Chemistry

Factors Affecting Reaction Rates

Factors affecting reaction rates include concentration, temperature, surface area, catalysts, and the nature of the reactants. Increasing the concentration of reactants or the temperature generally speeds up reactions, while larger surface areas and catalysts can also accelerate the rate. The nature of the reactants, such as their chemical structure and reactivity, also plays a significant role in determining reaction rates.

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  • AP® Chemistry Crash Course Book + Online
    eBook - ePub

    AP® Chemistry Crash Course Book + Online

    PART V

    RATES OF REACTION

    Passage contains an image

    Chapter 14

    Factors Affecting Rates of Reaction

    Note: It is likely that you will have been taught the contents of Chapter 15 before the contents of Chapter 14 , and that makes sense. This book is compiled in the order of the Big Ideas and Enduring Understandings outlined in the College Board course and exam description.

    I. Kinetics

    A.   Concept of Rate of Reaction 1.   Rate of a chemical reaction is a measure of the change in concentration of reactants or products over time.
    2.   Can be measured as the decrease of the reactant concentration per unit time.
    3.   Can be measured as the increase of the product concentration per unit time.
    4.   One important method employed to measure rate is using Beer’s law to determine the concentration of a colored solution as a reaction proceeds. B.   Conditions That Can Affect Rate
    1.   Increasing the surface area of a solid reactant can increase the rate by increasing the number of collisions between the reactant particles (see Chapter 15 ).
    2.   Catalyst increases the rate by lowering the activation energy of a reaction (see Chapter 17 ).
    3.   Increasing the temperature results in a faster reaction. The rate constant is temperature dependent and a rise in temperature will increase the rate constant (see below).
    4.   Concentration of reactants increases the amount of reactants colliding with each other, thus yielding product (see Chapter 15 ).
    C.   Use of Experimental (Concentration) Data and Graphical Analysis to Determine Reactant Order, Rate Laws, and Rate Constants 1.   General formula for rate equation. For the generic reaction
    Rate of reaction = k [A ]x [B ]y [C ]z
    where k is the rate constant and x , y , z are the orders with respect to A , B , and C , respectively, but are not necessarily the stoichiometric coefficients of A , B , and C .
    2.   Orders
        i.      The order with respect to a reactant is the exponent of the concentration term in the rate equation (a.k.a. the rate law
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  • AP® Chemistry All Access Book + Online + Mobile
    eBook - ePub

    AP® Chemistry All Access Book + Online + Mobile

    11

    Chemical Kinetics

    Rates of Reaction

    Chemical kinetics involves the measurement of how fast a chemical reaction occurs. Many of the reactions that are performed in AP Chemistry classes happen at a relatively fast rate: the reactants are mixed together and seconds, maybe minutes later, the products are formed and the reaction is over. Some reactions are spontaneous but still occur at a slow rate, such as the reaction of diamond with air to form carbon dioxide—which takes thousands of years to occur. There are a variety of factors that affect the rate of a chemical reaction, including concentration, the nature of the reactants, temperature, surface area, and the presence of a catalyst.

    Measuring Reaction Rates

    The reaction rate of a chemical reaction is a measure of the change in concentration per unit of time. Reaction rate can be expressed in terms of either the appearance of a product or the disappearance of a reactant. For example, the rate of the reaction below can be expressed as follows.
    In the preceding equation, the negative sign indicates that the reactant is decreasing in concentration. The multiplicative factor ½ in front of the HCl is utilized for stoichiometric reasons since 2 moles of HCl react for every 1 mole of products formed.

