How Chemical Reactions Take Place

Weknowfromelementary chemistry thatreactionstakeplacewhenmoleculescollidewith one another. We also know that reactions often take place faster athighertemperature and thatacatalystoftenimprovestheratefurther.Enzymesaretheprototypicalnaturalcatalysts andtheyworkbyorientingmoleculesalongspecificdirectionsthatarepreferredforreaction. 

Wecansay then that reactionshavestrongtemperature and orientational dependence, and thatcollisionsalonearenotenoughforreactiontotakeplace. 

Wehavenotdiscussedtheissueofthedimensionsoftherateconstant.Thereasonisthatthe dimensionschange with thechange in the rate dependence uponconcentration. Hence we havepostponedconsiderationofdimensionsuntilwereachthatpoint. 

Thisissimplestoichiometricallyandwecanassumethatitisirreversible,whichmeansthat reactionproceedstotheright-handsidecompletelyandthatproduct D doesnotreturnto A andB.Thecomponent mass balances become  

The next step is to find kinetics for the rate of reaction that canbe used as a constitutive 

We know from elementary chemistry that reactions take place when molecules collide with one another. We also know that reactions often take place faster at higher temperature and that a catalyst often improves the rate further. Enzymes are the prototypical natural catalysts and they work by orienting molecules along specific directions that are preferred for reaction. 

We can say then that reactions have strong temperature and orientational dependence, and that collisions alone are not enough for reaction to take place. 

We can get a quantitative sense for this by turning once again to the kinetic theory of gases to compute the number of collisions that take place per unit volume and time at fixed temperature and pressure. The collision number Zab between two molecules A and B is given as  

This is the product of the molecular cross section at collision, the mean speed, and the product of the number concentrations of A and B. We can compute this value for standard conditions  

Chapter 7 Reacting Systems—Kinetics and Batch Reactors 

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To understand both organic and biological chemistry, it s necessary to know not just what occurs, but also why and how chemical reactions take place. In this chapter, we ll start with an overview of the fundamental kinds of organic reactions, we ll see why reactions occur, and we ll see how reactions can be described. Once this background is out of the way, we ll then be ready to begin studying the details of organic chemistry. 

We are now ready to build a model of how chemical reactions take place at the molecular level. Specifically, our model must account for the temperature dependence of rate constants, as expressed by the Arrhenius equation it should also reveal the significance of the Arrhenius parameters A and Ea. Reactions in the gas phase are conceptually simpler than those in solution, and so we begin with them. 

Summary We ve been discussing chemical reactions for several chapters. In the Kinetics chapter you saw how chemical reactions take place and some of the factors that affect the reaction s speed. In this chapter we will discuss another aspect of chemical reactions equilibrium. 

As described above, the progress of reaction dynamics showed how chemical reactions take place. Is it possible to apply the methods and techniques of reaction dynamics to chemical synthesis If we can activate all the molecules at once, the chemical reaction takes place and finishes within several hundred femtoseconds. If it is possible, synthesis can be complete within several hundred femtoseconds. Such a method leads to the ultimate acceleration of chemical synthesis. 

Space and astronomy is the oldest of all sciences. Long before humans understood the composition of rocks and minerals, knew how chemical reactions take place, or even discovered how their own bodies were constructed and operated, they knew of the existence of bodies beyond the Earth s atmosphere other planets, stars, and a variety of strange objects for which they had only simple explanations. The knowledge that early astronomers had of the skies was quite remarkable. They were able to predict the motion of stars, the arrival of seasons, the appearance of eclipses, and other astronomical phenomena with an accuracy that is quite astonishing to modern scientists. 

Thermodynamics and kinetics enable us to explain why and how chemical reactions take place. This type of information is important in many areas of ceramics, but particularly in ceramic processing. Traditional processing of ceramic components is carried out at high temperatures because the kinetics would be too slow otherwise. Kinetics is often closely linked to economics. Processes that are slow are usually expensive. 

Another theory that also helps us understand how chemical reactions take place is called transition state theory (TST), the development of which is attributed primarily to the American chemist Henry Eyring (1901-81), who spent most of his career at the University of Utah in Salt Lake City. In TST, we picture an energy barrier that the reacting molecules must pass over before they can become products. Consider an analogy. 

Although thermodynamics can be used to predict the direction and extent of chemical change, it does not tell us how the reaction takes place or how fast. We have seen that some spontaneous reactions—such as the decomposition of benzene into carbon and hydrogen—do not seem to proceed at all, whereas other reactions—such as proton transfer reactions—reach equilibrium very rapidly. In this chapter, we examine the intimate details of how reactions proceed, what determines their rates, and how to control those rates. The study of the rates of chemical reactions is called chemical kinetics. When studying thermodynamics, we consider only the initial and final states of a chemical process (its origin and destination) and ignore what happens between them (the journey itself, with all its obstacles). In chemical kinetics, we are interested only in the journey—the changes that take place in the course of reactions. 

