Chemical Kinetics

Chemical kinetics is the study and discussion of chemical reactions with respect to reaction rates, effect of various variables, re-arrangement of atoms, formation of intermediates etc. There are many topics to be discussed, and each of these topics is a tool for the study of chemical reactions. By the way, the study of motion is called kinetics, from Greek kinesis, meaning movement.

At the macroscopic level, we are interested in amounts reacted, formed, and the rates of their formation. At the molecular or microscopic level, the following considerations must also be made in the discusion of chemical reaction mechanism.

• Molecules or atoms of reactants must collide with each other in chemical reactions.
• The molecules must have sufficient energy (discussed in terms of activation energy) to initiate the reaction.
• In some cases, the orientation of the molecules during the collision must also be considered.

Reaction Rates

Chemical reaction rates are the rates of change in concentrations or amounts of either reactants or products. For changes in amounts, the units can be one of mol/s, g/s, lb/s, kg/day etc. For changes in concentrations, the units can be one of mol/(L s), g/(L s), %/s etc.

With respect to reaction rates, we may deal with average rates, instantaneous rates, or initial rates depending on the experimental conditions.

Thermodynamics and kinetics are two factors that affect reaction rates. The study of energy gained or released in chemical reactions is called thermodynamics, and such energy data are called thermodynamic data. However, thermodynamic data have no direct correlation with reaction rates, for which the kinetic factor is perhaps more important. For example, at room temperature (a wide range of temperatures), thermodynamic data indicates that diamond shall convert to graphite, but in reality, the conversion rate is so slow that most people think that diamond is forever.

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.

A BRIEF HISTORY OF CHEMICAL KINETICS

(Ref.: "The World of Physical Chemistry," by K. J. Laidler, Oxford Univ. Press, 1993)

•1864: Guldberg and Waage (Norway) formulated their "law of mass action," according to which the reaction "forces" are proportional to the product of the concentrations of the reactants:

K=[R]^r [S]^s/([A]^a [B]^b)

where a, b, r and s are the stoichiometric coefficients in the chemical equation A+B=R+S. So the rate of the forward reaction is proportional to [A]^a [B]^b and that of the reverse reaction is proportional to {R]^r [S]^s.

•1884: Van't Hoff (The Netherlands) published his "Studies of Chemical Dynamics" (Études de dynamique chimique), in which he generalized and further developed the work of Wilhelmy, Harcourt and Esson. In particular, he introduced the differential method of analysis.

•1889: Arrhenius (Sweden) further analyzed the temperature dependence of reaction rate, k = A exp(-B/T), and gave it an "energy barrier" interpretation; this is now called the "Arrhenius equation."

In the 20th century there have been significant developments in the theory of chemical kinetics (determination of rate constants and reaction orders from "first principles"). It is not yet possible, however, to predict the kinetic parameters for real-world chemical processes, and in reactor design we must rely on carefully planned and executed experiments

•1920: Langmuir (USA) studied the kinetics of surface reactions and introduced what is now known as the "Langmuir isotherm," which was further developed by Hinshelwood (UK) into the "Langmuir-Hinshelwood mechanism" of heterogeneous reactions.

 SOME (MOSTLY PEDAGOGICAL) LANDMARKS IN THE HISTORY OF CHEMICAL REACTION ENGINEERING (CRE)

•1934: 1st edition of Perry's "Chemical Engineers' Handbook" is published, but it contains nothing on reaction kinetics or reactor design. The closest section, written by Stillman, Taylor and Graves, is entitled "Indicators, Quantitative Analysis, Catalysis, Organic Chemistry."