    Effect of Concentration

    In general, increasing the concentration of a reactant will increase the rate of a chemical reaction. One way to study the effect of concentration on reaction rate is through studying the rate using initial concentrations of the reactants (method of initial rates ). The overall dependence of concentration of reactants on rate can be expressed as an experimental rate law . For the following hypothetical reaction, the rate law has the form:
    where k is the rate constant , x and y are the orders of the individual reactants , and [A] and [B] are the concentrations of A and B, respectively. The values for k , x , and y can only be determined experimentally; you cannot figure them out from the balanced chemical equation. The orders of reaction are an indication of how a given reactant actually reacts in a chemical reaction. For example, a reactant that reacts in a second-order fashion will quadruple the rate of reaction when the concentration of that reactant is doubled (rate = k [2]2 = 4 times as fast). The sum of the individual orders of reaction will give the overall order of the reaction
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  • Phenomenological Creep Models of Composites and Nanomaterials
    eBook - ePub

    Phenomenological Creep Models of Composites and Nanomaterials

    Deterministic and Probabilistic Approach

    • Leo Razdolsky(Author)
    • 2019(Publication Date)
    • CRC Press
      (Publisher)
    3 the following relations are correct
    (5.9)
    v =
    1
    ( - 1)
    dC
    N 2
    d τ
    =
    1
    ( - 3)
    dC
    H 2
    d τ
    (
    n V
    ) =
    1 2
    dC
    NH 3
    d τ
    Thus, the rate of the chemical reaction is always a positive value and has the same value for the given reaction irrespective of the change in the concentration of which reagent is expressed.
    The main factors affecting the rate of chemical reactions are the temperature, the concentration of reactants, the presence of a catalyst in the system. According to the law of effective masses at T = const: the reaction rate is proportional to the product of the concentrations of the reacting substances in certain degrees, therefore, in general, the relationship of the velocity with these concentrations can be represented for the reaction
    a(A) + b(B) = c(C) + d(D)
    as follows:
    (5.10)
    v = K[C
    A
    n A
    ][C
    B
    n B
    ]
    In this equation, called the kinetic equation, the proportionality coefficient K is a rate constant, and the exponents nA and nB partial orders of reaction for the substance A and B. The sum of the exponents at the concentrations in the kinetic equation of the reaction is called the total (general) order nA +nB
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  • Physical Chemistry of Foods
    eBook - ePub

    Physical Chemistry of Foods

    • Pieter Walstra(Author)
    • 2002(Publication Date)
    • CRC Press
      (Publisher)

    4 Reaction Kinetics

    Chemical kinetics is generally discussed with respect to reactions between molecules (or ions or radicals) in a gas phase or in a very dilute solution. In foods, we often have other situations. The system never is gaseous, it is rarely very dilute, and it may have more than one phase containing reactants. Changes may occur within molecules, especially macromolecules. Reactions may be between particles, causing, for instance, their aggregation. Numerous other changes may occur, such as phase transitions, leading to a change in rheological properties, color, or other perceptible property. In nearly all such cases we are greatly interested in the rate at which these processes occur. This we cannot derive from the bond energies involved or from other thermodynamic considerations: these may tell us what the driving force is, but in general the rate results from a driving force divided by a resistance, and the resistance may be very large or highly variable.
    In this chapter, we will recall some basic aspects of chemical reaction kinetics in solution, starting from an oversimplified point of view and gradually bringing in more complications. We will not discuss theory aimed at explaining reaction rates on a molecular level (molecular reaction dynamics). Other rate processes will be discussed in Chapters 5 and 13 .

    4.1 REACTION ORDER

    Before coming to factors determining reaction rates, it is useful to review the manner in which concentrations depend on time.
    The reaction rate is usually given as the change in concentration c, i.e., as either + or -dc/dt. According to the units of c, it may be expressed in mol·L-1 ·s-1 (the most common way), mol·kg-1·s-1 , number ·m-3·s-1 , etc.
    For a zero-order reaction, the rate remains constant: see Table 4.1
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  • General Chemistry for Engineers
    eBook - ePub

