Chemistry
Reaction Rates
Reaction rates refer to the speed at which a chemical reaction takes place, typically measured by the change in concentration of reactants or products over time. Factors influencing reaction rates include temperature, concentration, surface area, and the presence of catalysts. Understanding reaction rates is crucial for optimizing industrial processes and developing new materials in chemistry.
Written by Perlego with AI-assistance
Related key terms
1 of 5
10 Key excerpts on "Reaction Rates"
eBook - PDF- Paul Flowers, Klaus Theopold, Richard Langley, William R. Robinson(Authors)
- 2015(Publication Date)
- Openstax(Publisher)
In this chapter, we will examine the factors that Chapter 12 | Kinetics 651 influence the rates of chemical reactions, the mechanisms by which reactions proceed, and the quantitative techniques used to determine and describe the rate at which reactions occur. 12.1 Chemical Reaction Rates By the end of this section, you will be able to: • Define chemical reaction rate • Derive rate expressions from the balanced equation for a given chemical reaction • Calculate Reaction Rates from experimental data A rate is a measure of how some property varies with time. Speed is a familiar rate that expresses the distance traveled by an object in a given amount of time. Wage is a rate that represents the amount of money earned by a person working for a given amount of time. Likewise, the rate of a chemical reaction is a measure of how much reactant is consumed, or how much product is produced, by the reaction in a given amount of time. The rate of reaction is the change in the amount of a reactant or product per unit time. Reaction Rates are therefore determined by measuring the time dependence of some property that can be related to reactant or product amounts. Rates of reactions that consume or produce gaseous substances, for example, are conveniently determined by measuring changes in volume or pressure. For reactions involving one or more colored substances, rates may be monitored via measurements of light absorption. For reactions involving aqueous electrolytes, rates may be measured via changes in a solution’s conductivity. For reactants and products in solution, their relative amounts (concentrations) are conveniently used for purposes of expressing Reaction Rates.
eBook - PDFChemistry
Principles and Reactions
- William Masterton, Cecile Hurley(Authors)
- 2020(Publication Date)
- Cengage Learning EMEA(Publisher)
▼ 274 ▼ Rate of Reaction 11 The rate at which the speeding train is traveling can be expressed in km/h. Similarly, the rate of a reaction can be expressed in M /h. Not every collision, not every punctilious trajectory by which billiard-ball complexes arrive at their calculable meeting places leads to reaction. Men (and women) are not as different from molecules as they think. —ROALD HOFFMANN Excerpt from Men and Molecules Chapter Outline 11-1 Meaning of Reaction Rate 11-2 Reaction Rate and Concentration 11-3 Reactant Concentration and Time 11-4 Models for Reaction Rate 11-5 Reaction Rate and Temperature 11-6 Catalysis 11-7 Reaction Mechanisms ▼ F or a chemical reaction to be feasible, it must occur at a reasonable rate. Conse-quently, it is important to be able to control the rate of reaction. Most often, this means making it occur more rapidly. When you carry out a reaction in the gen-eral chemistry laboratory, you want it to take place quickly. A research chemist trying to synthesize a new drug has the same objective. Sometimes, though, it is desirable to reduce the rate of reaction. The aging process, a complex series of biological oxida-tions, believed to involve “free radicals” with unpaired electrons such as O } H and O } O 2 is one we would all like to slow down. This chapter sets forth the principles of chemical kinetics , the study of Reaction Rates. The main emphasis is on those factors that influence rate. These include ■ ■ the concentrations of reactants (Sections 11-2 and 11-3). ■ ■ the process by which the reaction takes place (Section 11-4). ■ ■ the temperature (Section 11-5). ■ ■ the presence of a catalyst (Section 11-6). ■ ■ the reaction mechanism (Section 11-7). 11-1 Meaning of Reaction Rate To discuss reaction rate meaningfully, it must be defined precisely. The rate of reac-tion is a positive quantity that expresses how the concentration of a reactant or product changes with time.
