process is equal to the rate of the overall reverse process.
• 28.6: The Rate Constants of a Reversible Reaction Can Be Determined Using Relaxation Techniques
Many chemical reactions are complete in less than a few seconds, which makes the rate of reaction difficult to determine. In these cases, the relaxation methods can be used to determine the rate of the reaction.
• 28.7: Rate Constants Are Usually Strongly Temperature Dependent
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.
• 28.8: Transition-State Theory Can Be Used to Estimate Reaction Rate Constants
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.
• 28.E: Chemical Kinetics I - Rate Laws (Exercises)