    General Chemistry for Engineers

    • Jeffrey Gaffney, Nancy Marley(Authors)
    • 2017(Publication Date)
    • Elsevier
      (Publisher)
    reaction rate , defined as the change in concentration as a function of time, can often vary widely for different chemical reactions. While chemical thermodynamics predicts if a chemical reaction will occur, chemical kinetics predicts how fast a reaction will occur. So, it can be said that thermodynamics guides while kinetics decides if a reaction will occur in a time frame that will be useful. In addition, by determining the effects of temperature and concentrations on the reaction rate, chemical kinetics can provide a means of controlling and optimizing the speed of a chemical reaction.
    Chemical kinetics uses an experimental approach to study the rates and mechanisms of chemical reactions. It accomplishes this through careful measurements of the reactant and/or product concentrations as a function of time. For the general chemical reaction between reactants A and B producing the products C and D;
    a A + b B → c C + d D
    the changes in the concentrations of [A], [B], [C], and [D] are measured as a function of time and are described mathematically in the form of a derivative of each concentration with respect to time. The reactants are represented as negative derivatives because their concentrations decrease with time and the products are represented as positive derivatives because their concentrations increase with time:
    Reactant concentration change = − d A / dt and − d B / dt
    Product concentration change = d C / dt and d D / dt
    The change in the concentrations between the reactants A and B and the products C and D will depend upon the stoichiometric coefficients: a , b , c , and d . For the simple case of a  = b  = c  = d
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  • CLEP® Chemistry Book + Online
    eBook - ePub

    CLEP® Chemistry Book + Online

    CHAPTER 7

    Kinetics

    CHAPTER 7

    KINETICS

    REACTION RATE

    • Kinetics determines the speed of a reaction, and is dependent on the mechanism by which reactants turn into products. • Reaction rate is based on the rate of appearance of a product or disappearance of a reactant, and is expressed as a change in concentration over time. • Reaction rates are determined experimentally by measuring concentrations. • Reaction rates increase by increasing concentration or by increasing the temperature. • Reaction rates may also increase by increasing the surface area of the reactant—which is the same as increasing the concentration of possible participants in the reaction, or by adding a catalyst.

    RATE LAW AND REACTION ORDER

    • The rate law describes the rate of the reaction as a function of a rate constant, which is dependent on the temperature and the concentrations of the reactants.
    • All rate laws take the form of rate = k [reactants] x , where k is the rate constant, [reactants] refers to the molar concentration of reactants, and the exponent, x, is the reaction order .
    • For all single-step reactions , the rate law for a particular step can be surmised from the balanced equation for that step because the rate of the forward reaction is proportional to the concentration of available reactants.
    Example:
    Total reaction (single step): A → B + C      rate = k [A]
      (first order) Or
    Total reaction (single step): A + A → B      rate = k [A]2
      (second order) Or
    Total reaction (single step): A + B → C      rate = k [A] [B]
      (second order overall)
    • It cannot be assumed that a reaction involves only a single step. Therefore, the rate law (and determination of the order of reaction with respect to each reactant) can only be determined experimentally.
    Example: Use the experimental data in the table to determine the rate law for the following reaction, A + B → C Solution:
    When doubling [B] between the first and second trial (holding [A] constant), the reaction rate also doubles. Therefore, the rate of the reaction is first order with respect to [B]. When doubling [A] between the second and third trial (holding [B] constant), the reaction rate quadruples. Therefore, the rate changes as a square of a change in [A]; the reaction rate is second order with respect to [A]. The overall rate law of the reaction is the combination of the two orders: rate = k [A]2
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  • The Really Useful Book of Secondary Science Experiments
    eBook - ePub

    The Really Useful Book of Secondary Science Experiments

    101 Essential Activities to Support Teaching and Learning

    • Tracy-ann Aston(Author)
    • 2017(Publication Date)
    • Routledge
      (Publisher)
    EXPERIMENT 42 Fair testing: How does temperature affect the rate of a reaction?    

    LEARNING OBJECTIVES:

    Investigate how temperature affects the rate of a reaction.

    INTRODUCTION:

    The students react starch solution and potassium iodate solution in order to see the effect of temperature on the rate of the reaction.

    USEFUL PRIOR WORK:

    The students should understand the term rate of reaction.

