Chapter 6: Chemical Kinetics
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- 502625
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Introduction
In this chapter, we will discuss the fascinating field of chemical kinetics, which focuses on the rates at which chemical reactions occur and the factors that influence them. Unlike thermodynamics, which tells us whether a reaction is energetically favorable (as discussed in the following Chapter), kinetics answers the question of how fast a reaction will proceed. By understanding the principles of reaction rates, we can predict, measure, and even control the speed of chemical reactions, including understanding complexities in the mechanism of a reaction - the steps in which it occurs.
For example, catalytic converters accelerate the breakdown of harmful emissions like carbon monoxide (CO) and nitrogen oxides (NOx) into safer products such as carbon dioxide (CO2) and nitrogen gas (N2). A typical reaction is:
2 CO (g) + O2 (g) → 2 CO2 (g)
The catalyst, typically platinum, palladium, or rhodium, lowers the activation energy required for these reactions, ensuring they occur rapidly even at engine exhaust temperatures.
Figure 6.1: The purpose of using metals like platinum in catalytic converters is to create a surface on which chemisorption can occur. Chemisorption is a process resulting from a chemical bond between adsorbate molecules (A and B) and specific surface locations on a material, known as active sites. (CC BY-SA, 2011: Brazosport College via WikiMedia Commons)
In the case of catalytic converters (and other pollutant-"scrubbing" technologies), clusters of metal-oxide molecules react with gases like CO and NOx such that they become oxidized to products like CO2. Since the reaction needs a surface cluster in order to occur, the rate of the reaction is dictated by the number of metallic sites, allowing for a controlled mechanism that is more expedient and thus efficient.
- 6.1: The Rate of a Chemical Reaction
- Reaction rates are reported as either the average rate over a period of time or as the instantaneous rate at a single time. Reaction rates can be determined over particular time intervals or at a given point in time.
- 6.2: Reaction Mechanisms
- A balanced chemical reaction does not necessarily reveal either the individual elementary reactions by which a reaction occurs or its rate law. A reaction mechanism is the microscopic path by which reactants are transformed into products. Each step is an elementary reaction. Species that are formed in one step and consumed in another are intermediates. Each elementary reaction can be described in terms of its molecularity. The slowest step in a reaction mechanism is the rate-determining step.
- 6.3: Catalysis
- Catalysts participate in a chemical reaction and increase its rate. They do not appear in the reaction’s net equation and are not consumed during the reaction. Catalysts allow a reaction to proceed via a pathway that has a lower activation energy than the uncatalyzed reaction. In heterogeneous catalysis, catalysts provide a surface to which reactants bind in a process of adsorption. In homogeneous catalysis, catalysts are in the same phase as the reactants. Enzymes are biological catalysts.