Definition of a chemical reaction

Let us consider a closed system consisting of M elements and N constituents. Let Ai stand for the f-th constituent and Bj for the j-th element. Then, 

For conventional use this intuitive concept of a compound is sufficient, and there is no difficulty in extending it to a radical. Provided an electron is additionally defined as one of the set of elementary particles By, ions may also be considered to be independent constituents. 

When every compound is described as a line vector of constitutional coefficients, a system of N constituents may be defined by a matrix of N x M elements. Clearly the elements of this matrix must be non-negative integers. With regard to the physical sense of the matrix, none of its rows or columns may be composed exclusively of zeros. 

Let us consider a system composed of three elements (C, H, O) and five constituents (CH4, H2O, H2, CO, CO2). The matrix of constitution coefficients will take the form 

A number of reactions may take place in a system defined between the given N constituents. Assuming the number of possible reactions in the system to be R, the chemical conversion of the initial substances to products may be expressed by the relation 

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One of the simplest definitions of a chemical reaction is changes in the bonds . All reactions proceed with some bonds cleaving concurrently with others forming. Each... 

The mathematical definition of a chemical reaction rate has been a source of confusion in chemical and chemical engineering literature for many years. The origin of this confusion stems from laboratory bench-scale experiments that were carried out to obtain chettiical reaction rate data. These eai ly experiments were batch-type, in which the reaction vessel was closed and rigid consequently, the ensuing reaction took place at constant volume. The reactants were mixed together at time t - 0 and the concentration of one of the reactants, was measured at various times f. The rate of reaction was determined from the slope of a plot of as a function of time. Letting be the rate of formation of A per unit volume (e.g., g mol/s dm ), the investigators then defined and reported the chemical reaction rate as... 

Using your own words, give a definition of a chemical reaction. 

A Using your own words, give a definition of a chemical reaction. Using drawings of ball-and-stick models, illustrate an example of a chemical reaction. 

By our definition of a chemical reaction, reactants are consumed and products are produced. This is known as forward reaction. A reversible process is one in which the products can also he consumed to produce reactants, a process known as the reverse reaction. 

The are many ways to define the rate of a chemical reaction. The most general definition uses the rate of change of a themiodynamic state function. Following the second law of themiodynamics, for example, the change of entropy S with time t would be an appropriate definition under reaction conditions at constant energy U and volume V ... 

A catalyst is a substance that iacreases the rate of approach to equiUbrium of a chemical reaction without being substantially consumed itself. A catalyst changes the rate but not the equiUbrium of the reaction. This definition is almost the same as that given by Ostwald ia 1895. The term catalysis was coiaed ia ca 1835 by Ber2eHus, who recognized that many seemingly disparate phenomena could be described by a single concept. For example, ferments added ia small amounts were known to make possible the conversion of plant materials iato alcohol and there were numerous examples of both decomposition and synthesis reactions that were apparendy caused by addition of various Hquids or by contact with various soHds. 

Most of the reactions described in this chapter would fall under Huisgen s definition of a cycloaddition reaction (/, la). However some of the reactions described would not be considered cycloaddition reactions according to this restrictive definition. Therefore, the more liberal definition given by Baldwin will be used as a guideline, namely, Cycloadditions are chemical transformations giving at least one product having at least two new bonds as... 

The rate of a chemical reaction can be described in any of several different ways. The most commonly used definition involves the time rate of change in tlie amount of one of the components participating in tlie reaction tliis rate is usually based on some arbitrary factor related to tlie reacting system size or geometry, such as volume, mass, or interfacial area. Tlie definition shown in Eq. (4.6.7), wliich applies to homogeneous reactions, is a convenient one from an engineering point of view. 

The formation of activated species during mechanoehemieal degradation is, in general, not sufficiently documented both experimentally and with respect to the proposed mechanisms to give a definite proof of their existence. In the dilute state, the rate of energy transfer is high and it is reasonable to assume that any activated species, if present, will be thermalized well before the occurrence of a chemical reaction. 

What do we mean by the rate of a chemical reaction Informally, we consider a reaction to be fast if the products are formed rapidly, as occurs in a precipitation reaction or an explosion (Fig. 13.1). A reaction is slow if the products are formed over a long period of time, as happens in corrosion or the decay of organic material (Fig. 13.2). Our first task is to set up a precise, quantitative definition of the rate of a chemical reaction. 

The textbook definition of a reactive intermediate is a short-lived, high-energy, highly reactive molecule that determines the outcome of a chemical reaction. Well-known examples are radicals and carbenes such species cannot be isolated in general, but are usually postulated as part of a reaction mechanism, and evidence for their existence is usually indirect. In thermal reactivity, for example, the Wheland intermediate (Scheme 9.1) is a key intermediate in aromatic substitution. 

The tools for calculating the equilibrium point of a chemical reaction arise from the definition of the chemical potential. If temperature and pressure are fixed, the equilibrium point of a reaction is the point at which the Gibbs free energy function G is at its minimum (Fig. 3.1). As with any convex-upward function, finding the minimum G is a matter of determining the point at which its derivative vanishes. 

A series of observations in which the presence of an apparently unaffected substance initiated the progress of a chemical reaction prompted the Swedish chemist Berzelius in 1835 to introduce the term catalyst , but it was only towards the end of the last century that a dear definition could be given by Ostwald, namely a catalyst is a substance which affects the rate of a chemical reaction without appearing in the final products . This is achieved through the formation of intermediate compounds between the catalyst and the molecules involved in the reaction whereby an alternative path is offered which may be passed through with higher probability, i.e. 

