Chemistry
Rate of Reaction and Temperature
The rate of a chemical reaction generally increases with temperature. This is due to the higher kinetic energy of the particles, leading to more frequent and energetic collisions. As a result, more successful collisions occur, leading to a faster rate of reaction. Conversely, decreasing the temperature typically slows down the rate of reaction.
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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- 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
The Molecular Nature of Matter
- James E. Brady, Neil D. Jespersen, Alison Hyslop(Authors)
- 2014(Publication Date)
- Wiley(Publisher)
Understanding the reaction at this level of detail often allows more precise control of the reaction’s speed, and suggests ways to modify the reaction to produce new types of products, or to improve the reaction’s yield by preventing undesirable side reac- tions from occurring. LEARNING OBJECTIVES After reading this chapter, you should be able to: • understand and use the five conditions that affect how rapidly chemicals react • determine, from experimental data, the relative rates at which reactants disappear and products appear, and the rate of reaction, which is independent of the substance monitored • use experimental initial rate data to determine rate laws • use the basic results of integrated rate laws to determine the order of a reaction and calculate the time dependence of concentration for zero-, first-, and second-order reactions • explain the rate of chemical reactions based on a molecular view of collisions that includes frequency, energy, and orientation that make up collision theory • describe the basics of transition state theory including activated complexes and potential energy diagrams • use the Arrhenius equation to determine the activation energy of a reaction • use the concepts of reaction mechanisms to recognize reasonable mechanisms and suggest plausible mechanisms given experimental data • relate the properties of homogeneous and heterogeneous catalysts and how they act to increase reaction rates This Chapter in Context 13.1 | Factors that Affect the Rate of Chemical Change The rate of a given chemical change is the speed with which the reactants disappear and the products form. When a reaction is fast, more product is formed in a given period of time than in a slow reaction. This rate is measured by the amount of products produced or reactants consumed per unit time. Usually this is done by monitoring the concentrations of the reactants or products over time, as the reaction proceeds (see Figure 13.1).
eBook - PDF- George W. Roberts(Author)
- 2015(Publication Date)
- Wiley(Publisher)
Chapter 2 Reaction Rates—Some Generalizations LEARNING OBJECTIVES After completing this chapter, you should be able to 1. use the Arrhenius relationship to calculate how reaction rate depends on temperature; 2. use the concept of reaction order to express the dependence of reaction rate on the individual species concentrations; 3. calculate the frequency of bimolecular and trimolecular collisions; 4. determine whether the rate equations for the forward and reverse rates of a reversible reaction are thermodynamically consistent; 5. calculate heats of reaction and equilibrium constants at various temperatures (review of thermodynamics). In order to design a new reactor, or analyze the behavior of an existing one, we need to know the rates of all the reactions that take place. In particular, we must know how the rates vary with temperature, and how they depend on the concentrations of the various species in the reactor. This is the field of chemical kinetics. This chapter presents an overview of chemical kinetics and introduces some of the molecular phenomena that provide a foundation for the field. The relationship between kinetics and chemical thermodynamics is also treated. The information in this chapter is sufficient to allow us to solve some problems in reactor design and analysis, which is the subject of Chapters 3 and 4. In Chapter 5, we will return to the subject of chemical kinetics and treat it more fundamentally and in greater depth. 2.1 RATE EQUATIONS A ‘‘rate equation’’ is used to describe the rate of a reaction quantitatively, and to express the functional dependence of the rate on temperature and on the species concentrations. In symbolic form, r A ¼ r A ðT , all C i Þ where T is the temperature. The term ‘‘all C i ’’ is present to remind us that the reaction rate can be affected by the concentrations of the reactant(s), the product(s), and any other compounds that are present, even if they do not participate in the reaction.
eBook - PDFChemistry
Principles and Reactions
- William Masterton, Cecile Hurley(Authors)
- 2020(Publication Date)
- Cengage Learning EMEA(Publisher)
11-5 Reaction Rate and Temperature The rates of most reactions increase as the temperature rises (Figure 11.11). A person in a hurry to prepare dinner applies this principle by turning the dial on the oven to the highest possible setting. By storing the leftovers in a refrigerator, the chemical reactions responsible for food spoilage are slowed down. ▲ As a general and very approximate rule, it is often stated that an increase in temperature of 10°C doubles the reaction rate. If this rule holds, foods should deteriorate four times as rapidly at room temperature (25°C) as they do in a refrigerator at 5°C. The effect of temperature on reaction rate ▲ can be explained in terms of ki-netic theory. Recall from Chapter 5 that raising the temperature greatly increases the fraction of molecules having very high speeds and hence high kinetic energies (kinetic energy 5 mu 2 /2). These are the molecules that are most likely to react when they collide. The higher the temperature, the larger the fraction of molecules that can provide the activation energy required for reaction. This effect is apparent from Figure 11.12 where the distribution of kinetic energies is shown at two different temperatures. Notice that the fraction of molecules having a kinetic energy equal to or greater than the activation energy E a (shaded area) is considerably larger at the Hibernating animals lower their body temperature, slowing down life processes. T q: k q: rate q Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 294 CHAPTER 11 Rate of Reaction Figure 11.12 Temperature and activation energy.