•1950: 3rd edition of Perry's "Chemical Engineers' Handbook" is published. Section 4, entitled "Physical and Chemical Principles," written by Bryant, Elgin, Perry, Rossini and Whitwell, has a chapter on "Chemical reaction kinetics," containing a discussion of homogeneous and heterogeneous reactions (but not )

•1973: Section 4 of the 5th edition of "Chemical Engineers' Handbook," entitled "Reaction Kinetics, Reactor Design and Thermodynamics," written by Lin, Van Ness and Abbott, contains chapters on Fundamentals, Experimental techniques, Interpretation of laboratory and pilot-plant data, Scale-up methods and Reactor design.

•1984: Section 4 of the 6th edition "Perry's Chemical Engineers' Handbook," entitled "Reaction Kinetics, Reactor Design and Thermodynamics," written by Lin, Van Ness and Abbott, contains chapters on Fundamentals of chemical reaction systems, Experimental techniques for kinetic-data acquisition, Analyses of reaction kinetic data, Scale-up methods, and Reactor design (basic principles and data).

•1997: Section 7 in the 7th edition of "Perry's Chemical Engineers' Handbook," entitled "Reaction Kinetics," written by S. M. Walas, contains chapters on Reaction kinetics, Rate equations, Ideal reactors, Large scale operations, Acquisition of data, and Solved problems.  There is also a separate section on Chemical Reactors.

•1999: 3rd edition of Fogler's "Elements of Chemical Reaction Engineering."

This now appears to be the most "popular" textbook (see Shalabi et al., "Current Trends in Chemical Reaction Engineering Education" in Chem. Eng. Educ., 1996, pp. 146-149). A derivative of Levenspiel's classic textbook, and perhaps its successor, it emphasizes the multimedia approach: it has a CD-ROM, a web site and uses PolyMath quite a bit.

•2000: J. B. Butt publishes "Reaction Kinetics and Reactor Design," second edition, revised and expanded.

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. Nature and strength of bonds in reactant molecules greatly influences the rate of its transformation into products. The reactions which involve lesser bond rearrangement proceed faster than the reactions which involve larger bond rearrangement.

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 are 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 ananalogy, 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 a very important role in reactions according to the collision theory of chemical reactions, 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 more frequently by being in closer contact at any given point in time. Think of 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 it causes these collisions to happen more often, increasing the reaction rate (Figure 1.1).

Temperature

Temperature usually has a major effect on the rate of a chemical reaction. Molecules at a higher temperature have more thermal energy. Although collision frequency is greater at higher temperatures, this alone contributes only a very small proportion to the increase in rate of reaction. Much more important is the fact that the proportion of reactant molecules with sufficient energy to react (energy greater than activation energy: E > E_a) is significantly higher and is explained in detail by the Maxwell–Boltzmann distribution of molecular energies.

The 'rule of thumb' that the rate of chemical reactions doubles for every 10 °C temperature rise is a common misconception. This may have been generalized from the special case of biological systems, where the Q₁₀ (temperature coefficient) is often between 1.5 and 2.5.

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

Generic potential energy diagram showing the effect of a catalyst in an hypothetical endothermic chemical reaction. The presence of the catalyst opens a different reaction pathway (shown in red) with a lower activation energy. The final result and the overall thermodynamics are the same.

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 kineticsdescribe the rate of enzyme mediated reactions. A catalyst does not affect the position of the equilibria, as the catalyst speeds up the backward and forward reactions equally.

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.

Pressure

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.

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.

A Combination of Temperature and Isothermal Variations in the Viscosity of a Medium as a Method of Measuring a Low Activation Barrier. Relative Activation Energy of Stereochemical Inversion upon Denitrogenation of Bicyclic Azoalkane

IMPORTANCE: 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.

Chemical kinetics has many applications. Some of them are;

. Used as manufacturer of medicine,

. Used as synthesis of organic and inorganic compounds.

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. 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 Chemical equilibrium and chemical kinetics are impor-

tant concepts in general chemistry, both in secondary in higher education. The study of chemical equilibrium

aims at a better understanding of incomplete, reversible

chemical reactions that lead to a stable mixture of re-

education (e.g., UK and the Netherlands) as well as the stability of this dynamic equilibrium.