How do Chemical Reactions Take Place

What is a chemical reaction How does it take place These questions are the most fundamental questions of chemistry, and they are the last to be solved. In order to deal with flash chemistry, however, let us begin with a consideration of such fundamental questions. 

When we consider a chemical reaction, there are two viewpoints a macroscopic one and a molecular level one. It was only about a hundred years ago when the reality of molecules was established. In 1905, Einstein proposed a theory of Brownian motion, and later (1908-1912) Perrin proved it by experimental work. They showed that Brownian motion is caused by the collision of molecules on small particles (micrometer size). Although some scientists at the time considered that molecules only had a virtual existence that was useful to explain chemical phenomena, since then, no scientist has doubted the existence of molecules. Since that time a molecular point of view has become very popular in chemistry, although it is rather difficult to see molecules directly even with the present technology. 

Flash Chemistry Fast Organic Synthesis in Microsystems Jun-ichi Yoshida 2008 John Wiley Sons, Ltd. ISBN 978-0-470-03586-3 

---

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). 

The reactive resonances reveal the quasi-bound levels of the reaction complex with unique clarity and they do exist. Identification of the reactive resonances can help us with understanding how elementary chemical processes take place at a single quantum state level. F - - H2 and its isotopic analogs are the most beautiful examples. 

Why Do We Need to Know This Material The laws of thermodynamics govern chemistry and life. They explain why reactions take place and let us predict how much heat reactions release and how much work they can do. Thermodynamics plays a role in every part of our lives. For example, the energy released as heat can be used to compare fuels, and the energy resources of food lets us assess its nutritional value. The material in this chapter provides a foundation for the following chapters, in particular Chapter 7, which deals with the driving force of chemical reactions. 

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. 

Kinetic studies tell how fast enzymes act but by themselves say nothing about how enzymes catalyze reactions. They do not give the chemical mechanism of catalysis, the step-by-step process by which a reaction takes place. Most of the individual steps involve the simultaneous breaking of a chemical bond and formation of a new bond. Consider a simple displacement reaction, that of a hydroxyl ion reacting with methyl iodide to give the products methanol and iodide ions. 

The quotation is appropriate. By the turn of the century a lot was understood about the tendency for chemical reactions to occur, the types of chemical bonds that formed when they occurred, and the properties of the materials created, but not much was known about Sennert s passion—about how these chemical reactions occurred— at the moment of conception, as it were. The difficulty was, and is, that we cannot witness a single chemical reaction at the molecular level. The best we can do is see the results of many chemical reactions (the bulk) and from this bulk behavior attempt to divine what happened on the molecule-to-molecule level. As it turns out, one of the best handles we have on this problem is our ability to measure rates of chemical reactions, that is, the speed at which they take place—the subject of chemical kinetics. 

What are the principal chemical reactions that take place in the chemosphere to give it its name How do they influence stratospheric and tropospheric chemical reactions ... 

The molecular mechanics calculations discussed so far have been concerned with predictions of the possible equilibrium geometries of molecules in vacuo and at OK. Because of the classical treatment, there is no zero-point energy (which is a pure quantum-mechanical effect), and so the molecules are completely at rest at 0 K. There are therefore two problems that I have carefully avoided. First of all, I have not treated dynamical processes. Neither have I mentioned the effect of temperature, and for that matter, how do molecules know the temperature Secondly, very few scientists are interested in isolated molecules in the gas phase. Chemical reactions usually take place in solution and so we should ask how to tackle the solvent. We will pick up these problems in future chapters. 

Many times in geochemical modeling we want to understand not only what reactions proceed in an open chemical system, but where they take place (e.g., Steefel et al., 2005). In a problem of groundwater contamination, for example, we may wish to know not only the extent to which a contaminant might sorb, precipitate, or degrade, but how far it will migrate before doing so. 

In the introduction to this chapter we discussed how chemical reactions occurred. Recall that before a reaction can occur there must be a collision between one reactant with the proper orientation at the reactive site of another reactant that transfers enough energy to provide the activation energy. However, many reactions do not take place in quite this simple a way. Many reactions proceed from reactants to products through a sequence of reactions. This sequence of reactions is called the reaction mechanism. For example, consider the reaction... 

Why do some reactions occur slowly while others seem to take place instantaneously How do chemists measure, compare, and express the rates at which chemical reactions occur Can chemists predict and control the rate of a chemical reaction These questions will be answered in Chapter 6. 

A pacemaker obtains electrical energy from a tiny battery that lasts for about seven years before it must be replaced. But how do batteries supply electrical energy The answer lies in a branch of chemistry known as electrochemistry. In this unit, you will learn about the connection between chemical reactions and electricity. You will also learn about the chemical reactions that take place inside batteries. 

Many conditions are required for a chemical reaction to proceed. Conditions such as heat, light, and pressure must be just right for a reaction to take place. Furthermore, the reaction may proceed very slowly. Some reactions occur in a fraction of a second, while others occur very slowly. Consider the difference in the reaction times of gasoline igniting in a car s cylinder versus the oxidation of iron to form rust. The area of chemistry that deals with how fast reactions occur is known as kinetics (Chapter 12). Finally, not all reactions go to completion. The amount of product produced based on the chemical equation is known as the theoretical yield. The amount actually obtained expressed as a percent of the theoretical is the actual yield. In summary, it s best to think of a chemical equation as an ideal representation of a reaction. The equation provides a general picture of the reaction and enables us to do theoretical calculations, but in reality reactions deviate in many ways from that predicted by the equation. 

Most reactions do not occur in the simple fashion that we describe in a balanced chemical equation. Chemical equations show the reactants and products of the reaction but do nothing to describe how the former converts to the latter. A method that is used to show the intermediate processes that occur during a reaction is a reaction mechanism. A reaction mechanism lists the proposed changes that take place to the reactants as the product(s) are being formed. Usually this consists of two or three chemical reactions, referred to as elementary reactions or elementary steps, shown one on top of the other. For example, in the reaction of nitrogen dioxide and carbon monoxide, the balanced chemical equation looks like this ... 

Why are we interested in determining the rate law for a reaction How does it help us It helps us because we can work backward from the rate law to find the steps by which the reaction occurs. Most chemical reactions do not take place in a single step but result from a series of sequential steps. To understand a chemical reaction, we must learn what these steps are. For example, a chemist who is designing an insecticide may study the reactions involved in the process of insect growth to see what type of molecule may interrupt this series of reactions. Or an industrial chemist may be trying to make a reaction occur faster, using less expensive conditions. To accomplish this, he or she must know which step is slowest, because it is that step that must be speeded up. Thus a chemist is usually not interested in a rate law for its own sake but for what it tells about the steps by which a reaction occurs. We will develop a process for finding the reaction steps later in this chapter. 

You can begin to think of processes as chemical reactions even if you do not know the names of all the substances involved. Table 1 can help you begin to think like a chemist. It shows the word equations for chemical reactions you might see around your home. See how many other reactions you can find. Look for the signs you have learned that indicate a reaction might be taking place. Then, try to write them in the form shovm in the table. 

As will be discussed later, the rates at which chemical species are being formed (or depleted) depend on all the chemical reactions that actually take place in the reactor (reaction pathways). Hence, to design chemical reactors with multiple reactions, we consider all the chemical reactions that are taking place, including the dependent reactions. Therefore, it is necessary to express the dependent reactions in terms of the independent reactions. Next, we describe how to do so. 

---