Chapter 14 Chemical Kinetics CHEMISTRY The Central Science 9th Edition Chapter 14 Chemical Kinetics David P. White Prentice Hall © 2003 Chapter 14

Factors that Affect Reaction Rates Kinetics is the study of how fast chemical reactions occur. There are 4 important factors which affect rates of reactions: reactant concentration, temperature, action of catalysts, and surface area. Goal: to understand chemical reactions at the molecular level. Prentice Hall © 2003 Chapter 14

Reaction Rates Speed of a reaction is measured by the change in concentration with time. For a reaction A  B Suppose A reacts to form B. Let us begin with 1.00 mol A. Prentice Hall © 2003 Chapter 14

Reaction Rates Prentice Hall © 2003 Chapter 14

Reaction Rates At t = 0 (time zero) there is 1.00 mol A (100 red spheres) and no B present. At t = 20 min, there is 0.54 mol A and 0.46 mol B. At t = 40 min, there is 0.30 mol A and 0.70 mol B. Calculating, Prentice Hall © 2003 Chapter 14

Reaction Rates For the reaction A  B there are two ways of measuring rate: the speed at which the products appear (i.e. change in moles of B per unit time), or the speed at which the reactants disappear (i.e. the change in moles of A per unit time). Prentice Hall © 2003 Chapter 14

C4H9Cl(aq) + H2O(l)  C4H9OH(aq) + HCl(aq) Reaction Rates Change of Rate with Time For the reaction A  B there are two ways of Most useful units for rates are to look at molarity. Since volume is constant, molarity and moles are directly proportional. Consider: C4H9Cl(aq) + H2O(l)  C4H9OH(aq) + HCl(aq) Prentice Hall © 2003 Chapter 14

C4H9Cl(aq) + H2O(l)  C4H9OH(aq) + HCl(aq) Reaction Rates Change of Rate with Time C4H9Cl(aq) + H2O(l)  C4H9OH(aq) + HCl(aq) We can calculate the average rate in terms of the disappearance of C4H9Cl. The units for average rate are mol/L·s or M/s. The average rate decreases with time. We plot [C4H9Cl] versus time. The rate at any instant in time (instantaneous rate) is the slope of the tangent to the curve. Instantaneous rate is different from average rate. We usually call the instantaneous rate the rate. Prentice Hall © 2003 Chapter 14

C4H9Cl(aq) + H2O(l)  C4H9OH(aq) + HCl(aq) Reaction Rates Reaction Rate and Stoichiometry For the reaction C4H9Cl(aq) + H2O(l)  C4H9OH(aq) + HCl(aq) we know In general for aA + bB  cC + dD Prentice Hall © 2003 Chapter 14

Concentration and Rate In general rates increase as concentrations increase. NH4+(aq) + NO2-(aq)  N2(g) + 2H2O(l) Prentice Hall © 2003 Chapter 14

Concentration and Rate For the reaction NH4+(aq) + NO2-(aq)  N2(g) + 2H2O(l) we note as [NH4+] doubles with [NO2-] constant the rate doubles, as [NO2-] doubles with [NH4+] constant, the rate doubles, We conclude rate  [NH4+][NO2-]. Rate law: The constant k is the rate constant. Prentice Hall © 2003 Chapter 14

Concentration and Rate Exponents in the Rate Law For a general reaction with rate law we say the reaction is mth order in reactant 1 and nth order in reactant 2. The overall order of reaction is m + n + …. A reaction can be zeroth order if m, n, … are zero. Note the values of the exponents (orders) have to be determined experimentally. They are not simply related to stoichiometry. Prentice Hall © 2003 Chapter 14

Concentration and Rate Using Initial Rates to Determines Rate Laws A reaction is zero order in a reactant if the change in concentration of that reactant produces no effect. A reaction is first order if doubling the concentration causes the rate to double. A reacting is nth order if doubling the concentration causes an 2n increase in rate. Note that the rate constant does not depend on concentration. Prentice Hall © 2003 Chapter 14

The Change of Concentration with Time First Order Reactions Goal: convert rate law into a convenient equation to give concentrations as a function of time. For a first order reaction, the rate doubles as the concentration of a reactant doubles. Prentice Hall © 2003 Chapter 14