eBook - PDFFoundations of Chemistry
An Introductory Course for Science Students
- Philippa B. Cranwell, Elizabeth M. Page(Authors)
- 2021(Publication Date)
- Wiley(Publisher)
Chemists and chemical engineers must be able to control reaction conditions in order to obtain maximum yields by optimising factors such as temperature and pressure. Equally important is the time required to produce products. Fast reaction times reduce costs and energy requirements. Reaction times are critical when designing drugs and in drug deliv-ery. To be effective a drug must be sufficiently inert that it reaches its target before breaking down but must then react quickly and produce waste products that are readily removed from the body. The area of chemistry concerned with studying and controlling the rates of chemical reactions is known as chemical kinetics . Chemists study the rates at which chemical reactions occur so they can control them. The series of molecular processes that occur when a chemical reaction takes place is called the mechanism of the reaction, and understanding the mechanism helps chemists control the rate of reaction . 8.2 The rate of reaction 8.2.1 Defining the rate of a chemical reaction The rate of a chemical reaction is defined as the increase in concentration of one of the products of reaction divided by the time taken. Alternatively, it can be defined as the decrease in concentration of one of the reactants divided by the time: Rate of reaction = change in concentration of reactant or product time taken for the change A plot of concentration against time is given in Figure 8.1 for the hypothetical reaction of reactant A being converted to product B, as represented by the equa-tion A B. The rate can be expressed as: Rate of reaction = change in concentration of B time or Δ B Δ t 256 Chemical kinetics – the rates of chemical reactions The symbol Δ (Greek letter delta) means ‘ a change ’ , so Δ [B] represents a change in concentration of B and Δ t is the time taken for this change to occur. The units for reaction rate are therefore units of concentration divided by time: typically, mol dm -3 s -1 .
eBook - PDF- William R. Robinson, Edward J. Neth, Paul Flowers, Klaus Theopold, Richard Langley(Authors)
- 2016(Publication Date)
- Openstax(Publisher)
So why do we not see diamond spontaneously forming graphite? The answer is found in the topic of this Chapter 17 | Kinetics 895 chapter, chemical kinetics, which involves the rate at which reactions take place. It turns out that while the conversion of diamond into graphite is a spontaneous process, it occurs so slowly that we do not observe the conversion taking place on any time scale we know. The study of chemical kinetics concerns the second and third questions—that is, the rate at which a reaction yields products and the molecular-scale means by which a reaction occurs. In this chapter, we will examine the factors that influence the rates of chemical reactions, the mechanisms by which reactions proceed, and the quantitative techniques used to determine and describe the rate at which reactions occur. 17.1 Chemical Reaction Rates By the end of this section, you will be able to: • Define chemical reaction rate • Derive rate expressions from the balanced equation for a given chemical reaction • Calculate Reaction Rates from experimental data A rate is a measure of how some property varies with time. Speed is a familiar rate that expresses the distance traveled by an object in a given amount of time. Wage is a rate that represents the amount of money earned by a person working for a given amount of time. Likewise, the rate of a chemical reaction is a measure of how much reactant is consumed, or how much product is produced, by the reaction in a given amount of time. The rate of reaction is the change in the amount of a reactant or product per unit time. Reaction Rates are therefore determined by measuring the time dependence of some property that can be related to reactant or product amounts. Rates of reactions that consume or produce gaseous substances, for example, are conveniently determined by measuring changes in volume or pressure. For reactions involving one or more colored substances, rates may be monitored via measurements of light absorption.
eBook - PDFEngineering Chemistry
Fundamentals and Applications
- Shikha Agarwal(Author)
- 2016(Publication Date)
- Cambridge University Press(Publisher)
11.1 Introduction The study of chemical reactions has been of great interest to the chemists. They wish to investigate the manner in which the reaction occurs and the speed at which it takes place. They are also interested in studying the effect of various parameters like temperature, pressure and concentration on the rate of the reaction. Chemical kinetics deals with all the above and can be defined as the branch of chemistry which deals with • Rate of chemical reaction • Mechanism by which the reactants are converted into products and • Factors affecting the rate of a reaction It is observed that different reactions occur at different rates. Some reactions are fast and some reactions occur very slowly and take months or even years for their completion, while some reactions occur at a moderate rate. It is these reactions occurring at moderate speed which are of great interest to the chemists as their rates can easily be measured in the laboratory. Depending upon their speed the chemical reactions are categorized as follows 1. Fast Reactions Reactions that occur instantaneously as soon as the reactants are mixed together are termed as fast reactions. For example, the precipitation of the solution of silver nitrate and sodium chloride. AgNO 3 + NaCl → AgCl+ NaNO 3 Similarly, neutralization of acid and base also occurs as soon as the two substances are mixed. Generally, ionic reactions are fast reactions because they involve ions that are held together by electrostatic forces and no bonds are broken in them. The rates of these reactions are too fast to be determined by conventional methods. These methods cannot deal with reactions whose half lives are less than a second or so. Special techniques are employed to measure the rate of such reactions. 2. Very slow reactions Some reactions like rusting of iron occur so slowly that one can be misled into thinking that no reaction is occurring at all.
