Factors Affecting Reaction Rates

11 Key excerpts on "Factors Affecting Reaction Rates"

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  Chemistry

  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).

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  Food Engineering

  Principles and Practices

  • Sanjaya K. Dash, Pitam Chandra, Abhijit Kar(Authors)
  • 2023(Publication Date)
  • CRC Press

    (Publisher)

   The temperature dependence of a reaction rate is generally explained by the Arrhenius equation. As a rule of thumb for many reactions, every 10°C increase in temperature leads to a doubling of the reaction rate.

   In some rare situations, the reaction rate is not affected by the temperature (called non-Arrhenius behavior) or there may be a decrease in the reaction rate with an increase in temperature (anti-Arrhenius). The rate constant decreases with a temperature increase for those reactions that have no activation barrier (e.g., some radical reactions).

  • Surface area. The reaction rate increases with an increase in the surface area as more solid particles are exposed for the reaction. Stirring strongly influences the rates for heterogeneous reactions.
  • Order of reaction. It controls the reaction rate by managing the reactant concentration (or pressure).
  • Solvent. The properties of the solvent in a solution and the ionic strength affect the reaction rate.
  • Intensity of electromagnetic radiation. Electromagnetic radiation imparts energy to the reactants and may accelerate the reactions. An increase in the radiation intensity will increase the reaction rate.
  • Presence of catalyst. The rate of reaction is accelerated by the presence of a catalyst in both forward and reverse directions.

  Besides, reaction rates for the same molecule could be different because of the kinetic isotope effect if it has different isotopes.

  Out of all the factors mentioned above, the temperature is the most important one affecting the rate of reaction. Also, as we discussed, the rate equation consists of three parameters, viz., concentration; the rate constant, k; and the reaction order. Thus, all the above factors, except concentration and order, are considered in the rate constant, k.

  4.3 Influence of Temperature on Rate of Reaction

  The Arrhenius equation

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  Laboratory Manual for Principles of General Chemistry

  • J. A. Beran, Mark Lassiter(Authors)
  • 2022(Publication Date)
  • Wiley

    (Publisher)

  Jo A. Beran Objective Techniques and Safety Practices The following techniques and safety practices are used in the Experimental Procedure: Introduction and Applications Chemical kinetics is the study of chemical reaction rates, how reaction rates are con- trolled, and the pathway or mechanism by which a reaction proceeds from its reactants to its products. Reaction rates vary from the very fast, in which the reaction, such as the explo- sion of a hydrogen–oxygen mixture, is essentially complete in microseconds or even nanoseconds, to the very slow, in which the reaction, such as the setting of concrete, requires years to complete. The rate of a chemical reaction may be expressed as a change in the concentra- tion of a reactant (or product) as a function of time (e.g., per second)—the greater the change in the concentration per unit of time, the faster the rate of the reaction. Other parameters that can follow the change in concentration of a species as a function of time in a chemical reaction are color (expressed as absorbance, Figure 14.1), tempera- ture, pH, gas evolution (see opening photo), odor, and conductivity. The parameter chosen for following the rate of a particular reaction depends on the nature of the reac- tion and the species of the reaction. We will investigate four of five factors that can be controlled to affect the rate of a chemical reaction. The first four factors listed below are systematically studied in this experiment: • Nature of the reactants • Concentration of the reactants • Temperature of the chemical system • Surface area of the reactants • Presence of a catalyst Figure 14.1 The higher concentration of light-absorbing species, the more intense is the color of the solution. Species: any atom, molecule, or ion that may be a reactant or product of a chemical reaction Nature of the Reactants Some substances are naturally more reactive than others and therefore undergo rapid chemical changes.

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  Introduction to Chemical Engineering Kinetics and Reactor Design

  • Charles G. Hill, Thatcher W. Root(Authors)
  • 2014(Publication Date)
  • Wiley

    (Publisher)

  the rate of reaction observed is not identical with the intrinsic chemical reaction rate evaluated using the bulk fluid properties. The rate observed in the laboratory reflects the effects of both chemical and physical rate processes. The intrinsic rate may be thought of as the conversion rate that would exist if all physical rate processes occurred at infinitely fast rates.

