What Are Chemical Reactions

If two or more elements or chemical compounds come into contact with one another and there is enough energy present, a chemical change may take place. When a substance undergoes a chemical change, the chemical structure of that substance is altered. A chemical change can also be called a chemical reaction. During a chemical reaction, a new substance is formed. 

Burning a log in a fireplace is an example of a chemical reaction. When the wood burns, it reacts with the oxygen in the air. This chemical reaction forms ash, water vapor, and carbon dioxide gas. The wood and the oxygen in this chemical reaction are called reactants because they react together to form new substances. The new substances—ash, water vapor, and carbon dioxide—are called products because they are produced by the reaction. 

Some chemical reactions, such as the explosion of fireworks, happen very, very fast—in less than a second. Others, like the rusting of a car, might take years. 

There are also many ways substances can be changed without a chemical reaction taking place and changing the chemical composition of the substance. These changes are called physical changes. 

Cutting up a piece of paper is an example of a physical change. The paper is different It is now in small pieces instead of the larger piece it started out as. But the chemical composition of the smaller pieces of paper is the same as the chemical composition of the 

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Many exothermic reactions are spontaneous. A critical question facing chemists in the late eighteenth century was how to tell spontaneous reactions from nonspontaneous ones without performing an experiment. What characteristics must the reactants have to proceed without the prod of added energy In other words, what drives chemical reactions ... 

Besides the two most well-known cases, the local bifurcations of the saddle-node and Hopf type, biochemical systems may show a variety of transitions between qualitatively different dynamic behavior [13, 17, 293, 294, 297 301]. Transitions between different regimes, induced by variation of kinetic parameters, are usually depicted in a bifurcation diagram. Within the chemical literature, a substantial number of articles seek to identify the possible bifurcation of a chemical system. Two prominent frameworks are Chemical Reaction Network Theory (CRNT), developed mainly by M. Feinberg [79, 80], and Stoichiometric Network Analysis (SNA), developed by B. L. Clarke [81 83]. An analysis of the (local) bifurcations of metabolic networks, as determinants of the dynamic behavior of metabolic states, constitutes the main topic of Section VIII. In addition to the scenarios discussed above, more complicated quasiperiodic or chaotic dynamics is sometimes reported for models of metabolic pathways [302 304]. However, apart from few special cases, the possible relevance of such complicated dynamics is, at best, unclear. Quite on the contrary, at least for central metabolism, we observe a striking absence of complicated dynamic phenomena. To what extent this might be an inherent feature of (bio)chemical systems, or brought about by evolutionary adaption, will be briefly discussed in Section IX. 

What problems face the theory of combustion The theory of combustion must be transformed into a chapter of physical chemistry. Basic questions must be answered will a compound of a given composition be combustible, what will be the rate of combustion of an explosive mixture, what peculiarities and shapes of flames should we expect We shall not be satisfied with an answer based on analogy with other known cases of combustion. The phenomena must be reduced to their original causes. Such original causes for combustion are chemical reaction, heat transfer, transport of matter by diffusion, and gas motion. A direct calculation of flame velocity using data on elementary chemical reaction events and thermal constants was first carried out for the reaction of hydrogen with bromine in 1942. The problem of the possibility of combustion (the concentration limit) was reduced for the first time to thermal calculations for mixtures of carbon monoxide with air. Peculiar forms of propagation near boundaries which arise when normal combustion is precluded or unstable were explained in terms of the physical characteristics of mixtures. 

Unfortunately, chemical identification of the relaxation process is by far the most difficult and ambiguous part of the analysis. There are no direct chemical observations. The magnitude of the relaxation time can often distinguish between physical processes and chemical reactions, but this does not reveal what the chemical reactions are. 

Are chemical reactions that occur under the conditions of synthesis and those promoted by femtosecond laser photolysis the same or not If not, what is the difference Such naive questions may come to mind. In... 

Thermochemistry is the study of the energy of the entire system at every instant of a reaction. It is therefore part of the information gained when deciphering The study of the energy of chemical a complete mechanism. Before examining the mechanisms and thermochemistry of acid-base reactions, we must examine what causes chemical reactions to occur and introduce some basic energy principles that are key to chemical reactivity. 

What about chemical reactions Why do chemical systems eventually reach equilibrium The answer is analogous to that for the rock There are balanced forces acting on the chemical species in the system. These forces are actually energies—chemical potentials of the different chemical species involved in the system at equilibrium. The next section introduces chemical equilibrium in those terms. 

What chemical species in the stratosphere is essential for life on Earth What are the reactions by which it is made, and how may it be threatened ... 

The chemist s goal is to control chemistry - to direct chemical reactions to yield the products that are desired and to prevent the formation of those that are not, and to do it on a convenient time scale and at an acceptable cost in energy. To achieve this it is necessary to understand how fast and by what mechanisms chemical reactions occur. These are the subjects of chemical kinetics and chemical dynamics the former is... 

Chemists usually learn about reactions according to fiinctional groups for example, How can I make an aldehyde and what reactions are known for aldehydes " This is clearly not a very good starting point for classifying reactions. The poor state of affairs in the definition of reaction types is further quite vividly illustrated by the fact that many chemical reactions are identified by being named after their inventor Diels-Alder reaction, Michael addition, Lobry-de Bruyn-van Ekenstein rear-... 

Acids and bases are a big part of organic chemistry but the emphasis is much different from what you may be familiar with from your general chemistry course Most of the atten tion m general chemistry is given to numerical calculations pH percent loniza tion buffer problems and so on Some of this returns m organic chemistry but mostly we are concerned with the roles that acids and bases play as reactants products and catalysts m chemical reactions We 11 start by reviewing some general ideas about acids and bases... 

The circle m a hexagon symbol was first suggested by the British chemist Sir Robert Robinson to represent what he called the aromatic sextet —the six delocalized TT electrons of the three double bonds Robinson s symbol is a convenient time saving shorthand device but Kekule type formulas are better for counting and keeping track of electrons especially m chemical reactions... 

This chapter is divided into two parts The first and major portion is devoted to carbohydrate structure You will see how the principles of stereochemistry and confer matronal analysis combine to aid our understanding of this complex subject The remain der of the chapter describes chemical reactions of carbohydrates Most of these reactions are simply extensions of what you have already learned concerning alcohols aldehydes ketones and acetals... 

Each of the following reactions has been reported m the chemical literature and proceeds m good yield What are the principal organic products of each reaction" In some of the exercises more than one diastereomer may be theoretically possible but m such instances one diastereomer is either the major product or the only product For those reactions m which one diastereomer is formed preferentially indicate its expected stereochemistry... 

Finally, a consideration of equilibrium chemistry can only help us decide what reactions are favorable. Knowing that a reaction is favorable does not guarantee that the reaction will occur. How fast a reaction approaches its equilibrium position does not depend on the magnitude of the equilibrium constant. The rate of a chemical reaction is a kinetic, not a thermodynamic, phenomenon. Kinetic effects and their application in analytical chemistry are discussed in Chapter 13. 

In this procedure, the question of what chemical reactions are involved never enters directly into any of the equations. However, the choice of a set of species is entirely eqmvalent to the choice of a set of independent reactions among the species. In any event, a set of species or an equivalent set of independent reactions must always be assumed, and different assumptions produce different results. 

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