eBook - PDFEngineering Chemistry
Fundamentals and Applications
- Shikha Agarwal(Author)
- 2019(Publication Date)
- Cambridge University Press(Publisher)
13.13 Variation of Reaction Rates with Temperature – Arrhenius Equation The rate of a chemical reaction increases with the rise in temperature and it has been found that it becomes almost double for every 10 °C rise in temperature. This is known as the temperature coefficient, which may be defined as the ratio of rate constants of the reaction at two temperatures differing by 10 °C. Thus, Temperature coefficient = Rate constant ( 10 C) Rate constant at C T T + ° ° Figure 13.8 Variation of reaction rate with temperature The curve shows that at higher temperature the curve is shifted towards the right indicating that at higher temperatures the molecules have higher energies. Since the rate of reaction depends on effective collisions, that is, collisions with sufficient energy and proper orientation (to be discussed in collision theory of reaction rates); hence, as seen from the graph the number of effective collision doubles on increasing the temperature by 10 °C; therefore, the rate of reaction also doubles. Arrhenius equation and calculation of activation energy Swedish chemist Arrhenius in 1889 gave a method for expressing the influence of temperature on reaction velocity. He proposed a quantitative relationship between rate constant and temperature: / Ea RT k Ae − = (1) where k is the rate constant, A is the pre-exponential factor which is related to the frequency of collision, E a is the activation energy or the energy barrier which the reactants must cross to form products, R is the gas constant and T is the temperature in K. Chemical Kinetics 723 Taking logarithm of both sides, ln ln a E k A RT = − (2) Converting to the base 10 ( 10 ln 2.303 log x x = ), we get 2.303 log 2.303 log a E k A RT = − log log 2.303 a E k A RT = − (3) The above equation shows that the value of k decreases as activation energy increases; hence, the rate of reaction decreases with the rise in activation energy.
eBook - PDFProblems in Metallurgical Thermodynamics and Kinetics
International Series on Materials Science and Technology
- G. S. Upadhyaya, R. K. Dube, D. W. Hopkins(Authors)
- 2013(Publication Date)
- Pergamon(Publisher)
CHAPTER 9 KINETICS 9.1 Introduction A chemical or metallurgical reaction is thermodynamically possible only when there is a decrease in free energy. All the equations which we use in the thermodynamic treatment of a metallurgical reaction refer to equilibrium conditions. A reaction may be thermodynamically possible, but in practice the reaction may not proceed to completion in a measurable period of time. In other words, the thermodynamic treatment does not provide information on the rate of reaction. For this reason, another theoretical approach -'kinetics 1 - has been used to study the rate of reaction. The rate or velocity of a reaction may be defined as the rate of decrease of the concentration of a reactant or as the rate of increase of a product of the reaction. If a reactant of initial concentration C has a concentra-tion C at any time t , the rate is expressed as (-dC/dt). If the con-centration of the product is x at any time t , the rate is expressed as (dx/dt). 9.2 Effect of Concentration on the Reaction Rate The rate of a chemical reaction is proportional to the concentration of the reacting substances. The sum of the powers to which the concentration of the reacting atoms or molecules must be raised to determine the rate of reaction, is known as the 'order of reaction'. The order of reaction does not bear any relation to the molecularity of the reaction. The expressions for the rates of reactions of different orders can be evaluated as follows. 203 204 PROBLEMS IN METALLURGICAL THERMODYNAMICS AND KINETICS 9.2.1 First-Order Reaction In a first-order reaction, for example, A = X + Y, the rate of reaction is given by -£ = kC, (9.1) at where C is the concentration of A at any time t , and k is a constant known as the velocity constant, rate constant, or specific reaction rate. On integrating Eq.(9.1) within the limits C = C at t = 0, and C = C at t = t, k = 1^91 log A . , (9.2) t C - x o where x is the amount of A reacted in time t.