  Situations in which both physical (e.g., mass transfer, diffusion, or heat transfer) and chemical rate processes influence the conversion rate are discussed in Chapter 12; the present chapter is concerned only with those situations for which the effects of physical rate processes are unimportant. This approach permits us to focus our concern on the variables that influence intrinsic chemical reaction rates (i.e., temperature, pressure, composition, and the presence or absence of catalysts in the system).

  In reaction rate studies one's goal is a phenomenological description of a system in terms of a limited number of empirical constants. Such descriptions permit one to predict the time-dependent behavior of similar systems. In these studies the usual procedure is to try to isolate the effects of the different variables and to investigate each independently. For example, one encloses the reacting system in a thermostat to maintain it at a constant temperature.

  Several generalizations can be made about the variables that influence reaction rates. Those that follow are in large measure adapted from Boudart's text (1).

  1. The rate of a chemical reaction depends on the temperature, pressure, and composition of the system under investigation.
  2. Certain species that do not appear in the stoichiometric equation for the reaction under study can markedly affect the reaction rate, even when they are present in only trace amounts. These materials are known as catalysts or inhibitors

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  Chemistry: Atoms First

  • William R. Robinson, Edward J. Neth, Paul Flowers, Klaus Theopold, Richard Langley(Authors)
  • 2016(Publication Date)
  • Openstax

    (Publisher)

  Catalysis will be discussed in greater detail later in this chapter as it relates to mechanisms of reactions. Link to Learning Chapter 17 | Kinetics 903 Figure 17.8 The presence of a catalyst increases the rate of a reaction by lowering its activation energy. Chemical reactions occur when molecules collide with each other and undergo a chemical transformation. Before physically performing a reaction in a laboratory, scientists can use molecular modeling simulations to predict how the parameters discussed earlier will influence the rate of a reaction. Use the PhET Reactions & Rates interactive (http://openstaxcollege.org/l/16PHETreaction) to explore how temperature, concentration, and the nature of the reactants affect reaction rates. 17.3 Rate Laws By the end of this section, you will be able to: • Explain the form and function of a rate law • Use rate laws to calculate reaction rates • Use rate and concentration data to identify reaction orders and derive rate laws As described in the previous module, the rate of a reaction is affected by the concentrations of reactants. Rate laws or rate equations are mathematical expressions that describe the relationship between the rate of a chemical reaction and the concentration of its reactants. In general, a rate law (or differential rate law, as it is sometimes called) takes this form: rate = k[ A] m [B] n [C] p … in which [A], [B], and [C] represent the molar concentrations of reactants, and k is the rate constant, which is specific for a particular reaction at a particular temperature. The exponents m, n, and p are usually positive integers (although it is possible for them to be fractions or negative numbers). The rate constant k and the exponents m, n, and p must be determined experimentally by observing how the rate of a reaction changes as the concentrations of the reactants are Link to Learning 904 Chapter 17 | Kinetics This OpenStax book is available for free at http://cnx.org/content/col12012/1.7

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  Chemistry 2e

  • Paul Flowers, Klaus Theopold, Richard Langley, William R. Robinson(Authors)
  • 2019(Publication Date)
  • Openstax

    (Publisher)

  INTRODUCTION CHAPTER 12 Kinetics 12.1 Chemical Reaction Rates 12.2 Factors Affecting Reaction Rates 12.3 Rate Laws 12.4 Integrated Rate Laws 12.5 Collision Theory 12.6 Reaction Mechanisms 12.7 Catalysis The lizard in the photograph is not simply enjoying the sunshine or working on its tan. The heat from the sun’s rays is critical to the lizard’s survival. A warm lizard can move faster than a cold one because the chemical reactions that allow its muscles to move occur more rapidly at higher temperatures. A cold lizard is a slower lizard and an easier meal for predators. From baking a cake to determining the useful lifespan of a bridge, rates of chemical reactions play important roles in our understanding of processes that involve chemical changes. Two questions are typically posed when planning to carry out a chemical reaction. The first is: “Will the reaction produce the desired products in useful quantities?” The second question is: “How rapidly will the reaction occur?” A third question is often asked when investigating reactions in greater detail: “What specific molecular-level processes take place as the reaction occurs?” Knowing the answer to this question is of practical importance when the yield or rate of a reaction needs to be controlled. 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. This chapter examines the factors Figure 12.1 An agama lizard basks in the sun. As its body warms, the chemical reactions of its metabolism speed up. CHAPTER OUTLINE that influence the rates of chemical reactions, the mechanisms by which reactions proceed, and the quantitative techniques used to describe the rates at which reactions occur.

