Reaction Rates
Reaction rates refer to the speed at which reactants are converted into products in a chemical reaction. The study of reaction rates, or chemical kinetics, focuses on the factors influencing these rates, including the concentration of reactants, temperature, pressure, and the presence of catalysts. In a closed system, the likelihood of reactant particles colliding and reacting increases with higher concentrations, leading to faster reactions. The complexity of reaction mechanisms means that each reaction can have unique rates, with factors such as activation energy and molecular geometry playing significant roles.
To determine reaction rates, scientists often analyze the concentrations of reactants and products over time using various analytical methods, such as chromatography and spectrometry. The rate at which a reaction occurs can provide insights into its mechanism and can indicate whether a catalyst is involved. Additionally, reactions can reach a state of equilibrium, where the rates of the forward and reverse reactions are equal, maintaining constant concentrations of reactants and products. Understanding reaction rates is particularly important in biological systems, where it impacts processes such as metabolism and the effects of drugs, highlighting the significance of kinetics in both chemistry and health sciences.
Reaction Rates
FIELDS OF STUDY: Physical Chemistry; Chemical Engineering; Biochemistry
ABSTRACT
The study of chemical kinetics examines the behavior of chemical reaction systems over time. The rate of a chemical reaction depends on the concentration of the reactants and products and is affected by the presence and activity of materials that catalyze the particular process. This is of central importance in biological systems.
Visualizing the Rate of Reaction
Reactions rates are typically studied as a reaction is carried out in a closed system. The higher the concentration of reactant materials in the system, the more rapidly the reaction can occur in a specific amount of time. The motion of atoms and molecules is often compared to the motion of a blindfolded person walking about in a room. The more people there are in the room, the more likely it is that the blindfolded person will bump into someone else. Similarly, the more reactant atoms and molecules there are in a reaction mixture, the more often they will bump into each other and react. In its simplest definition, chemical kinetics is counting the number of times collisions between atoms and molecules in a reaction mixture result in a reaction taking place over a specific amount of time.
The occurrence of reactions is governed by a number of factors, including the activation energy of the reaction, temperature, pressure, the geometry of the molecules, and the reduction-oxidation (redox) potential of the reacting components. Accordingly, different reactions occur at different rates. One could, for example, compare the rates at which 1-chloropropane and 1-bromopropane undergo the same substitution reaction under the same conditions and with the same reagent. Even though these are similar halogen elements, they exhibit different rates of reaction. Furthermore, the same element can exhibit measurably different rates of reaction if the reaction is carried out using two different isotopes, such as protium and deuterium. This process is termed the "isotope effect."
Determining the Rate of a Reaction
Reaction rates are determined by direct observation and measurement. Typically, from the moment the reagents of a reaction are mixed, small samples of the live reaction mixture are extracted at intervals and analyzed. The disappearance of starting materials and the appearance of product materials over time shows the extent to which the reaction has progressed. Analytical methods such as high-performance liquid chromatography, gas chromatography, nuclear magnetic resonance spectrometry, and ultraviolet-visible spectrophotometry are routinely used to monitor the progress of a reaction. The measured changes of the absorbance peaks indicate the relative amounts of each component. Relating these to the known quantity of each component at the beginning of the reaction indicates the actual quantities of each. The measured results will always exhibit a mathematical relationship to the concentration of each individual component of the reaction mixture.
The rate of a particular reaction can provide insight into the mechanism by which the reaction proceeds. All reactions occur only at the rate of the slowest elementary step in the reaction mechanism. When more than one proposed mechanism is possible for an overall reaction, the rate of the reaction can demonstrate whether or not a catalyst is involved in the mechanism. The order of the reaction is also an important indicator of the probable mechanism. A reaction that exhibits first-order kinetics, for example, cannot occur by a mechanism that depends on just one single reactant because second-order reactions require the participation of two components in the rate-determining step.
The general form of the relationship for a first-order reaction, such as A → B, is
rate = k[A]
where k is the unique rate constant for that particular reaction and the rate is the measured rate of the reaction, specific to the conditions under which the reaction has been carried out. A reaction of the form A + B → C will have a rate equation of the form
rate = k[A][B]
A reaction of the form 2A + B → C has the kinetic expression
rate = k[A]2[B]
and so on for each individual reaction. The rate expression reflects the balanced chemical equation for the reaction, with the coefficients of each reactant becoming the exponent of its concentration term in the rate equation. Determining the rate equation for an unknown reaction can thus reveal information about the overall reaction process.
The rate expression for a complex reaction can quickly become too complex to be useful. In such cases, it is beneficial to determine the rate of reaction using a reaction mixture that has been saturated with so much of one of the components that its concentration effectively does not change throughout the course of the reaction. In this way, it is possible to force a more complex reaction to behave as though it were a first-order reaction. This method is particularly useful for determining the maximum rate of a reaction that involves a catalyst and especially for the study of enzyme-catalyzed reactions. Because the rate of such reactions is quite variable (because the amount of enzyme-substrate activated complex that is present in the reaction mixture is inconsistent), saturating the reaction mixture ensures that the amount of enzyme-substrate complex is constant and at its maximum value throughout the course of the reaction.
Equilibrium and Rate of Reaction
For reactions that are readily reversible, equilibrium will be attained at some point, unless the products are somehow isolated from the mixture or more reactant materials are added. At equilibrium, the relative proportions of reactants and products are constant and do not change over time. The reaction does not cease, but rather is exactly balanced by the reverse reaction, and reactants are forming products at exactly the same rate that products are reverting back to reactants. In rate measurements, the relative concentrations of components change rapidly at the beginning of the reaction and trend to constant values as the system approaches equilibrium.
Reaction Rates in Biological Systems
Understanding the rates of reactions in biological systems is crucial to the design and safe use of pharmaceuticals and other compounds. All materials are chemical in nature, and the ingestion of anything at all, from simple water to the most complex drugs, must pass through the metabolic system. The rate at which metabolic reactions process each substance determines the length of time that the material has an effect. It also determines the length of time required to prevent overdosing. A common application of this principle involves the consumption of alcohol, which is metabolized at a specific rate. Consuming alcohol faster than it can be metabolized results in overdosing, leading to impaired judgment, which can have fatal consequences, and even to alcohol poisoning, which may be equally deadly.
PRINCIPAL TERMS
- chemical kinetics: the branch of chemistry that studies the various factors that affect rates of chemical reactions.
- closed system: a physical or chemical reaction system defined by certain boundary conditions that prevent any components, reactants, or products from entering or exiting the system.
- equilibrium: the state that exists when the forward activity of a process is exactly equal to the reverse activity of that process.
- rate-determining step: in any multistep process, the step that proceeds at the slowest rate compared to the other steps in the process, thus determining the maximum rate at which the overall process takes place.
- reaction mechanism: the sequence of electron and orbital interactions that occurs during a chemical reaction as chemical bonds are broken, made, and rearranged.
Bibliography
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Skoog, Douglas A., Donald M. West, and F. James Holler. Fundamentals of Analytical Chemistry. 9th ed. Orlando: Harcourt, 2014. Print.