Factors That Influence Reaction Rates

Concepts Skills to Review Before You Stucl) This Chapter 

Reactions occur at a wide range of rates. Some, like a neutralization, a precipitation, or an explosive redox process, seem to be over as soon as the reactants make contact—in a fraction of a second. Others, such as the reactions involved in cooking or rusting, take a moderate length of time, from minutes to months. Still others take much longer the reactions that make up the human aging process continue for decades, and those involved in the formation of coal from dead plants take hundreds of millions of years. 

Knowing how fast a chemical change occurs can be essential. How quickly a medicine acts or blood clots can make the difference between life and death. How long it takes for cement to harden or polyethylene to form can make the difference between profit and loss. In general, the rates of these diverse processes depend on the same variables, most of which chemists can manipulate to maximize yields within a given time or to slow down an unwanted reaction. 

In this chapter, we first discuss reaction rate and then focus on the reaction mechanism, the steps a reaction goes through as reactant bonds are breaking and product bonds are forming. 

Let s begin our study of kinetics with a qualitative look at the key factors that affect how fast a reaction proceeds. Under any given set of conditions, each reaction has its own characteristic rate, which is determined by the chemical nature of the reactants. At room temperature, for example, hydrogen reacts explosively with fluorine but extremely slowly with nitrogen  

---

Alginate impression materials are chemically reactive mixtures. AH factors that influence reaction rates are, therefore, important in the use of these materials, ie, correct proportioning temperature of the water, powder, and mixing equipment and spatulation rate and duration. 

Many different approaches have been reported in the last decade toward a better understanding of the medium factors that influence reaction rates. Fundamental studies have been devoted to probe the reaction at a microscopic level in order to obtain information on the nature of several specific solvent-solute interactions on S Ar and to attempt a description of these effects quantitatively. Recent works have shown the wide applicability of a single parameter scale such as the Ex(30) Dimroth and Reichardt37, as well as other multi-parameter equations. 

Nitrogen, N2, is very unreactive. The Haber process is the economically important industrial process by which atmospheric N2 is converted to ammonia, NH3, a soluble, reactive compound. Innumerable dyes, plastics, explosives, fertilizers, and synthetic fibers are made from ammonia. The Haber process provides insight into kinetic and thermodynamic factors that influence reaction rates and the positions of equilibria. In this process the reaction between N2 and H2 to produce NH3 is never allowed to reach equilibrium, but moves toward it. 

In THE PREVIOUS chapters of this book we have discussed chemical principles, such as the laws of thermodynamics, the formation of chemical bonds, the behavior of different phases of matter, the factors that influence reaction rates and equilibria, and so forth. In... 

Factors That Influence Reaction Rates (Section 8.5)... 

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. 

The two factors that influence reaction rates are concentration and temperature. The rate of a reaction increases with increasing concentration. The rate of a reaction increases with increasing temperature. 

Classroom context. The lessons that we studied occurred over two days. On the first day, a double lesson (80 minutes) was given, starting with a recapitulation of the particulate nature of chemical reactions and factors that influence reaction rate, followed by activation energy, reaction profile diagrams, and the conditions for chemical equilibrium. The next day, a single lesson (40 minutes) elaborated the concept of dynamic equilibrium. No teaeher demonstrations or student practical work were included in the lessons. The topies were presented and discussed in an interactive way with Neil and his students asking many questions. 

---