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Chapter 14.doc

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Human Biology

Chapter 14- Chemical Kinetics Nerosanth Selvarajah 14.1- The Rate of a Chemical Reaction • The rate of a reaction describes how fast the concentration of a reactant or product changes with time A + B  C + D Rate of Formation = ∆[C]/ ∆t Rate of Disappearance = -∆[A]/ ∆t • Rate of disappearance is a negative quantity because concentration decreases with time o The concentration at the end of a time period is less than t was at the start of the period 14.2 Measuring Reaction Rates • To determine rate of reaction, we need to measure changes in concentration over time • Reaction rate is not constant; the lower the remaining concentration of the reactant, the more slowly the reaction proceeds • Instantaneous Rate of Reaction- is the exact rate of a reaction at some precise point in the reaction. It is obtained from the slope of a tangent line to a concentration-time graph • Initial Rate of Reaction- is the rate of a reaction immediately after the reactants are brought together 14.3 Effect of Concentration on Reaction Rates: The Rate Law • Rate Law/ Rate Equation-for a reaction relates the reaction rate to the concentrations of the reactants m n Rate= k[A] [B] [Pg. 578] • Term order is related to the exponents in the rate law • The overall order of reaction is the sum of all the exponents: m + n + … • Rate constant (k)- is the proportionality constant in a rate law that permits the rate of a reaction to be related to the concentrations of the reactants o its values depend on the specific reaction, presence of a catalyst and temperature o the larger the value of k, the faster a reaction goes • order of the reaction establishes the general form of the rate law & the appropriate units of k • if reaction is first order in one of the reactants, doubling the initial concentration of that reactant causes the initial rate of reaction to double o zero order in reactant- no effect on initial rate of reaction o first order in reactant- initial rate of reaction doubles o second order in reactant- initial rate of reaction quadruples o third order in reactant- initial rate of reaction increases eightfold • order of reaction (indicated through rate law) establishes units of rate constant, k [k = M min -1 14.4 Zero-Order Reactions • zero-order reaction has a rate law in which the sum of the exponents, m + n+… is equal to 0 o reaction proceeds at a rate that is independent of reactant concentrations Rate of Reaction = k [A] = k = constant  concentration-time graph is a straight line with a negative slope  rate of reaction which is equal to k and remains constant throughout the reaction is the negative of the slope of the line  units of k are the same as units of rate of a reaction: mol/L*t = M/s • Integrated Rate Law- expresses the concentration of a reactant as a function of time [A]t= -kt + [A]o 14.5 First-Order Reactions  First-order reaction has a rate law in which sum of the exponents, m + n + …is equal to 1 ln ([A]t/ [A]o) = -kt or ln [A]t= -kt + ln[A] o  an easy test for a first-order reaction is to plot the natural logarithm of a reactant concentration versus time & see if graph is linear k = -slope  Half-Life of a reaction is the time required for one-half of a reactant to be consumed; time during which the amount of reactant or its concentration decreases to one-half of its initial value t1/2= ln 2 / k  Half-life is constant for a first-order reaction; it is also independent of the initial concentration used  In Reactions involving gases, Rates are often measured in terms of gas pressure  Radioactive decay is a first-order process 14.6 Second-Order Reactions Equation of Straight Line Graph: 1/ [A]t= kt + 1/[A] 0  Half life depends on both the rate constant & the initial concentration [A] 0 o Half –life is not a constant; its value depends on the concentration of reactant at the start of each half-life interval  Because the starting concentration is always one-half that of the previous half-life, each successive half-life is twice as long as the one before it t1/2= 1/ k[A] 0  Pseudo-First-Order Reaction- a second-order reaction that is made to behave like a first-order reaction by holding one reactant concentration constant 14.7 Reaction Kinetics: A Summary REFER TO PAGE 589 14.8 Theoretical Models for Chemical Kinetics Collision Theory:  Colli30on Theory- the number of molecular collisions per unit time; typical collision frequency is of the order of 10 collisions/second  Only a fraction of the collisions among gaseous molecules lead to chemical reaction  Cannot expect every collision to result in a reaction  Activation Energy- a reactions minimum energy above the average kinetic energy that molecules must bring to their collisions for a chemical reaction to occur  Rate of a reaction wi
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