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  Molecular Driving Forces

  Statistical Thermodynamics in Biology, Chemistry, Physics, and Nanoscience

  • Ken Dill, Sarina Bromberg(Authors)
  • 2010(Publication Date)
  • Garland Science

    (Publisher)

  19 Chemical Kinetics & Transition States Chemical Reaction Rates Depend on Temperature

  Now, we focus on the kinetics of chemical reactions. To predict how the rate of a chemical reaction depends on its molecular structures, you can use the same statistical thermodynamics approach that we used in Chapter 13 to model equilibria. Chemical reactions typically speed up more strongly with temperature than physical processes do. To understand this, you need one additional concept: the transition state or activation barrier.

  The Mass Action Laws Describe Mechanisms in Chemical Kinetics

  Consider a chemical reaction in which a product P is produced from reactants A, B, and C, with stoichiometric coefficients a, b, and c:

  a A + b B + c C → P .

  (19.1)

  In general, the reaction rate depends on the concentrations of the reactants, the temperature and pressure, and the coefficients a, b, and c. Experiments often measure how reaction rates depend on the concentrations of the reactants. Such experiments can provide valuable information about the mechanism of the reaction.

  The kinetic law of mass action, first developed by CM Guldberg and P Waage in 1864, says that reaction rates should depend on stoichiometry in the same way that equilibrium constants do. According to this law, the initial rate of product formation, d[P]/dt for the reaction in Equation (19.1) , depends on the reactant concentrations:

  d [ P ] d t = k f [ A ] a [ B ] b [ C ] c ,

  (19.2)

  where kf is the rate coefficient for the forward reaction. However, kinetic mechanisms often do not follow the thermodynamic stoichiometries. If a reaction is a single step, called an elementary reaction, then such expressions apply. The main types of elementary reactions are unimolecular decay of a molecule (a = 1, b = c = 0) or a bimolecular reaction when two molecules come together (a = b = 1, c = 0). However, chemical reactions typically involve multiple steps and kinetic intermediate states. For such non-elementary reactions, you cannot express the rate equation in such simple stoichiometric terms. At present, rate laws can only be determined from experiments. We do not address reaction mechanisms here; they are described in chemical kinetics textbooks such as [1 , 2

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  Combustion

  • Irvin Glassman, Richard A. Yetter, Nick G. Glumac(Authors)
  • 2014(Publication Date)
  • Academic Press

    (Publisher)

  2 ].

  2.2. Rates of Reactions and Their Temperature Dependence

  All chemical reactions, whether of the hydrolysis, acid–base, or combustion type, take place at a definite rate and depend on the conditions of the system. The most important of these conditions are the concentration of the reactants, the temperature, radiation effects, and the presence of a catalyst or inhibitor. The rate of the reaction may be expressed in terms of the concentration of any of the reacting substances or of any reaction product; that is, the rate may be expressed as the rate of decrease of the concentration of a reactant or the rate of increase of a reaction product.

  A stoichiometric relation describing a one-step chemical reaction of arbitrary complexity can be represented by the equation [3 ,4 ]

  ∑

  j = 1

  n

  ν j ′

  M j

  ⇄

  ∑

  j = 1

  n

  ν j ″

  M j

  (2.1)

  where

  ν j ′

  are the stoichiometric coefficients of the reactants,

  ν j ″

  are the stoichiometric coefficients of the products, M is an arbitrary specification of all chemical species, and n is the total number of species involved. If a species represented by Mj does not occur as a reactant or product, its ν j equals zero. Consider, as an example, the recombination of H atoms in the presence of H atoms, that is, the reaction

  H + H + H →

  H 2

  + H

  n = 2 ,

  M 1

  = H ,

  M 2

  =

  H 2

  ;

  v 1 ′

  = 3 ,

  v 1 ″

  = 1 ,

  v 2 ″

  = 1

  The reason for following this complex notation will become apparent shortly. The law of mass action, which is confirmed experimentally, states that the rate of a chemical reaction, defined as RR

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  Problems 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.