The Change of Concentration with Time First Order Reactions A plot of ln[A]t versus t is a straight line with slope -k and intercept ln[A]0. In the above we use the natural logarithm, ln, which is log to the base e. Prentice Hall © 2003 Chapter 14

The Change of Concentration with Time First Order Reactions Prentice Hall © 2003 Chapter 14

The Change of Concentration with Time Second Order Reactions For a second order reaction with just one reactant A plot of 1/[A]t versus t is a straight line with slope k and intercept 1/[A]0 For a second order reaction, a plot of ln[A]t vs. t is not linear. Prentice Hall © 2003 Chapter 14

The Change of Concentration with Time Second Order Reactions Prentice Hall © 2003 Chapter 14

The Change of Concentration with Time Half-Life Half-life is the time taken for the concentration of a reactant to drop to half its original value. For a first order process, half life, t½ is the time taken for [A]0 to reach ½[A]0. Mathematically, Prentice Hall © 2003 Chapter 14

The Change of Concentration with Time Half-Life For a second order reaction, half-life depends in the initial concentration: Prentice Hall © 2003 Chapter 14

Temperature and Rate The Collision Model Most reactions speed up as temperature increases. (E.g. food spoils when not refrigerated.) When two light sticks are placed in water: one at room temperature and one in ice, the one at room temperature is brighter than the one in ice. The chemical reaction responsible for chemiluminescence is dependent on temperature: the higher the temperature, the faster the reaction and the brighter the light. Prentice Hall © 2003 Chapter 14

Temperature and Rate The Collision Model As temperature increases, the rate increases.

Temperature and Rate The Collision Model Since the rate law has no temperature term in it, the rate constant must depend on temperature. Consider the first order reaction CH3NC  CH3CN. As temperature increases from 190 C to 250 C the rate constant increases from 2.52  10-5 s-1 to 3.16  10-3 s-1. The temperature effect is quite dramatic. Why? Observations: rates of reactions are affected by concentration and temperature. Prentice Hall © 2003 Chapter 14

Temperature and Rate The Collision Model Goal: develop a model that explains why rates of reactions increase as concentration and temperature increases. The collision model: in order for molecules to react they must collide. The greater the number of collisions the faster the rate. The more molecules present, the greater the probability of collision and the faster the rate. Prentice Hall © 2003 Chapter 14

Temperature and Rate The Collision Model The higher the temperature, the more energy available to the molecules and the faster the rate. Complication: not all collisions lead to products. In fact, only a small fraction of collisions lead to product. The Orientation Factor In order for reaction to occur the reactant molecules must collide in the correct orientation and with enough energy to form products. Prentice Hall © 2003 Chapter 14

Temperature and Rate The Orientation Factor Consider: Cl + NOCl  NO + Cl2 There are two possible ways that Cl atoms and NOCl molecules can collide; one is effective and one is not. Prentice Hall © 2003 Chapter 14

Temperature and Rate The Orientation Factor Prentice Hall © 2003 Chapter 14

Temperature and Rate Activation Energy Arrhenius: molecules must posses a minimum amount of energy to react. Why? In order to form products, bonds must be broken in the reactants. Bond breakage requires energy. Activation energy, Ea, is the minimum energy required to initiate a chemical reaction. Prentice Hall © 2003 Chapter 14

Temperature and Rate Activation Energy Consider the rearrangement of methyl isonitrile: In H3C-NC, the C-NC bond bends until the C-N bond breaks and the NC portion is perpendicular to the H3C portion. This structure is called the activated complex or transition state. The energy required for the above twist and break is the activation energy, Ea. Once the C-N bond is broken, the NC portion can continue to rotate forming a C-CN bond. Prentice Hall © 2003 Chapter 14

Temperature and Rate Activation Energy The change in energy for the reaction is the difference in energy between CH3NC and CH3CN. The activation energy is the difference in energy between reactants, CH3NC and transition state. The rate depends on Ea. Notice that if a forward reaction is exothermic (CH3NC  CH3CN), then the reverse reaction is endothermic (CH3CN  CH3NC). Prentice Hall © 2003 Chapter 14

