Concentration affects, reaction rates

You can use simple collision theory to begin to understand why factors such as concentration affect reaction rate. If a collision is necessary for a reaction to occur, then it makes sense that the rate of the reaction will increase if there are more collisions per unit time. More reactant particles in a given volume (that is, greater concentration) will increase the number of collisions between the particles per second. Figure 6.7 illustrates this idea. 

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 numerical methods in this book can be applied to all components in the system, even inerts. When the reaction rates are formulated using Equation (2.8), the solutions automatically account for the stoichiometry of the reaction. We have not always followed this approach. For example, several of the examples have ignored product concentrations when they do not affect reaction rates and when they are easily found from the amount of reactants consumed. Also, some of the analytical solutions have used stoichiometry directly to ease the algebra. This section formalizes the use of stoichiometric constraints. 

Read the entire laboratory activity. Form a hypothesis about how an increase in temperature will affect reaction rate. Form a second hypothesis about how an increase in concentration will affect reaction rate. Record your hypotheses on page 130. 

In Lab 17.1, you learned about the effect of temperature and concentration on reaction rate. Another factor that affects reaction rate is the amount of surface area of the reactants. If a chemical reaction is to take place, the molecules of reactants must collide. Changing the amount of surface area modifies the rate of collision, and, thus, the rate of reaction. If surface area increases, collision frequency increases. If surface area decreases, so does the number of collisions. In this lab, you will examine the effect of surface area on rate of reaction. You will also determine how a combination of factors can affect reaction rate. 

The rate of reaction may depend upon reactant concentration, product concentration, and temperature. Cases in which the product concentration affects the rate of reaction are rare and are not covered on the AP exam. Therefore, we will not address those reactions. We will discuss temperature effects on the reaction later in this chapter. For the time being, let s just consider those cases in which the reactant concentration may affect the speed of reaction. For the general reaction aA + bB+...->c C + dD +. . . where the lower-case letters are the coefficients in the balanced chemical equation the upper-case letters stand for the reactant and product chemical species and initial rates are used, the rate equation (rate law) is written ... 

In this section, you learned how to express reaction rates and how to analyze reaction rate graphs. You also learned how to determine the average rate and instantaneous rate of a reaction, given appropriate data. Then you examined different techniques for monitoring the rate of a reaction. Finally, you carried out an investigation to review some of the factors that affect reaction rate. In the next section, you will learn how to use a rate law equation to show the quantitative relationships between reaction rate and concentration. 

The rate of a chemical reaction depends on several factors, as you learned in section 6.1. One of the factors that affect reaction rate is the concentrations of the reactants. You know that the rates of most chemical reactions increase when the concentrations of the reactants increase. Is there a more specific relationship In this section, you will explore the quantitative relationships between the rate of a reaction and the concentrations of the reactants. 

Wirz and coworkers also studied photorelease from o-nitrotoluene derivative 89 (Scheme 46). The authors proposed a mechanism of photorelease from these compounds that is similar to that proposed for 83. The authors demonstrated that the reaction medium, including solvent, pH, and buffer concentration, affects the rate of release (Scheme 45). For example, in methanol (pH up to 8), the rate of release from... 

In the previous chapter, we discussed the rate of reactions and the factors affecting reaction rates. We also learned that there is a relation between the concentration of reactants and the rate. 

Substrate Concentration Affects the Rate of Enzyme-Catalyzed Reactions... 

Supercritical fluid solvents can act in a variety of ways to affect reaction rates. Since the reaction rate is the product of the rate constant and the concentrations of the reactants, one must consider the solvent effect on the rate constant itself (discussed below), as well as changes in concentrations. It is this second possibility that has not been addressed until this study i.e., the possible influence of changes in the local concentrations of the reactants in the compressible region near the critical point... 

The rate at which a chemical reaction occurs in homogeneous systems (single-phase) depends primarily on temperature and the concentrations of reactants and products. Other variables, such as catalyst concentration, initiator concentration, inhibitor concentration, or pH, also can affect reaction rates. In heterogeneous systems (multiple phases), chemical reaction rates can become more complex because they may not be governed solely by chemical kinetics but also by the rate of mass and/or heat transfer, which often play significant roles. 

As can be seen from Table 14 the addition order affects reaction rate, molar mass and PDI. The authors suggested that in-situ activation results in the formation of two types of active species the relative concentrations of which were governed by the addition sequence. Addition order (1) EASC + NdV + DIBAH promotes the formation of insoluble species which produced polymer with a broad, bimodal MMD (PDI = 7.5) at low catalyst activity. Addition order (3) DIBAH + NdV + EASC leads to the formation of more soluble catalyst species which exhibit increased catalyst activity and produce BR with a monomodal MMD (PDI = 3.4). The influence of the addition order on cis-1,4-content is negligible. 