The term catalysis was coined by Berzelius over 150 years ago when he had noticed changes in substances when they were brought in contact with small amounts of certain species called "ferments". Many years later in 1895 Ostwald came up with the definition that we use until today A catalyst is a substance that changes the rate of a chemical reaction without itself appearing into the products. This means that according to Ostwald a catalyst can also slow down a reaction The definition used today reads as follows A catalyst is a substance which increases the rate at which a chemical reaction approaches equilibrium without becoming itself permanently involved. 

As used in the act, the term chemical substance means any organic or inorganic substance of a particular molecular identity, including any combination of such substances occurring in whole or in part as a result of a chemical reaction or occurring in nature and any element or uncombined radical. Items not considered chemical substances are listed in the definition section of the act. The term mixture means any combination of two or more chemical substances if the combination does not occur in nature and is not, in whole or in part, the result of a chemical reaction except that such term does include any combination that occurs, in whole or in part, as a result of a chemical reaction if none of the chemical substances comprising the combination is a new chemical substance and if the combination could have been manufactured for commercial purposes without a chemical reaction at the time the chemical substances comprising the combination were combined. 

When chemists investigate the mechanism of a reaction, they are not so lucky. Determining the mechanism of a chemical reaction is a bit like figuring out how a clock works just by looking at its face and hands. For this reason, reaction mechanisms are proposed rather than definitively stated. Much of the experimental evidence that is obtained to support a mechanism is indirect. Researchers need a lot of creativity as they propose and test mechanisms. 

The definition of a chemical engineer is one who handles the engineering of chemical reactions. Separations, fluid flow, and transport are details (admittedly sometimes very... 

Lavoisier had formed a clear, consistent, and suggestive mental picture of chemical changes. He thought of a chemical reaction as always the same under the same conditions, as an action between a fixed and measurable quantity of one substance, having definite and definable properties, with fixed and measurable quantities of other substances, the properties of each of which were definite and definable. 

A catalyst was defined by J. J. Berzelius in 1836 as a compound, which increases the rate of a chemical reaction, but which is not consumed by the reaction. This definition allows for the possibility that small amounts of the catalyst are lost in the reaction or that the catalytic activity is slowly lost. 

The phenomenon under consideration was studied systematically in the beginning of the 19th century. In 1815, Davy performed experiments that dealt with catalytic combustion on platinum gauzes. The term catalysis , however, was introduced by Berzelius in 1836. He first defined a catalyst (Berzelius, 1836) as a compound, which increases the rate of a chemical reaction, but which is not consumed during the reaction. This definition was later amended by Ostwald (1853-1932) in 1895 to involve the possibility that small amounts of the catalyst are lost in the reaction or that the catalytic activity is slowly decreased A catalyst is a substance that increases the rate of approach to equilibrium of a chemical reaction without being substantially consumed in the reaction. It was more than a century after Berzelius first definition that Marcel Prettre s introduced the notion of yield The catalyst is a substance that increases the rate of a chemical transformation without modifying the yield, and that is found intact among the final products of the reaction. ... 

Definition of an enzyme Definition of an active it Enzymes are protein catalysts that increase the velocity of a chemical reaction, and are not consumed during the reaction they catalyze. Each enzyme has an active site where the substrate binds and is converted to product. 

The definition of a photochemical reaction depends on the definition of light . Indeed, a photochemical reaction is a chemical reaction induced by light, a reaction in which the energy of light is used to promote molecules... 

SMART MATERIALS. From a technical and simple point of view, a smart material is a material that responds to its environment in a timely manner. To expand on this definition, a smart material is one that receives, transmits, or processes a stimulus and responds by producing a useful effect, which may include a signal that the material is acting upon it. Stimuli may include strain, stress, temperature, chemicals, an electric field, a magnetic field, hydrostatic pressures, different types of radiation, and other forms of stimuli. Transmission or processing of the stimulus may be in the form of an absorption of a photon, of a chemical reaction, of an... 

Equations 27 and 28 permit a simple comparison to be made between the actual composition of a chemical system in a given state (degree of advancement) and the composition at the equilibrium state. If Q K, the affinity has a positive or negative value, indicating a thermodynamic tendency for spontaneous chemical reaction. Identifying conditions for spontaneous reaction and direction of a chemical reaction under given conditions is, of course, quite commonly applied to chemical thermodynamic principle (the inequality of the second law) in analytical chemistry, natural water chemistry, and chemical industry. Equality of Q and K indicates that the reaction is at chemical equilibrium. For each of several chemical reactions in a closed system there is a corresponding equilibrium constant, K, and reaction quotient, Q. The status of each of the independent reactions is subject to definition by Equations 26-28. 

The classical definition of a catalyst, according to Oswald, is a substance that increases the rate of a chemical reaction without undergoing any permanent... 

Most countries with chemical inventories have excluded by-products, small product tests and laboratory quantities, and naturally occurring substances from their reporting schemes. One of the issues associated with inventories is the definition of a chemical. The definition of polymers is a complex issue that will vary depending upon the nature of the chemicals from which they are formed, their relative proportions, the order of reaction, and reaction conditions (duration, temperature, catalysts, etc.).14 Most inventories include polymers and treat all different polymeric substances and monomers as individual chemicals. Some countries, including the United States, have decided not to require updated reporting on the production of such polymers. None of the national chemical inventories include formulated mixtures, or preparations, such as paints and household cleaners, or articles such as automobiles, computers, and paper. 

In the case of a chemical reaction, we need a more precise thermodynamic definition. The thermodynamic quantities that are important for chemical reactions are AH, AS, and AG. 

Since about 1930, but especially in the last few years, the concept of non-stoichiometric compounds has been a stimulating influence in inorganic chemistry. It has opened up new ideas it has led to much new work on the constitution and the valence problems of inorganic compounds it has shed light on the mechanism of reactions taking place in the solid state. Indeed, it goes to the roots of our ideas about the definition of a chemical compound. 

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