Reactant concentration, chemical reaction rate affected

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. 

Complexity in multiphase processes arises predominantly from the coupling of chemical reaction rates to mass transfer rates. Only in special circumstances does the overall reaction rate bear a simple relationship to the limiting chemical reaction rate. Thus, for studies of the chemical reaction mechanism, for which true chemical rates are required allied to known reactant concentrations at the reaction site, the study technique must properly differentiate the mass transfer and chemical reaction components of the overall rate. The coupling can be influenced by several physical factors, and may differently affect the desired process and undesired competing processes. Process selectivities, which are determined by relative chemical reaction rates (see Chapter 2), can thenbe modulated by the physical characteristics of the reaction system. These physical characteristics can be equilibrium related, in particular to reactant and product solubilities or distribution coefficients, or maybe related to the mass transfer properties imposed on the reaction by the flow properties of the system. 

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. 

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 rate of the ion transfer is governed by the complexation reaction, whose rate constant should not depend on potential since it is a purely chemical reaction. However, the concentrations of the reactant change with potential. Typically, the ionophore is uncharged, so that a change in the potential drop affects only the concentration of the ions at the interface. If the thickness X of the interface is neglected, the concentration of the ion at the interface is given by ... 

The rate of an exothermic chemical reaction determines the rate of energy release, so factors which affect reaction kinetics are important in relation to possible reaction hazards. The effects of proportions and concentrations of reactants upon reaction rate are governed by the Law of Mass Action, and there are many examples where changes in proportion and/or concentration of reagents have transformed an... 

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 ... 

The five factors that can affect the rates of chemical reaction are the nature of the reactants, the temperature, the concentration of the reactants, the physical state of the reactants, and the presence of a catalyst. 

The collision theory describes how the change in concentration of one reactant affects the rate of chemical reactions. In this laboratory experiment, you will observe how concentration affects the reaction rate. 

There are many different aspects to the field of turbulent reacting flows. Consider, for example, the effect of turbulence on the rate of an exothermic reaction typical of those occurring in a turbulent flow reactor. Here, the fluctuating temperatures and concentrations could affect the chemical reaction and heat release rates. Then, there is the situation in which combustion products are rapidly mixed with reactants in a time much shorter than the chemical reaction time. (This latter example is the so-called stirred reactor, which will be discussed in more detail in the next section.) In both of these examples, no flame structure is considered to exist. 

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. 

In this section, you learned how to relate the rate of a chemical reaction to the concentrations of the reactants using the rate law. You classified reactions based on their reaction order. You determined the rate law equation from empirical data. Then you learned about the half-life of a first-order reaction. As you worked through sections 6.1 and 6.2, you may have wondered why factors such as concentration and temperature affect the rates of chemical reactions. In the following section, you will learn about some theories that have been developed to explain the effects of these factors. 

Another factor that affects the rate of a chemical reaction is the concentration of reactants. As noted, most reactions take place in solutions. It is expected that as the concentration of reactants increases more collisions occur. Therefore, increasing the concentrations of one or more reactants generally leads to an increase in reaction rate. The dependence of reaction rate on concentration of a reactant is determined experimentally. A series of experiments is usually conducted in which the concentration of one reactant is changed while the other reactant is held constant. By noting how fast the reaction takes place with different concentrations of a reactant, it is often possible to derive an expression relating reaction rate to concentration. This expression is known as the rate law for the reaction. 

In Equation 3.1, the suffix i usually designates a reaction product. Ihe rate r,-is negative, in case i is a reactant. Several factors, such as temperature, pressure, the concentrations of the reactants, and also the existence of a catalyst affect the rate of a chemical reaction. In some cases, what appears to be one reaction may in fact involve several reaction steps in series or in parallel, one of which may be rate limiting. 

The preceding method uses initial rates rather than rates at a later stage of the reaction because chemical reactions are reversible and we want to avoid complications from the reverse reaction reactants <— products. As the product concentrations build up, the rate of the reverse reaction increases. If the reverse rate becomes comparable to the forward rate, the measured rate will depend on the concentrations of both reactants and products. At the beginning of the reaction, however, the product concentrations are zero, and therefore the products can t affect the measured rate. When we measure an initial rate, we are measuring the rate of only the forward reaction, so only reactants (and catalysts see Section 12.12) can appear in the rate law. 