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  Chemistry, 5th Edition

  • Allan Blackman, Steven E. Bottle, Siegbert Schmid, Mauro Mocerino, Uta Wille(Authors)
  • 2022(Publication Date)
  • Wiley

    (Publisher)

  Qualitatively, the rate of the uni- molecular reaction is therefore proportional to the concentration of activated molecules, which in turn is proportional to the concentration of unenergised molecules. Temperature effects: the Arrhenius equation The concept of activation energy allows us to explain why the rate of a reaction increases so much with increasing temperature. The two plots in figure 15.17 correspond to different temperatures for the same Pdf_Folio:764 764 Chemistry mixture of reactants. Each curve is a plot of the different fractions of all collisions (vertical axis) that have a certain kinetic energy of collision (horizontal axis). The total area under a curve represents the total number of collisions. Notice what happens to the plots when the temperature is increased; the maximum point shifts to the right and the curve flattens somewhat. The shaded areas under the curves in figure 15.17 represent the sum of all the fractions of the total collisions that equal or exceed the activation energy. This sum — we could call it the reacting fraction — is much greater at the higher temperature than at the lower temperature, which can be seen in the significantly larger area under the curve beyond the activation energy with even a modest increase in temperature. In other words, at the higher temperature, a much greater fraction of the collisions results in a chemical change; the reaction is faster. FIGURE 15.17 Kinetic energy distributions for a reaction mixture at two different temperatures, T 1 and T 2 Fraction of collisions with a given kinetic energy T 1 T 2 T 2 > T 1 Kinetic energy of collision activation energy E a (minimum KE needed for reaction to occur) The sizes of the shaded areas under the curves (from the curve to the baseline) are proportional to the total fraction of the collisions that involve the minimum activation energy or more. In fact, the fractions of effective collisions increase exponentially with temperature.

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  Principles of Enzymology for the Food Sciences

  • John R. Whitaker(Author)
  • 2018(Publication Date)
  • Routledge

    (Publisher)

  max determined), the effect of temperature on the rate-determining step in conversion of enzyme-substrate complex to products can be determined. This ratedetermining step may involve covalent bond formation, covalent bond breakage, dissociation of product from enzyme, or enzyme conformational changes. The rate-determining step may be different with different substrates, at different pH values, and may change with temperature. If more than one step in a reaction contributes to the observed rate of a reaction, the observed effect of temperature will be a composite of effect of temperature on each of those steps.

  A. Quantitation of Effect of Temperature on Rates of Reactions

  A number of methods are available for quantitatively expressing the effect of temperature on rate of transformation of substrate to product. Each of these methods will be examined briefly for what it has to offer.

  1 Q 10

  A term often used in biology is that of Q 10 · Q 10 is defined as the increase in rate of a reaction for a 10°C increase in temperature:

  Q 10

  =

  rate

  T + 10 °

  rate T

  (13)

  It is determined by observing the rates of a reaction at two temperatures 10°C apart. The Q 10 values for most chemical and enzymatic reactions fall within the range 1.5 to 3. Although the difference between a Q 10 of 2 and 3 does not look large, consider the relative rates (based on 1 at 0°C) of 243 versus 32 at 50°C for a (210 of 3 and 2, respectively (Table 3 ). Such comparisons point out that the relationship between Q 10 and rate is logarithmic rather than linear:

  Table 3 Relative Rates at Different Q 10 Valuesa

  Δ rate =

  Q 10 n

  ,

  where n = Δ T / 10 ° C

  (14)

  When Q 10 = 2, a 10°C rise in temperature will double the rate of a reaction. It takes a doubling of reactant concentration to double the rate of a first-order process. Therefore, the effect of temperature on rates of reactions are extremely important.

  2 Arrhenius Equation

  The first quantitative, experimental formulation of the dependence of reaction rates on temperature was made by Hood. His formulations were later extended and made into a general equation by Arrhenius. The dependence of the specific reaction rate constant, k , on temperature is given by

  k = A

  e

  −

  E a

  / R T

  (15)

  which may be rewritten as [taking logarithms of both sides of Eq. (15 )]

  log k = log A −

  E a

  2.3 R T

  (16)

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