Reactant concentration, chemical reaction

You have learned that the rate of a chemical reaction is affected by the concentration of the reactant or reactants. The rate law describes the way in which reactant concentration affects reaction rate. A rate law may be simple or very complicated, depending on the reaction. 

The graphing calculator can run a program that can tell you the order of a chemical reaction, provided you indicate the reactant concentrations and reaction rates for two experiments involving the same reaction. 

Stoichiometric coefficients (Zrk) are generally considered positive for products and negative for reactants. Each chemical reaction is associated with its kinetics representing dependence of net rate of reaction on concentrations of participating species and temperature. Dependence on concentrations of participating species is represented by order of reaction, o . The rate is represented by two parameters, frequency factor, ko, and activation energy, AE (see textbooks such as Levenspiel, 1972 for more discussion on these two parameters). The net rate of formation or consumption of component k due to reaction n is usually written ... 

We often use solutions to supply the reactants for chemical reactions. Solutions allow the most intimate mixing of the reacting substances at the molecular level, much more than would be possible in solid form. (A practical example is drain cleaner, shown in the photo.) We sometimes adjust the concentrations of solutions to speed up or slow down the rate of a reaction. In this section we study methods for expressing the quantities of the various components present in a given amount of solution. 

The concentrations of the reactants. Most chemical reactions proceed faster if the concentration of one or more of the reactants is increased. For example, steel wool bums with difficulty in air, which contains 20% O2, but brnsts into a brilliant white flame in pure oxygen (Figure 14.2 T). As concentration increases, the frequency with which molecules collide increases, leading to increased rates. 

Another continuous operation that is very popular in industry, but is not used very much in the chemical laboratory, is the continuous stirred tank reactor (CSTR) (it is silently assumed that the CSTR is operated in the steady state). The reactants are fed continuously into a well stirred vessel, and a constant product flow leaves the reactor through an exit port that can be located anywhere in the reactor wall. If the reactor contents are indeed well mixed, the reactants entering the vessel are diluted immediately, and the reaction proceeeds at relatively low reactant concentrations. The pr uct flow leaving the reactor must then have the same composition as the reaction mixture. This may appear to be an illogical way of carrying out a chemical reaction, but it has several distinct advantages, at least for reactions that are intrinsically rapid. The most important advantage is the thermal stability, especially in the case of exothermic reactions. Since the reaction proceeds at low reactant concentrations, the reaction rate per unit volume is relatively low, and so is the heat evolution. These are both constant in time. 

This example illustrates the conflict between reactant concentration and reaction temperature for a chemical reaction, and this is a particularly important feature of reactive distillation. Because of the reversible reaction, the reactive zone should be placed where the reactants are most abimdant. In the type 11 example considered in this section, this occurs in the upper section of the reactive distillation column, which is the low temperature zone. It is interesting to note that the reaction temperature is the lowest (reflux drum) when the product of the two-reactant concentration is the highest. Figure 17.13a indicates that... 

Transient, or time-resolved, techniques measure tire response of a substance after a rapid perturbation. A swift kick can be provided by any means tliat suddenly moves tire system away from equilibrium—a change in reactant concentration, for instance, or tire photodissociation of a chemical bond. Kinetic properties such as rate constants and amplitudes of chemical reactions or transfonnations of physical state taking place in a material are tlien detennined by measuring tire time course of relaxation to some, possibly new, equilibrium state. Detennining how tire kinetic rate constants vary witli temperature can further yield infonnation about tire tliennodynamic properties (activation entlialpies and entropies) of transition states, tire exceedingly ephemeral species tliat he between reactants, intennediates and products in a chemical reaction. 

Krypton clathrates have been prepared with hydroquinone and phenol. 85Kr has found recent application in chemical analysis. By imbedding the isotope in various solids, kryptonates are formed. The activity of these kryptonates is sensitive to chemical reactions at the surface. Estimates of the concentration of reactants are therefore made possible. Krypton is used in certain photographic flash lamps for high-speed photography. Uses thus far have been limited because of its high cost. Krypton gas presently costs about 30/1. 

Several important points about the rate law are shown in equation A5.4. First, the rate of a reaction may depend on the concentrations of both reactants and products, as well as the concentrations of species that do not appear in the reaction s overall stoichiometry. Species E in equation A5.4, for example, may represent a catalyst. Second, the reaction order for a given species is not necessarily the same as its stoichiometry in the chemical reaction. Reaction orders may be positive, negative, or zero and may take integer or noninteger values. Finally, the overall reaction order is the sum of the individual reaction orders. Thus, the overall reaction order for equation A5.4 isa-l-[3-l-y-l-5-l-8. 

Design possibilities for electrolytic cells are numerous, and the design chosen for a particular electrochemical process depends on factors such as the need to separate anode and cathode reactants or products, the concentrations of feedstocks, desired subsequent chemical reactions of electrolysis products, transport of electroactive species to electrode surfaces, and electrode materials and shapes. Cells may be arranged in series and/or parallel circuits. Some cell design possibiUties for electrolytic cells are... 

Chemical engineering inherited the definition for the reaction rate from chemical kinetics. The definition is for closed systems, like batch reactors, in which most of the classical kinetic studies were done. Inside a batch reactor little else besides chemical reaction can change the concentration of reactant A. In a closed system, for the reaction of... 

The chemical reaction rate is generally a function of a reactant concentration and temperature. In the case of an exothermic reaction, unless the heat of reaction is removed, an increase in temperature may result in a runaway reaction. For most homogeneous reaction, the rate is increased by a factor of 2 or 3 for every 10°C rise in temperature. This is represented by... 

Another means is available for studying the exchange kinetics of second-order reactions—we can adjust a reactant concentration. This may permit the study of reactions having very large second-order rate constants. Suppose the rate equation is V = A caCb = kobs A = t Ca, soAtcb = t For the experimental measurement let us say that we wish t to be about 10 s. We can achieve this by adjusting Cb so that the product kc 10 s for example, if A = 10 M s , we require Cb = 10 M. This method is possible, because there is no net reaction in the NMR study of chemical exchange. 

The rate of a chemical reaction is influenced by pressure, temperature, concentration of reactants, kinetic factors such as agitation, and the presence of a catalyst. Since the viability of a plant depends not only on reaction efficiencies but also on the capital cost factor and the cost of maintenance, it may be more economic to alter a process variable in order that a less expensive material of construction can be used. The flexibility which the process designer has in this respect depends on how sensitive the reaction efficiency is to a change in the variable of concern to the materials engineer. 

The rate of a chemical reaction is proportional to the concentration of the reactants, and for a reversible isothermal homogeneous reaction ... 

Every chemical reaction can go in either forward or reverse direction. Reactants can go forward to products, and products can revert to reactants. As you may remember from your general chemistry course, the position of the resulting chemical equilibrium is expressed by an equation in which /Cec], the equilibrium constant, is equal to the product concentrations multiplied together, divided by the reactant concentrations multiplied together, with each concentration raised to the power of its coefficient in the balanced equation. Eor the generalized reaction... 

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