Temperature and Rate Activation Energy How does a methyl isonitrile molecule gain enough energy to overcome the activation energy barrier? From kinetic molecular theory, we know that as temperature increases, the total kinetic energy increases. We can show the fraction of molecules, f, with energy equal to or greater than Ea is where R is the gas constant (8.314 J/mol·K). Prentice Hall © 2003 Chapter 14

Temperature and Rate Activation Energy Prentice Hall © 2003 Chapter 14

Temperature and Rate The Arrhenius Equation Arrhenius discovered most reaction-rate data obeyed the Arrhenius equation: k is the rate constant, Ea is the activation energy, R is the gas constant (8.314 J/K-mol) and T is the temperature in K. A is called the frequency factor. A is a measure of the probability of a favorable collision. Both A and Ea are specific to a given reaction. Prentice Hall © 2003 Chapter 14

Temperature and Rate Determining the Activation Energy If we have a lot of data, we can determine Ea and A graphically by rearranging the Arrhenius equation: From the above equation, a plot of ln k versus 1/T will have slope of –Ea/R and intercept of ln A. Prentice Hall © 2003 Chapter 14

Temperature and Rate

Temperature and Rate Determining the Activation Energy If we do not have a lot of data, then we recognize Prentice Hall © 2003 Chapter 14

Reaction Mechanisms The balanced chemical equation provides information about the beginning and end of reaction. The reaction mechanism gives the path of the reaction. Mechanisms provide a very detailed picture of which bonds are broken and formed during the course of a reaction. Elementary Steps Elementary step: any process that occurs in a single step. Prentice Hall © 2003 Chapter 14

Reaction Mechanisms Elementary Steps Molecularity: the number of molecules present in an elementary step. Unimolecular: one molecule in the elementary step, Bimolecular: two molecules in the elementary step, and Termolecular: three molecules in the elementary step. It is not common to see termolecular processes (statistically improbable). Prentice Hall © 2003 Chapter 14

Reaction Mechanisms Multistep Mechanisms Some reaction proceed through more than one step: NO2(g) + NO2(g)  NO3(g) + NO(g) NO3(g) + CO(g)  NO2(g) + CO2(g) Notice that if we add the above steps, we get the overall reaction: NO2(g) + CO(g)  NO(g) + CO2(g) Prentice Hall © 2003 Chapter 14

Reaction Mechanisms Multistep Mechanisms If a reaction proceeds via several elementary steps, then the elementary steps must add to give the balanced chemical equation. Intermediate: a species which appears in an elementary step which is not a reactant or product. Prentice Hall © 2003 Chapter 14

Reaction Mechanisms Rate Laws for Elementary Steps The rate law of an elementary step is determined by its molecularity: Unimolecular processes are first order, Bimolecular processes are second order, and Termolecular processes are third order. Rate Laws for Multistep Mechanisms Rate-determining step: is the slowest of the elementary steps. Prentice Hall © 2003 Chapter 14

Reaction Mechanisms Rate Laws for Elementary Steps Prentice Hall © 2003 Chapter 14

Reaction Mechanisms Rate Laws for Multistep Mechanisms Therefore, the rate-determining step governs the overall rate law for the reaction. Mechanisms with an Initial Fast Step It is possible for an intermediate to be a reactant. Consider 2NO(g) + Br2(g)  2NOBr(g) Prentice Hall © 2003 Chapter 14

Reaction Mechanisms Mechanisms with an Initial Fast Step 2NO(g) + Br2(g)  2NOBr(g) The experimentally determined rate law is Rate = k[NO]2[Br2] Consider the following mechanism Prentice Hall © 2003 Chapter 14

Reaction Mechanisms Mechanisms with an Initial Fast Step The rate law is (based on Step 2): Rate = k2[NOBr2][NO] The rate law should not depend on the concentration of an intermediate (intermediates are usually unstable). Assume NOBr2 is unstable, so we express the concentration of NOBr2 in terms of NOBr and Br2 assuming there is an equilibrium in step 1 we have Prentice Hall © 2003 Chapter 14