- Frank R. Foulkes(Author)
- 2012(Publication Date)
- CRC Press(Publisher)
1. In general, the concentrations initially change quite rapidly with time, and then level off as they 1 Unless the reaction started 75,000 years ago! 2 Sometimes, for gas phase reactions, we plot pressure vs. time instead of concentration vs. time; but plotting concentration is more common. products reactants Concn time c o 0 eqm Fig. 1 26-2 CHEMICAL REACTION KINETICS approach equilibrium. 3 Usually, as products begin to form, there is the possibility that some of them will start reacting back to reactants again: C + D A A + B In general the process is a dynamic one in which the net rate is given by: Reaction Rate Net = Forward Rate – Reverse Rate . . . [1] At equilibrium, the net rate equals zero; i.e., we have a dynamic equilibrium for which Forward Rate = Reverse Rate . . . [2] The reverse rate generally starts to become significant as equilibrium is approached. 26.3 EXPRESSION OF Reaction Rates When the reactants are first brought together (i.e., before any significant amount of product has formed), the net rate is approximately just the forward rate. Consider the reaction (in a constant volume reactor) of hydrogen with oxygen to form water: 2H 2(g) + O 2(g) A 2H 2 O (g) The rates of change with time of the concentrations of the various reactants and products are related according to < 1 2 d [H ] dt 2 = < d [O ] dt 2 = + 1 2 d [H O] dt 2 . . . [3] That is, H 2 is used up twice as fast as O 2 , and H 2 O is produced at the same rate at which H 2 is consumed. In general, for the reaction aA + bB A cC + dD Rate (mol L –1 s –1 ) = r = < 1 a d[A] dt = < 1 b d[B] dt = + 1 c d[C] dt = + 1 d d[D] dt . . . [4] Example 26-1 The rate of the reaction A + 2B A 3C + D was reported as 2.0 mol L –1 s –1 . State the rates of consumption or formation of each participant. 3 The introductory treatment of chemical kinetics given in this chapter deals mainly with constant volume batch processes in closed systems.
eBook - PDFElementary Chemical Reactor Analysis
Butterworths Series in Chemical Engineering
- Rutherford Aris(Author)
- 2013(Publication Date)
- Butterworth-Heinemann(Publisher)
Sometimes there is less than this, and the construction of a reactor becomes an exercise in the art of experienced guessing and scale-up. The health of the chemical industry 53 4 54 Reaction Rates Chap. 4 is a tribute to the skillful practice of this art, but the paucity of background information emphasizes the need for a general understanding of reaction and reactor behavior. The student can at least begin to get a feel for this from reaction rate expressions conceived in the conjunction of chemical theory and experiment and developed in the spirit of applied mathematics. But he should be warned that this will give him only the beginning of wisdom; it can never presume to displace the knowledge that comes of experience. In summary, all that is needed to develop a coherent description of reac-tor analysis is provided by a formula for r(£, T, P) that is in accord with chemical realities. What can be obtained in an industrial situation may, for reasons of time and money, fall far short of this precision; but whatever expression can be obtained must play the same role in practical design as does r in the theoretical analysis. 4.2 Homogeneous Reaction Rates Before developing the general expression that is often used for homogeneous reactions let us look at a simple and by now familiar reaction. Our standard way of writing the reaction A 2 + A 3 ^ Ai is A 1 -A 2 -A 3 = 0, (4.2.1) where the species A 2 and A 3 are the reactants and A x is the product. We know all about the stoichiometry of this reaction (Sec. 2.4) and about its equilibrium (Sec. 3.7) and wish presently to describe its kinetics. Consider first the situa-tion when the reactants are present in concentrations c 2 and c 3 but no product has yet been formed. The rate of reaction will depend on the concentrations of the reactants and on temperature.
- Frederick Bettelheim, William Brown, Mary Campbell, Shawn Farrell(Authors)
- 2019(Publication Date)
- Cengage Learning EMEA(Publisher)
200 Reaction Rates and Chemical Equilibrium 7 CONTENTS 7.1 Measuring Reaction Rates 7.2 Molecular Collisions and Reactions 7.3 Activation Energy and Reaction Rate 7.4 Rate of a Chemical Reaction 7.5 Equilibrium 7.6 The Equilibrium Constant How To… Interpret the Value of the Equilibrium Constant K 7.7 Le Chatelier’s Principle 7.1 Measuring Reaction Rates In this chapter, we are going to look at two closely related topics—Reaction Rates and chemical equilibrium. Knowing whether a reaction takes place quickly or slowly can give important information about the process in ques-tion. If the process has health implications, the information can be especially crucial. Sooner or later, many reactions will appear to stop, but that simply means that two reactions that are the reverse of each other are proceeding at the same rate. When this is the case, the reaction is said to be at equi-librium. The study of chemical equilibrium gives information about how to control reactions, including those that play key roles in life processes. We will address chemical equilibrium later in this chapter. Some chemical reactions take place rapidly; others are very slow. For example, glucose and oxygen gas react with each other to form water and carbon dioxide: C 6 H 12 O 6 ( s ) Glucose 1 6O 2 ( g ) 6CO 2 ( g ) 1 6H 2 O( , ) This reaction is extremely slow, however. A sample of glucose exposed to O 2 in the air shows no measurable change even after many years. In contrast, consider what happens when you take one or two aspirin tablets for a slight headache. Very often, the pain disappears in half an hour or so. Thus, the aspirin must have reacted with compounds in the body within that time. When a glowing ribbon of magnesium is thrust into a beaker of carbon dioxide (from the sublimation of dry ice at the bottom of the beaker), the metal bursts into a brilliant white flame, producing a smoke of magnesium and carbon. Copyright 2020 Cengage Learning. All Rights Reserved.