eBook - PDFReaction Rate Constant Computations
Theories and Applications
- Keli Han, Tianshu Chu(Authors)
- 2013(Publication Date)
- Royal Society of Chemistry(Publisher)
The temperatures in the three types of system, (a), (b) and (c), vary widely, from up to several thousand K in (a) to as low as 10 K in (c), emphasising the need either for measurements over a correspondingly wide range of temperatures, or for theoretical methods capable either of calculating the rate constants over similar ranges of temperature or, at least, of extrapolating the values of the experimental rate constants that may have been determined over a limited range of temperatures—or even a single temperature—to the temperatures appropriate to the models of a particular environment. 1.2 A Little History The systematic study of chemical kinetics, that is, of the rates of chemical reactions and their dependence on temperature, dates back to the middle of the 19th century. During the next 60 years, a number of expressions were proposed to express the temperature-dependence of the rate constant, k ( T ). These efforts have been reviewed by Laidler, 1 who pointed out the difficulty of distinguishing between the various proposals of how k ( T ) varies with temperature when the available values of k ( T ) cover only a small temperature range. After the early years of the 20th century, attention focused on what is generally referred to as the Arrhenius equation: k T ð Þ¼ A exp ð E act = RT Þ ð 1 : 2 Þ where A is best referred to as the pre-exponential factor and E act is the acti-vation energy, and a modified form of this equation in which additional temperature-dependence is allowed for in the pre-exponential term: k T ð Þ¼ A 0 T m exp ð E 0 act = RT Þ ð 1 : 3 Þ These equations came to be favoured over other temperature-dependent expressions for the rate constant largely because, in Laidler’s words, the other expressions were ‘theoretically sterile’, whereas eqn (1.2) could be rationalised on the basis of the reactants requiring some minimum amount of energy to undergo reaction.
eBook - PDF- Allan Blackman, Steven E. Bottle, Siegbert Schmid, Mauro Mocerino, Uta Wille(Authors)
- 2022(Publication Date)
- Wiley(Publisher)
(c) Which step is the rate-determining step? Explain. 15.124 Which of the five factors that affect the rate of a reaction is illustrated in each of the following? LO2 (a) Food is sometimes frozen before it is used. (b) Small sticks of wood are often used to start a fire. (c) In hospitals, the speed of the healing process is often increased in an oxygen tent. 15.125 Why would you expect the rate of the reaction: LO2 Ag + (aq) + Br − (aq) → AgBr(s) at room temperature to be much faster than the rate of the reaction: CH 4 (g) + 2O 2 (g) → CO 2 (g) + 2H 2 O(l) at room temperature? 15.126 Consider the reaction Zn(s) + 2HCl(aq) → ZnCl 2 (aq) + H 2 (g). What would be the effect on the rate of reaction if each of the following was done? LO6 (a) powdered zinc was used instead of a solid piece of zinc metal (b) the concentration of HCl(aq) was halved (c) the temperature was lowered Pdf_Folio:794 794 Chemistry MATHS FOR CHEMISTRY In chemistry, we often study particular aspects of a chemical reaction over a period of time, or as we change the temperature. For example, we might be interested in how the concentration of a reactant in a chemical reaction changes with time, or how the rate of a chemical reaction changes as the temperature is increased. One of the best ways of displaying trends in these data is to plot them on a graph. Generally, we do this in two dimensions, by plotting a graph of y (called the dependent variable) versus x (called the independent variable). Consider the following set of (x,y) data. (0, 2), (1, 4), (2, 6), (3, 8), (4, 10), (5, 12), (6, 14), (7, 16), (8, 18), (9, 20), (10, 22) Plotting these on an (x,y) graph gives the following. y-axis x-axis 0 0 2 4 6 8 10 12 5 10 15 20 25 It is immediately obvious that a straight line can be drawn through the data points, and we say that there is therefore a linear relationship between x and y.
eBook - PDFChemistry
The Molecular Nature of Matter
- Neil D. Jespersen, Alison Hyslop(Authors)
- 2021(Publication Date)
- Wiley(Publisher)
Next, is the amount of time reasonable? Five minutes to cook something is reasonable, as long as the temperature is high enough. 13.8 Mechanisms of Reactions A balanced equation generally describes only a net overall change. Usually, however, the net change is the result of a series of simple reactions that are not at all evident from the equation. Consider, for example, the combustion of propane, C 3 H 8 : C 3 H 8 (g) + 5O 2 (g) ⟶ 3CO 2 (g) + 4H 2 O(g) Anyone who has ever played billiards knows that this reaction simply cannot occur in a single, simultaneous collision between one propane molecule and five oxygen molecules. Just get- ting only three balls to come together with but one “click” on a flat, two-dimensional sur- face is extremely improbable. How unlikely it must be, then, for the simultaneous collision in 688 CHAPTER 13 Chemical Kinetics three-dimensional space of six reactant molecules, one of which must be C 3 H 8 and the other five O 2 . Instead, the combustion of propane proceeds very rapidly by a series of much more probable steps. The series of individual steps that add up to the overall observed reaction is called the reaction mechanism. Information about reaction mechanisms is one of the divi- dends paid by the study of reaction rates. Each individual step in a reaction mechanism is a simple chemical reaction called an elementary process. An elementary process is a reaction involving collisions between molecules. As you will soon see, a rate law can be written from the equation for an elemen- tary process, using coefficients as exponents for the concentration terms without requiring experiments to determine the exponents. For most reactions, the individual elementary processes cannot actually be observed because they involve substances of fleeting exis- tence; instead, we only see the net reaction. Therefore, the mechanism a chemist writes is really a model about what occurs step-by-step as the reactants are changed to the products.
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