Reaction Mechanisms Mechanisms with an Initial Fast Step By definition of equilibrium: Therefore, the overall rate law becomes Note the final rate law is consistent with the experimentally observed rate law. Prentice Hall © 2003 Chapter 14

Catalysis A catalyst changes the rate of a chemical reaction. There are two types of catalyst: homogeneous, and heterogeneous. NO molecules are catalysts for the destruction of ozone. Homogeneous Catalysis The catalyst and reaction is in one phase. Prentice Hall © 2003 Chapter 14

Catalysis Homogeneous Catalysis Hydrogen peroxide decomposes very slowly: 2H2O2(aq)  2H2O(l) + O2(g) In the presence of the bromide ion, the decomposition occurs rapidly: 2Br-(aq) + H2O2(aq) + 2H+(aq)  Br2(aq) + 2H2O(l). Br2(aq) is brown. Br2(aq) + H2O2(aq)  2Br-(aq) + 2H+(aq) + O2(g). Br- is a catalyst because it can be recovered at the end of the reaction. Prentice Hall © 2003 Chapter 14

Catalysis Homogeneous Catalysis Generally, catalysts operate by lowering the activation energy for a reaction. Prentice Hall © 2003 Chapter 14

Catalysis

Catalysis Homogeneous Catalysis Catalysts can operate by increasing the number of effective collisions. That is, from the Arrhenius equation: catalysts increase k be increasing A or decreasing Ea. A catalyst may add intermediates to the reaction. Example: In the presence of Br-, Br2(aq) is generated as an intermediate in the decomposition of H2O2. Prentice Hall © 2003 Chapter 14

Catalysis Homogeneous Catalysis When a catalyst adds an intermediate, the activation energies for both steps must be lower than the activation energy for the uncatalyzed reaction. Heterogeneous Catalysis The catalyst is in a different phase than the reactants and products. Typical example: solid catalyst, gaseous reactants and products (catalytic converters in cars). Most industrial catalysts are heterogeneous. Prentice Hall © 2003 Chapter 14

Catalysis Heterogeneous Catalysis First step is adsorption (the binding of reactant molecules to the catalyst surface). Adsorbed species (atoms or ions) are very reactive. Molecules are adsorbed onto active sites on the catalyst surface. Prentice Hall © 2003 Chapter 14

Catalysis Prentice Hall © 2003 Chapter 14

C2H4(g) + H2(g)  C2H6(g), H = -136 kJ/mol. Catalysis Heterogeneous Catalysis Consider the hydrogenation of ethylene: C2H4(g) + H2(g)  C2H6(g), H = -136 kJ/mol. The reaction is slow in the absence of a catalyst. In the presence of a metal catalyst (Ni, Pt or Pd) the reaction occurs quickly at room temperature. First the ethylene and hydrogen molecules are adsorbed onto active sites on the metal surface. The H-H bond breaks and the H atoms migrate about the metal surface. Prentice Hall © 2003 Chapter 14

Catalysis Heterogeneous Catalysis Enzymes When an H atom collides with an ethylene molecule on the surface, the C-C  bond breaks and a C-H  bond forms. When C2H6 forms it desorbs from the surface. When ethylene and hydrogen are adsorbed onto a surface, less energy is required to break the bonds and the activation energy for the reaction is lowered. Enzymes Enzymes are biological catalysts. Most enzymes are protein molecules with large molecular masses (10,000 to 106 amu). Prentice Hall © 2003 Chapter 14

Catalysis Enzymes Enzymes have very specific shapes. Most enzymes catalyze very specific reactions. Substrates undergo reaction at the active site of an enzyme. A substrate locks into an enzyme and a fast reaction occurs. The products then move away from the enzyme. Prentice Hall © 2003 Chapter 14

Catalysis Enzymes Only substrates that fit into the enzyme lock can be involved in the reaction. If a molecule binds tightly to an enzyme so that another substrate cannot displace it, then the active site is blocked and the catalyst is inhibited (enzyme inhibitors). The number of events (turnover number) catalyzed is large for enzymes (103 - 107 per second). Prentice Hall © 2003 Chapter 14

Catalysis Enzymes Prentice Hall © 2003 Chapter 14

End of Chapter 14 Chemical Kinetics Prentice Hall © 2003 Chapter 14