FACTORS DETERMINING THE RATE OF A CHEMICAL REACTION

What factors determine the rate of a chemical reaction Give examples of reactions proceeding at various rates. 

These observations remind us of Chapter 8, in which we considered the factors that determine the rate of a chemical reaction. Of course, the same ideas apply here. We can draw qualitative information about the mechanism of the reaction by applying the collision theory. With quantitative study of the effects of temperature and concentration on the rate, we should be able to construct potential energy diagrams like those shown in Figure 8-6 (p. 134). 

As we saw in Section 12.10, the activation energy Ea is one of the most important factors affecting the rate of a chemical reaction. Its value can be determined using the Arrhenius equation if values of the rate constant are known at different temperatures. Taking the natural logarithm of both sides of the Arrhenius equation, we obtain the logarithmic form... 

The second major kinetic factor determining the flux of a chemical reaction is the rate constant itself. An enzyme reaction may be represented, in the simplest case, as follows (ignoring the back reaction of product) ... 

A number of factors influence the rate of a chemical reaction. The concentration of reactants present is one critical factor, and the experimental determination of this effect results in an expression referred to as the rate law. We can work with rate laws in either of two forms the differential rate law and the integrated rate law. The differential rate law tells us the rate if we know the appropriate concentrations, whereas the integrated rate law predicts the concentration of a reactant as a function of time. 

Enzymatic Catalysis. Enzymes are biological catalysts. They increase the rate of a chemical reaction without undergoing permanent change and without affecting the reaction equiUbrium. The thermodynamic approach to the study of a chemical reaction calculates the equiUbrium concentrations using the thermodynamic properties of the substrates and products. This approach gives no information about the rate at which the equiUbrium is reached. The kinetic approach is concerned with the reaction rates and the factors that determine these, eg, pH, temperature, and presence of a catalyst. Therefore, the kinetic approach is essentially an experimental investigation. 

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. 

The rate of a chemical reaction is of a zero order if it is independent of the concentrations of the participating substances. The rate of reaction is determined by such limiting factors as ... 

Describe the effect of temperature on the rate of a chemical reaction and use appropriate methods to determine the Arrhenius activation energy and Arrhenius 4-factor from experimental data. (Questions 6.1, 6.2 and Exercise 6.1)... 

When this equation is valid, a straight-line plot of (1 - Cf versus dCldt is obtained for the value of n, and k is calculated from the slope. Rate constants determined for more than one temperature can be used to calculate the Arrhenius activation energy constant. The Arrhenius equation predicts the rate of a chemical reaction at a given temperature (Kelvin temperature). The equation is a function of a frequency factor or preexponential factor A, mathematical quantity e, gas constant R, temperature T in kelvins, and activation energy E. 

Rates of Chemical Reactions The rate of a chemical reaction is the amount of reactant(s) that goes to prod-uct(s) in a given period of time. In general, reaction rates increase with increasing reactant concentration and increasing temperature. Since reaction rates depend on the concentration of reactants, and since the concentration of reactants decreases as a reaction proceeds, reaction rates usually slow down as a reaction proceeds. Rates of Chemical Reactions The rate of a chemical reaction determines how fast a reaction will reach its equilibrium. Chemists want to understand the factors that influence reaction rates so that they can control them. 

Write the mathematical equation used to determine the average rate of a chemical reaction. What factor is held constant What are the variables ... 

The single most important factor that determines the rate of a reaction is concentration — primarily the concentrations of the reactants, but sometimes of other species that may not even appear in the reaction equation. The relation between the rate of a reaction and the concentration of chemical species is termed the rate law it is the cornerstone of reaction mechanisms. The rate law alone allows much insight into the mechanism. This is usually supplemented by an examination of other factors which can also be revealing. (For these, see Chap. 2)... 

The sole factor determining the value of a rate or equilibrium constant is the difference in free energy between the reactants and either a transition state, in the case of a reaction rate, or the products, in the case of an equilibrium. Rate and equilibrium constants cannot therefore properly be correlated with any static property of the reactants themselves. This point cannot be emphasized too strongly in view of the many attempts that have been made to find such correlations. One might add that attempts of this kind are in any case basically unsatisfactory in that they ignore the transition state entirely and so can throw no light on its structure even if correlations of this kind can be established, they do not throw any light on the detailed mechanism of chemical process—and it is just in this field that quantum theory has most to offer to chemists. 

Our goal in this chapter is to understand how to determine reaction rates and to consider the factors that control these rates. What factors determine how rapidly food spoils, for instance, or how does one design a fast-setting material for dental fiUings What determines the rate at which steel rusts, or how can we remove hazardous pollutants in automobile exhaust before the exhaust leaves the tailpipe Although we will not address these specific questions, we will see that the rates of all chemical reactions are subject to the same principles. 

Kinetics is the study of the speed of a chemical reaction or process and is a critical element in toxicology. Most processes described in the discipline follow zero- and first-order kinetics. Recall that rate laws are employed to determine concentrations as a function of time. Although there are other variables and factors, the elimination of a xenobiotic substance from the body is fundamentally dependent on the kinetics of the reactions involved. Elimination processes often involve catalysts (enzymes), the role of which is central in determining the overall speed of the elimination, since a catalyst reduces the energy of activation. Eg. 

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. 

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