10: Chemical Kinetics
- Page ID
- 453696
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- 10.1: 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!
- 10.2: 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!)
- 10.3: 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.
- 10.4: 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.
- 10.5: 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.
- 10.6: 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.
- 10.7: Complex Rate Laws
- It is essential to specify that the order of a reaction and its molecularity are equal only for elementary reactions. Reactions that follow complex laws are composed of several elementary steps, and they usually have non-integer reaction orders, for at least one of the reactants.
- 10.8: Reaction Mechanisms
- A reaction mechanism is a set of elementary reactions steps, that when taken in aggregate define a chemical pathway that connects reactants to products. An elementary reaction is one that proceeds by a single process, such a molecular (or atomic) decomposition or a molecular collision.
- 10.9: The Connection between Reaction Mechanisms and Reaction Rate Laws
- The great value of chemical kinetics is that it can give us insights into the actual reaction pathways (mechanisms) that reactants take to form the products of reactions. Analyzing a reaction mechanism to determine the type of rate law that is consistent (or not consistent) with the specific mechanism can give us significant insight.
- 10.10: The Rate Determining Step Approximation
- The rate determining step approximation is one of the simplest approximations one can make to analyze a proposed mechanism to deduce the rate law it predicts. Simply stated, the rate determining step approximation says that a mechanism can proceed no faster than its slowest step.
- 10.11: The Steady-State Approximation
- One of the most commonly used and most attractive approximations is the steady state approximation. This approximation can be applied to the rate of change of concentration of a highly reactive (short lived) intermediate that holds a constant value over a long period of time.
- 10.12: The Equilibrium Approximation
- In many cases, the formation of a reactive intermediate (or even a longer lived intermediate) involves a reversible step. This is the case if the intermediate can decompose to reform reactants with a significant probability as well as moving on to form products. In many cases, this will lead to a pre-equilibrium condition in which the equilibrium approximation can be applied.
- 10.14: Collisions with Other Molecules
- A major concern in the design of many experiments is collisions of gas molecules with other molecules in the gas phase. For example, molecular beam experiments are often dependent on a lack of molecular collisions in the beam that could degrade the nature of the molecules in the beam through chemical reactions or simply being knocked out of the beam.
- 10.15: 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.
- 10.16: The Lindemann Mechanism
- The Lindemann mechanism was one of the first attempts to understand unimolecular reactions. Lindemann mechanisms have been used to model gas phase decomposition reactions. Although the net formula for a decomposition may appear to be first-order (unimolecular) in the reactant, a Lindemann mechanism may show that the reaction is actually second-order (bimolecular).
- 10.17: Some Reaction Mechanisms Involve Chain Reactions
- Chain reactions usually consist of many repeating elementary steps, each of which has a chain carrier. Once started, chain reactions continue until the reactants are exhausted. Fire and explosions are some of the phenomena associated with chain reactions. The chain carriers are some intermediates that appear in the repeating elementary steps. These are usually free radicals.
- 10.18: Catalysis
- There are many examples of reactions that involve catalysis. One that is of current importance to the chemistry of the environment is the catalytic decomposition of ozone
- 10.19: The Michaelis-Menten Mechanism
- The Michaelis-Menten mechanism (Michaelis & Menten, 1913) is one which many enzyme mitigated reactions follow. The basic mechanism involves an enzyme (E, a biological catalyst) and a substrate (S) which must connect to form an enzyme-substrate complex (ES) in order for the substrate to be degraded (or augmented) to form a product (P).
- 10.20: Isotherms are Plots of Surface Coverage as a Function of Gas Pressure at Constant Temperature
- The Langmuir isotherm was developed by Irving Langmuir in 1916 to describe the dependence of the surface coverage of an adsorbed gas on the pressure of the gas above the surface at a fixed temperature. Whilst the Langmuir isotherm is one of the simplest, it still provides a useful insight into the pressure dependence of the extent of surface adsorption.
- 10.21: Atoms and Molecules can Physisorb or Chemisorb to a Surface
- We can address the question of what happens when a molecule becomes adsorbed onto a surface at two levels; specifically we can aim to identify the nature of the adsorbed species and its local adsorption geometry (i.e., its chemical structure and co-ordination to adjacent substrate atoms) the overall structure of the extended adsorbate/substrate interface (i.e., the long range ordering of the surface).
- 10.22: Using Langmuir Isotherms to Derive Rate Laws for Surface-Catalyzed Gas-Phase Reactions
- It is possible to predict how the kinetics of certain heterogeneously-catalysed reactions might vary with the partial pressures of the reactant gases above the catalyst surface by using the Langmuir isotherm expression for equilibrium surface coverages.
- 10.23: The Structure of a Surface is Different from that of a Bulk Solid
- The kinetics and thermodynamics of the chemical and physical processes that occur on the surface of a solid are greatly dependent on the structure of the surface. Few, if any surfaces are perfectly flat, and thus the cavities, protrusions, ridges, and edges of the surface must be treated differently when studying chemisorption and physisorption.
- 10.24: The Haber-Bosch Reaction Can Be Surface Catalyzed
- The Haber-Bosch process for the synthesis of ammonia is one of the most important catalyzed syntheses in the chemical industry. The process takes advantage of the low activation energies required for the dissociative chemisorption of nitrogen and hydrogen molecules that have been physisorbed onto a metal oxide surface.