    BACKGROUND SCIENCE:

    Chemical reactions occur when the particles of the reactants collide with enough energy to form new products. This energy is referred to as the activation energy. The rate of the reaction refers to the rate at which the reactants are used up and the products are formed. If the temperature of the reactants is increased this will have two effects. Firstly, the particles will have more kinetic energy, meaning they will be moving at a greater rate and therefore more likely to collide with each other. Secondly, the particles will be colliding with more energy, meaning they are more likely to reach the activation energy level when they do collide. Both of these factors will increase the rate of the reaction.

    NATIONAL CURRICULUM LINKS:

    Chemical reactions

    • chemical reactions as the rearrangement of atoms
    • representing chemical reactions using formulae and using equations.

    MATERIALS NEEDED:

    Beakers, water baths, measuring cylinders, timers, starch solution, potassium iodate solution.

    SAFETY AND TECHNICAL NOTES:

    • Potassium iodate is oxidising.
    • Set up at least five temperatures for the students to test ranging from 20–60ºC.
    • Eye protection should be worn for this investigation.

    METHOD:

    To be done in advance by the teacher

    Set up the water baths and set to the necessary temperatures. Place bottles or beakers of starch solution and potassium iodate solution into the water baths so they can come up to the correct temperature.

    Students:

    1
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  • How Enzymes Work
    eBook - ePub

    How Enzymes Work

    From Structure to Function

    Chapter 3

    Factors That Affect Enzyme Activity

    The previous chapters described briefly that the enzyme activity is affected by the concentration of substrate and of product. This and the following chapters describe the effect of factors affecting the enzyme activity more in detail. What can we get by studying the effect of various factors on the activity of enzyme? These studies will lead to a construction of the reaction mechanism, and will give us the thermodynamic parameters of the reaction.

    3.1    Enzyme Concentration

    In Chapter 2 , we have known that the enzymatic reaction is expressed by the Michaelis–Menten mechanism:
    E + S
    ⇄
    k
    − 1
    k
    + 1
        ES
    →
    k
    + 2
    E + P
    		(3.1)
    Under the steady state of reaction, the rate of the reaction is expressed by the Michaelis–Menten equation:
    v =
    k
    + 2
    e 0
    s
    K m
    + s
     
    		(3.2)
    K m
    =
    k
    − 1
    +
    k
    + 2
    k
    + 1
    Thus, as Eq. 3.2 shows, plotting v vs. the enzyme concentration gives a linear relation (line A in Fig. 3.1 ) when the concentration of substrate is sufficiently larger than that of the concentration of enzyme.
    Figure 3.1 The effect of the concentration of enzyme on the enzymatic activity.
    Sometimes, the data like lines B and C will be obtained. In these cases, the Michaelis–Menten kinetics could not be applied. At least two cases are conceivable when these data were obtained. One is artifacts caused by the experimental conditions, and the other is the nature of the enzyme itself. In the first case, the enzyme preparation and/or the reaction mixture may contain an inhibitor or an activator of enzyme. In the case of line B, the enzyme preparation may contain an activator of enzyme, but the activator may dissociate at the low concentration of the enzyme. Or the reaction mixture may contain an inhibitor of enzyme, thus inhibiting at low concentration of enzyme. However, high concentrations of enzyme consume the inhibitor and prevent the inhibition. Similarly, in the case of line C, the enzyme preparation may contain the inhibitor of enzyme, thus the inhibitor dissociates from the enzyme at the low concentration of enzyme, showing relatively high activity. Or the reaction mixture may contain the activator of the enzyme, leading line C. In these cases, removal of the inhibitor or activator in the reaction mixture including enzyme itself may cause the linearity in the plot of the activity vs. enzyme concentration. Even though an inhibitor or an activator was removed, the enzyme still gives the data like lines B or C. This means that the enzyme has its own nature to show such a result. These considerations lead the important conclusion that the kinetic experiment must be performed using the highly purified enzyme.
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