Everything about Chemical Kinetics totally explained
Chemical kinetics, also known as reaction kinetics, is the study of
rates of chemical processes. Chemical kinetics includes investigations of how different experimental conditions can influence the speed of a chemical reaction and yield information about the
reaction's mechanism and
transition states, as well as the construction of mathematical models that can describe the characteristics of a chemical reaction. In 1864,
Peter Waage and
Cato Guldberg pioneered the development of chemical kinetics by formulating the
law of mass action, which states that the speed of a chemical reaction is proportional to the quantity of the reacting substances.
Rate of reaction
Chemical kinetics deals with the experimental determination of
reaction rates from which
rate laws and
rate constants are derived. Relatively simple
rate laws exist for
zero order reactions (for which reaction rates are independent of concentration),
first order reactions, and
second order reactions, and can be derived for others. In consecutive reactions the
rate-determining step often determines the kinetics. In consecutive first order reactions, a
steady state approximation can simplify the
rate law. The
activation energy for a reaction is experimentally determined through the
Arrhenius equation and the
Eyring equation. The main factors that influence the
reaction rate include: the
physical state of the reactants, the
concentrations of the reactants, the
temperature at which the reaction occurs, and whether or not any
catalysts are present in the reaction.
Factors affecting reaction rate
Nature of the Reactants
Depending upon what substances are reacting, the time varies. Acid reactions, the formation of
salts, and
ion exchange are fast reactions. When covalent bond formation takes place between the molecules and when large molecules are formed, the reactions tend to be very slow.
Physical State
The
physical state (
solid,
liquid, or
gas) of a reactant is also an important factor of the rate of change. When reactants are in the same
phase, as in
aqueous solution, thermal motion brings them into contact. However, when they're in different phases, the reaction is limited to the interface between the reactants. Reaction can only occur at their area of contact, in the case of a liquid and a gas, at the surface of the liquid. Vigorous shaking and stirring may be needed to bring the reaction to completion. This means that the more finely divided a solid or liquid reactant, the greater its
surface area per unit
volume, and the more contact it makes with the other reactant, thus the faster the reaction. To make an analogy, for example, when one starts a fire, one uses wood chips and small branches—one doesn't start with large logs right away. In organic chemistry
On water reactions are the exception to the rule that homogeneous reactions take place faster than heterogeneous reactions.
Concentration
Concentration plays an important role in reactions according to the
collision theory of chemical reactions, this is because molecules must collide in order to react together. As the concentration of the reactants increases, the
frequency of the molecules colliding increases, striking each other faster by being in closer contact at any given point in time. Imagine two reactants being in a closed container. All the molecules contained within are colliding constantly. By increasing the amount of one or more of the reactants you cause these collisions to happen more often, increasing the reaction rate (Figure 1.1).
Temperature
Temperature usually has a major effect on the speed of a reaction. Molecules at a higher temperature have more
thermal energy. When reactants (reactant + reactant → product) in a chemical reaction are heated, the more energetic atoms or molecules have a greater probability to collide with one another. Thus, more collisions occur at a higher temperature, making a product in a chemical reaction. More importantly however, is the fact that at higher temperatures molecules have more vibrational energy, that is, atoms are vibrating much more violently, so raising the temperature not only increases the number of collisions but also collisions that can result in rearrangement of atoms within the reactant molecules. For example, a
refrigerator slows down the speed of the rate of reaction since it cools the molecules. On the other hand, an
oven gives heat (energy) to the molecules which in turn speeds up the rate of reaction, cooking the food faster.
A reaction's kinetics can also be studied with a
temperature jump approach. This involves using a sharp rise in temperature and observing the relaxation rate of an equilibrium process.
Catalysts
A
catalyst is a substance that accelerates the rate of a chemical reaction but remains
chemically unchanged afterwards. The catalyst increases rate reaction by providing a different
reaction mechanism to occur with a lower
activation energy. In
autocatalysis a reaction product is itself a catalyst for that reaction leading to
positive feedback. Proteins that act as catalysts in biochemical reactions are called
enzymes.
Michaelis-Menten kinetics describe the
rate of enzyme mediated reactions.
In certain organic molecules specific substituents can have an influence on reaction rate in
neighbouring group participation.
Agitating or mixing a solution will also accelerate the rate of a chemical reaction, as this gives the particles greater kinetic energy, increasing the number of collisions between reactants and therefore the possibility of successful collisions.
Increasing the pressure in a gaseous reaction will increase the number of collisions between reactants, increasing the rate of reaction. This is because the
activity of a gas is directly proportional to the partial pressure of the gas. This is similar to the effect of increasing the concentration of a solution.
A catalyst doesn't affect the position of the equilibria, as the catalyst speeds up the backward and forward reactions equally.
Equilibrium
While chemical kinetics is concerned with the rate of a chemical reaction,
thermodynamics determines the extent to which reactions occur. In a
reversible reaction, chemical equilibrium is reached when the rates of the forward and reverse reactions are equal and the concentrations of the
reactants and
products no longer change. This is demonstrated by, for example, the
Haber-Bosch process for combining nitrogen and hydrogen to produce ammonia.
Chemical clock reactions such as the
Belousov-Zhabotinsky reaction demonstrate that component concentrations can oscillate for a long time before finally reaching equilibrium.
Free energy
In general terms, the
free energy change (ΔG) of a reaction determines if a chemical change will take place, but kinetics describes how fast the reaction is. A reaction can be very
exothermic and have a very positive
entropy change but won't happen in practice if the reaction is too slow. If a reactant can produce two different products, the thermodynamically most stable one will generally form except in special circumstances when the reaction is said to be under
kinetic reaction control. The
Curtin-Hammett principle applies when determining the product ratio for two reactants interconverting rapidly, each going to a different product. It is possible to make predictions about reaction rate constants for a reaction from
Free-energy relationships.
The
kinetic isotope effect is the difference in the rate of a chemical reaction when an atom in one of the reactants is replaced by one of its
isotopes.
Chemical kinetics provides information on
residence time and
heat transfer in a
chemical reactor in
chemical engineering and the
molar mass distribution in
polymer chemistry.
Applications
The mathematical models that describe chemical reaction kinetics provide chemists and chemical engineers with tools to better understand and describe chemical processes such as food decomposition, microorganism growth, stratospheric ozone decomposition, and the complex chemistry of biological systems. These models can also be used in the design or modification of chemical reactors to optimize product yield, more efficiently separate products, and eliminate environmentally harmful by-products. When performing
catalytic cracking of heavy hydrocarbons into gasoline and light gas, for example, kinetic models can be used to find the temperature and pressure at which the highest yield of heavy hydrocarbons into gasoline will occur.
Further Information
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