Chemical Kinetics PDF

Chemical Kinetics  Reaction Rates Reaction rate is the change in the concentration of a reactant or a product with time (M/s). A B Δ[A] Δ[A] = change in concentration of A over rate = -...

Chemical Kinetics  Reaction Rates Reaction rate is the change in the concentration of a reactant or a product with time (M/s). A B Δ[A] Δ[A] = change in concentration of A over rate = - Δt time period Δt Δ[B] Δ[B] = change in concentration of B over rate = Δt time period Δt Because [A] decreases with time, Δ[A] is negative. Chung (Peter) Chieh University of Waterloo  Reaction Rates C4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq) A plot of concentration vs. time for this reaction yields a curve like this. The slope of a line tangent to the curve at any point is the instantaneous rate at that time. John D. Bookstaver St. Charles Community College  Reaction Rates and Stoichiometry To generalize, for the reaction aA + bB cC + dD Reactants (decrease) Products (increase) John D. Bookstaver St. Charles Community College  Rate in Terms of Concentration Each reaction has its own equation that expresses its rate as a function of the concentrations of the involved species (e.g., reactants, products, catalysts). This is called its Rate Law John D. Bookstaver St. Charles Community College  Rate Laws In general, rates of reactions increase as concentrations increase since there are more collisions occurring between reactants. The overall concentration dependence of reaction rate is given in a rate law or rate expression. For reactions follow simple rate laws: v = k [A]m [B]n… - [A], [B]: reactant concentrations - The exponents m and n: reaction order (w.r.t. specific reactant) - The constant k: rate constant - The overall reaction order is the sum of the reaction orders: m+n Dan Reid Champaign CHS  Determination of Rate Laws Rate laws, rate constants, and orders are determined experimentally. The order of a reactant is NOT generally related to its stoichiometric coefficient in a balanced chemical equation. F2 (g) + 2ClO2 (g) 2FClO2 (g) v = k [F2][ClO2] 1 Chung (Peter) Chieh University of Waterloo  Expression of Rate Laws Reactions with simple rate laws: Reactions with complex rate laws*: * imply multi-step reactions (sequence of elementary steps) however, the overall rate cannot involve intermediate species Reaction Kinetics (Vallance)  Elementary Reactions Always follow simple rate laws Reactant order reflects molecularity (# of molecules involved in reaction) Reaction Kinetics (Vallance)  Elementary Reactions The reaction mechanism gives the path of the reaction. The stoichiometry can be used to determine the rate law! The molecularity of a process tells how many molecules are involved in the process. Law of Mass Action: The rate of a simple (one step) reaction is directly proportional to the concentration of the reacting substances. Dan Reid Champaign CHS  Integrated Rate Laws Goal: express concentration as a function of time Why: fit to the form of typical experimental data How: integrate the expression of rate law k k k k differential rate integrated rate expression expression Reaction Kinetics (Vallance)  Order of Reaction A reaction is 0th order in a reactant if the change in concentration of that reactant produces no effect. A reaction is 1st order if doubling the concentration causes the rate to double. A reaction is 2nd order if doubling the concentration causes a quadruple increase in rate. -3rd order…doubling concentration leads to 23 (or 8 times) the rate. - extremely rare. Dan Reid Champaign CHS  Zero-Order Reaction Δ[A]t reaction rate = - = k (constant) Δt Integrated rate law: [A]t = -kt + [A]0 This equation has the general form for a straight line, y=mx+b, so a plot of [A]t vs. t is a straight line with slope (-k) and intercept [A]0. (slope= −k) [A]t Time (s) Chung (Peter) Chieh University of Waterloo  First-Order Reaction Integrated rate law: ln[A]t = −kt + ln[A]0 where [A]t = concentration of [A] after some time, t k= reaction rate constant in units of s-1 t= time in seconds [A]o = initial concentration of A This equation has the general form for a straight line, y=mx+b, so a plot of ln[A]t vs. t is a straight line with slope (-k) and intercept ln[A]0. (slope= −k) ln[A]t Time (s) John D. Bookstaver St. Charles Community College  Second-Order Reaction Integrated rate law: 1/[A]t = kt + 1/[A]0 where [A]t = concentration of [A] after some time, t k= reaction rate constant in units of M-1s-1 t= time in seconds [A]o = initial concentration of A This equation has the general form for a straight line, y=mx+b, so a plot of l/[A]t vs. t is a straight line with slope (k) and intercept of 1/[A]0. (slope= k) 1/[A]t Dan Reid Time (s) Champaign CHS  Reaction Half-Life Half-life is the time taken for the concentration of a reactant to drop to half its original value. Substitute into integrated rate laws: t½ is the time taken for [A] to reach ½[A]0 NOTE: For a first-order process, the half-life does not depend on [A]0. Reaction Kinetics (Vallance) Dan Reid Champaign CHS  Multi-Step Process In a multistep process, one of the steps will be slower than all others. The overall reaction cannot occur faster than this slowest, rate-determining step. John D. Bookstaver St. Charles Community College  Activation Energy The minimum amount of energy required to initiate a reaction: Activation energy (Ea) John D. Bookstaver St. Charles Community College