Chapter 14 Chemical Kinetics

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.

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.

Reaction Rates

Reaction Rates At t = 0 (time zero) there is 1.00 mol A (100 red spheres) and no B present. At t = 10 min, there is 0.74 mol A and 0.36 mol B. At t = 40 min, there is 0.30 mol A and 0.70 mol B. Calculating,

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).

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)

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.

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

Example to consider For a hypothetical reaction A + 2B 3C + 2D Suppose that at one point in the reaction [A]=0.4658M, and 125 s later, [A]=0.4282M. During this time period, what is the average Rate or reactions expressed in M/s and rate of formation of C, expressed in M/min.

Concentration and Rate In general rates increase as concentrations increase. NH4+(aq) + NO2-(aq)  N2(g) + 2H2O(l)

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.

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.

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.

D 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.

D 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.

D Concentration with Time First Order Reactions

An example to consider For a first-order reaction of H2O2(aq), given that k=3.66x10-3s-1 and [H2O2]0 =0.882M, determine The time at which [H2O2]=0.600M and The concentration of [H2O2] after 225 s.

D 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.

D Concentration with Time Second Order Reactions

D 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,

D Concentration with Time Half-Life For a second order reaction, half-life depends in the initial concentration:

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.

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.