Because the reactions we consider in this section are single-step and therefore elementary reactions, the rate law specified in Section 2.3 as Equation 2.1 is obtained for the rate of formation of the substitution product Nu—R in Figure 2.4. It says that these reactions are bimolecular substitutions. They are consequently referred to as SN2 reactions. The bimolecu-larity makes it possible to distinguish between this type of substitution and SN1 reactions, which we will examine in Section 2.5 nucleophile concentration affects the rate of an SN2 reaction, but not an SN1 reaction. 

A reaction may be classified according to the number of molecules whose concentrations affect the rate of the reaction. This determines the order of the reaction. Aspirin decomposes according to the reaction in Figure 14-1 and the rate constant of this hydrolysis is directly proportional to the concentration of aspirin. No other dependencies of the rate are observed under the conditions of this experiment so that the reaction may be called pseudo-first order and may be described by the equation... 

We can draw a very useful general conclusion from this simple binary system that is applicable to more complex processes changes in production rate can be achieved only by changing conditions in the reactor. This means something that affects reaction rate in the reactor must vary holdup in liquid-phase reactors, pressure in gas-phase reactors, temperature, concentrations of reactants (and products in reversible reactions), and catalyst activity or initiator addition rate. Some of these variables affect the conditions in the reactor more than others. Variables with a large effect are called dominant. By controlling the dominant variables in a process, we achieve what is called partial control. The term partial control arises because we typically have fewer available manipulators than variables we would like to control. The setpoints of the partial control loops are then manipulated to hold the important economic objectives in the desired ranges. 

Concentration and temperature also affect reaction rate. 

How does the change in the reactant concentration affect the rate of the chemical reactions ... 

An interesting case of competitive adsorption is provided by the pH effect for an acidic enzyme catalyzed reaction (e.g. lipase). If the pH is too low protonation of an essential amino acid residue of the enzyme may occur with loss in activity. At very high pH again loss in the number of active enzyme molecules may occur, but now because of possible deprotonation of essential residues. Since we are dealing with competitive adsorption effects, at low substrate concentrations, where the rate is controlled by a maximum in the rate will occur at the pH where competitive adsorption is the least. As can be deduced from (3.39) at high substrate concentration the reaction rate will not be affected. 

In practice, the choice of substrate concentrations is limited by such considerations as the solubility of the substrates, the viscosity and high initial absorbance of concentrated solutions, and the relative costs of the reagents. Furthermore, the selection of appropriate substrate concentrations is only one of the factors to be considered in formulating an optimal assay system for the measurement of a specific enzyme activity. Critical choices must also be made with respect to other, frequently interdependent factors that affect reaction rate, such as the concentrations of activators and the nature and pH of the buffer system. The traditional empirical approach to optimization has been replaced by newer techniques of simplex co-optimization and response-... 

Methods in which some property related to substrate concentration (such as absorbance, fluorescence, chemiluminescence, etc.) is measured at two fixed times during the course of the reaction are known as two-point kinetic methods. They are theoreticahy the most accurate for the enzymatic determination of substrates. However, these methods are technically more demanding than equifibrium methods and all the factors that affect reaction rate, such as pH, temperature, and amount of enzyme, must be kept constant from one assay to the next, as must the timing of the two measurements. These conditions can readily be achieved in automatic analyzers. A reference solution of the analyte (substrate) must be used for calibration. To ensure first-order reaction conditions, the substrate concentration must be low compared to the K, (i.e., in the order of less than 0.2 X K, . Enzymes with high K , values are therefore preferred for kinetic analysis to give a wider usable range of substrate concentration. 

The collision theory allows us to explain the factors that affect reaction rate. There is a wide range of energies among molecules in any sample, and generally only the most energetic molecules can undergo reaction. An increase in temperature increases the number of molecules that have sufficient energy to react. An increase in concentration or pressure... 

Skill 9.5 Describing how temperature, concentrations, and catalysts affect reaction rates... 

Rate coefficient includes all factors that affect reaction rate, except for concentration, which is explicitly accounted for. Rate coefficient is therefore not constant because of that reason the name reaction rate coefficient is preferred over reaction rate constant. The rate coefficient is mainly affected by temperature as described by Arrhenius equation but also, ionic strength, surface area of the adsorbent (for heterogeneous reactions), light irradiation, and other physicochemical properties, depending on the considered reaction. 

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