eBook - PDFChemistry
The Molecular Nature of Matter
- James E. Brady, Neil D. Jespersen, Alison Hyslop(Authors)
- 2014(Publication Date)
- Wiley(Publisher)
Experiments yielded the following results: Initial Concentrations (mol L -1 ) Initial Rate of Formation of C (mol L -1 s -1 ) [A] [B] 0.40 0.30 1.00 Ž 10 -4 0.60 0.30 2.25 Ž 10 -4 0.80 0.60 1.60 Ž 10 -3 (a) What is the rate law for the reaction? (b) What is the value of the rate constant? (c) What are the units for the rate constant? (d) What is the overall order of this reaction? (Hint: Solve for the exponent of [A], then use it to solve for the exponent of [B].) 13.4 | Integrated Rate Laws The rate law tells us how the speed of a reaction varies with the concentrations of the reac- tants. Often, however, we are more interested in how the concentrations change over time. For instance, if we were preparing some compound, we might want to know how long it will take for the reactant concentrations to drop to some particular value, so we can decide when to isolate the products. The relationship between the concentration of a reactant and time can be derived from a rate law using calculus. By summing or “integrating” the instantaneous rates of a reaction from the start of the reaction until some specified time, t, we can obtain integrated rate laws that quantitatively give concentration as a function of time. The form of the integrated rate law depends on the order of the reaction. The mathematical expressions that relate concentra- tion and time in complex reactions can be complicated, so we will concentrate on using integrated rate laws for a few simple first- and second-order reactions with only one reactant. First-Order Reactions A first-order reaction is a reaction that has a rate law of the type rate = k 3 A 4 Practice Exercise 13.13 Practice Exercise 13.14 644 Chapter 13 | Chemical Kinetics Using calculus, 2 the following equation can be derived that relates the concentration of A and time: ln 3 A 4 0 3 A 4 t = kt (13.5) The symbol “ln” means natural logarithm.
eBook - PDFChemistry
The Molecular Nature of Matter
- Neil D. Jespersen, Alison Hyslop(Authors)
- 2021(Publication Date)
- Wiley(Publisher)
This is rather like the situation in a crowded supermarket with only a single checkout lane open. It doesn’t matter how many people join the line; the line will move at the same rate no matter how many people are standing in it. An example of a zero- order chemical reaction is the elimination of ethyl alcohol in the body, by the liver. Regardless of the blood alcohol level, the rate of alcohol removal by the body is constant, because the number of available catalyst molecules present in the liver is constant. Another zero-order reaction is the decomposition of gaseous ammonia into H 2 and N 2 on a hot platinum surface. The rate at which ammonia decomposes is the same, regardless of its concentration in the gas. The rate law for a zero-order reaction is simply rate = k where the rate constant k has units of mol L −1 s −1 . The rate constant depends on the amount, quality, and available surface area of the catalyst. For example, forcing the ammonia through hot platinum powder (with a high surface area) would cause it to decompose faster than simply passing it over a hot platinum surface. In all of the rate laws, the rate constant, k, indicates how fast a reac- tion proceeds. If the value for k is large, the reaction proceeds rapidly, and if k is small, the reaction is slow. The units for k must be such that the rate calculated from the rate law has units of mol L −1 s −1 . A list of units for k, as it depends on the overall order of the reaction, is given in Table 13.2. NOTE When an exponent in an equation is found to be 1, it is usually omitted. 1 The reason for describing the order of a reaction is to take advantage of a great convenience—namely, the mathematics involved in the treatment of the data is the same for all reactions having the same order. We will not go into this very deeply, but you should be familiar with this terminology; it’s often used to describe the effects of concentration on Reaction Rates.
Index pages curate the most relevant extracts from our library of academic textbooks. They’ve been created using an in-house natural language model (NLM), each adding context and meaning to key research topics.