We stressed in Section 13.3 that we cannot in general write a rate law from a chemical equation. The reason is that all but the simplest reactions are the outcome of several, and sometimes many, steps called elementary reactions. Each elementary reaction describes a distinct event, often a collision of particles. To understand how a reaction takes place, we have to propose a reaction mechanism, a sequence of elementary reactions describing the changes that we believe take place as reactants are transformed into products. 

There are two principal chemical concepts we will cover that are important for studying the natural environment. The first is thermodynamics, which describes whether a system is at equilibrium or if it can spontaneously change by undergoing chemical reaction. We review the main first principles and extend the discussion to electrochemistry. The second main concept is how fast chemical reactions take place if they start. This study of the rate of chemical change is called chemical kinetics. We examine selected natural systems in which the rate of change helps determine the state of the system. Finally, we briefly go over some natural examples where both thermodynamic and kinetic factors are important. This brief chapter cannot provide the depth of treatment found in a textbook fully devoted to these physical chemical subjects. Those who wish a more detailed discussion of these concepts might turn to one of the following texts Atkins (1994), Levine (1995), Alberty and Silbey (1997). 

In this section, you used collision theory and transition state theory to explain how reaction rates are affected hy various factors. You considered simple reactions, consisting of a single-step collision between reactants. Not all reactions are simple, however. In fact, most chemical reactions take place via several steps, occurring in sequence. In the next section, you will learn about the steps that make up reactions and discover how these steps relate to reaction rates. 

Nearly all chemical reactions take place in solution. Further it is well-known that the solvent can have strong influence on reactions. Due to the influence of the solvent a question arises How many molecules are needed to reproduce the effects of the solvent on the reaction (115) Different molecular dynamics approaches for solvent effects have been reported in literature. There are a... 

Regardless of how fast a chemical reaction takes place, it usually reaches an equilibrium position at which there appears to be no further change, because the reactants are being reformed from the products at the same rate at which they are reacting to form the products. This position of equilibrium commonly is characterized at a given temperature by a constant K,., called the equilibrium constant (see Chapter 16). The equilibrium constant commonly is measured at several different temperatures for a given reaction, because these values of are related to the Kelvin temperature (T) at which they are measured the relationship is... 

So far, we have not seen how to calculate the work done by a system except for the simple case of raising a weight. At this point, we start to consider real chemical reactions taking place in containers of various kinds and begin to explore how energy flows from the reaction system into the surroundings, or vice versa. 

In most cases in the real world of chemistry both currently and historically, the reverse order is followed that is, the mechanism is deduced with certainty only after the stereoselectivity has been determined. Because the stereochemical outcome of a chemical reaction is experimentally determined, it provides a powerful tool for examining the intimate spatial details of transition states and for determining how a reaction takes place at the molecular level. As such stereochemical studies have had a huge impact on the elucidation of reaction mechanisms. 

The chemical reactions taking place in fireworks, fireflies, and fuel cells are just the tip of the iceberg. But what exactly is a chemical reaction, and how does one happen ... 

Life depends on chemical reactions. Chemical reactions take place when plants and animals grow, digest their food, and even when they rest. Some of the chemical reactions that occur in nature take place in the most extreme conditions on the planet, such as near deep-sea vents or in Antarctica. Some reactions are so complex that scientists are not yet sure how they happen. 

Another classification of chemical reactors is according to the phases being present, either single phase or multiphase reactors. Examples of multiphase reactors are gas liquid, liquid-liquid, gas solid or liquid solid catalytic reactors. In the last category, all reactants and products are in the same phase, but the reaction is catalysed by a solid catalyst. Another group is gas liquid solid reactors, where one reactant is in the gas phase, another in the liquid phase and the reaction is catalysed by a solid catalyst. In multiphase reactors, in order for the reaction to occur, components have to diffuse from one phase to another. These mass transfer processes influence and determine, in combination with the chemical kinetics, the overall reaction rate, i.e. how fast the chemical reaction takes place. This interaction between mass transfer and chemical kinetics is very important in chemical reaction engineering. Since chemical reactions either produce or consume heat, heat removal is also very important. Heat transfer processes determine the reaction temperature and, hence, influence the reaction rate. 

The observation of MFEs in f-pairs is important as this is how most ordinary chemical reactions take place. Most recombination reactions of organic free radicals do not occur through photochemically generated RPs with initially pure spin states, but instead through the random encounter of radicals in solution (think of the classic termination reaction in free radical chain reactions). 

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