If the concentrations of the stoichiometrically-limiting reactant in the two phases are in equilibrium and if the chemical potential is the driving force, then, from thermodynamics, it is clear that the reaction rate is unaffected by the nature of the phase with which the solid is in contact, provided that no mass- and heat-transfer gradients exist and no blockage of the catalyst sites by the impurities occurs. However, the competitive adsorption of impurities in the liquid, even if these are inert to reaction, can markedly affect catalytic behavior. 

Any of six factors can affect the rate (1) the nature of the reactants, (2) the temperature, (3) the presence of a catalyst, (4) the concentration of reactants in solution, (5) the pressure of gaseous reactants, and (6) the state of subdivision of solid reactants. For a reaction to occur, the atoms, molecules, or ions must come into contact with one another with enough energy to rearrange chemical bonds in some way. Increased concentration, gas pressure, or surface area of a solid tends to get the particles to collide more frequently, and increased temperature tends to get them to collide more frequently and with greater energy to accomplish more effective collisions. Catalysts work in very many different ways. 

Concentration of Chemical Species Units of (moles/Iiter). Reaction rates are determined by both the concentrations of the reactants and the products of a reaction. Slurry chemicals either supply reactants or remove products, and hence as the concentration of the slurry chemicals increases, the reaction rates increase. Note, however, that the overall CMP process is composed of several steps and several different reactions. For any given set of process conditions, one of these steps will be limiting the total CMP removal rate. If an individual reaction is not part of this rate limiting step, the polish rate will be unaffected by changing the reaction rate. Only the reactions that are part of the rate limiting step will affect the polish rate. 

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

What happens if you add more reactant to a system in chemical equilibrium This increase in the reactant s concentration is a stress on the system. The system will respond to decrease the concentration of the reactant by changing some of the reactant into product. Therefore, the rate of the forward reaction must be greater than the rate of the reverse reaction. Because the forward reaction is increasing, the equilibrium is said to shift right. The reactant concentration will continue to drop until the reaction reaches equilibrium. Then, the forward and reverse reaction rates will be equal. Remember that changes in the amounts of solids and pure liquids do not affect values. 

As a first step in assessing the potential importance of nanoparticle reactions, we compare the volume and surface areas of these particles with the same values from other condensed phases with known chemical effects. We first consider nanoparticle volumes. As an upper limit, we consider an urban air parcel containing 20-nm diameter nanoparticles at a number concentration of 10 cm. Under this scenario, the nanoparticle volume is a small fraction (10 of the total air parcel volume. Thus the nanoparticle reaction rate (in units of mol m -air s ) would have to be ca. 10 times as fast as the equivalent gas phase reaction (mol m -air s ) to have a comparable overall rate in the air parcel. For comparison, clouds typically have liquid water contents of 10 to 10 (volume fraction) and can have significant effects upon atmospheric chemistry (Seinfeld and Pandis 1998). For simplicity of argument, if the medium of the cloud droplets and nanoparticles are assumed similar (e.g., dilute aqueous), then the fundamental rate constants in each medium are similar. Under this condition, reactant concentrations in the nanoparticles would need to be enhanced by 10, as compared to the cloud droplets, to have equal rates. Based on this analysis, it appears unlikely that reactions occurring in the bulk of nanoparticles could affect the composition of the gas phase. 

The majority of the degradation reactions of pharmaceuticals take place at finite rates and are chemical in nature. Solvent, concentration of reactants, temperature, pH of the medium, radiation energy, and the presence of catalysts are important factors that affect these reactions. The order of the reaction is characterized by the manner in which the reaction rate depends on the reactant concentration. The degradation of most pharmaceuticals is classified as zero order, first order, or pseudo-first order, although the compounds may degrade by complicated mechanisms, and the true expression may be of higher order or be complex and noninteger. 

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