化学专业英语《Chemistry English》课程教学资源（PPT课件）Lecture 05

Kinetics: the factsRATE OFREACTIONis the change in concentration of reactant or product in a giventimefor the general reaction:A+B→Cthe rate is:-d[A]/dt or-d[B]/dt or +djC]/dtRATELAW ORKINETICS OFAREACTIONis the equationrelating therate of reaction at anytime to theconcentration of reactants at that timee.g. rate is proportional to [A]X [B]2 or rate oc [A][B]?So rate = k[A[B]2where k is the constant of proportionality orrate constant for this reactionThis leads to the concept of order1

1 Kinetics: the facts is the change in concentration of reactant or product in a given time is the equation relating the rate of reaction at any time to the concentration of reactants at that time e.g. rate is proportional to [A]×[B]2 or rate ∝ [A][B]2 So rate = k[A][B]2 where k is the constant of proportionality or rate constant for this reaction This leads to the concept of order. RATE OF REACTION RATE LAW OR KINETICS OF A REACTION for the general reaction: A + B → C the rate is: -d[A]/dt or –d[B]/dt or +d[C]/dt

Kinetics: the factsORDERis the number of concentration factors in the rate equation.In the example above the order with respect to Ais 1 and withrespect to B is 2; the overall order is 1 + 2 = 3.The order of a reaction can only be found by experiment andcannot be worked out from the eguation of the reaction.Common orderszero order: rate is unchanged with concentration term:rate αc [A]°first order: rate is directly proportional to one concentration term:rate α [A]]second order: rate is proportional to two concentration terms:rate α [A]'[B]' or rate αc [A]?2

2 Kinetics: the facts Common orders zero order: rate is unchanged with concentration term: rate ∝ [A]0 first order: rate is directly proportional to one concentration term: rate ∝ [A]1 second order: rate is proportional to two concentration terms: rate ∝ [A]1 [B]1 or rate ∝ [A]2 ORDER is the number of concentration factors in the rate equation. In the example above the order with respect to A is 1 and with respect to B is 2; the overall order is 1 + 2 = 3. The order of a reaction can only be found by experiment and cannot be worked out from the equation of the reaction

Kinetics: the factsEXPERIMENTSTO FIND ORDER:discontinuouscontinuousmany separate experiments withoneexperimentdifferent starting concentrationsmany readings asone reading per experimentexperiment goesone.g. clock reactions:e.g. gas syringes:thiosulphate and acidsampling experiments3

3 Kinetics: the facts EXPERIMENTS TO FIND ORDER: discontinuous continuous many separate experiments with one experiment different starting concentrations one reading per experiment many readings as experiment goes on e.g. clock reactions; e.g. gas syringes; thiosulphate and acid sampling experiments

Kinetics: the factsEACTORSTHATAEEECTTHERATEOFREACTIONReaction rateis affectedby :.the concentration of the reactants (and pressure in gas phasereactions)the particle size in heterogeneous reactions (those involving solidswith gases or liquids). the temperature of the reacting system - typically the ratedoubles forevery 1ooC rise in temperature.(some reactions are affected by light energy instead of heat)the addition of a suitable catalystRATEDETERMININGSTEPSIna multi-stepreaction,the slowest step controlstherateCHAIN REACTIONSare reactions in which each step produces the reactant for the nextstep4

4 Kinetics: the facts FACTORS THAT AFFECT THE RATE OF REACTION Reaction rate is affected by : ● the concentration of the reactants (and pressure in gas phase reactions) ● the particle size in heterogeneous reactions (those involving solids with gases or liquids) ● the temperature of the reacting system – typically the rate doubles for every 10oC rise in temperature. (some reactions are affected by light energy instead of heat) ● the addition of a suitable catalyst RATE DETERMINING STEPS In a multi-step reaction, the slowest step controls the rate. CHAIN REACTIONS are reactions in which each step produces the reactant for the next step

Kinetics: the factsDETERMININGORDERSANDRATE CONSTANTSFor discontinuous experiments: inspect the data to see howchanging concentration affects the rate (see example on p.4O).Once order is found, write a rate equation then substitute one setof concentrations in to findthe rate constant.For continuous experimentseither: 1.Plot the reactant concentration against time2. Is it a straight line? If so then the order is zero.3. If not order is first or second, measure and tabulate halflives.4. Are they constant? If so , then the order is firstand rate constant is k =log.2/halflife5. If not, work out the initial concentration Cotimes thehalflifefor several values ofhalf life.6. Are they constant? If so then the order is second.5

5 Kinetics: the facts DETERMINING ORDERS AND RATE CONSTANTS For discontinuous experiments: inspect the data to see how changing concentration affects the rate (see example on p.40). Once order is found, write a rate equation then substitute one set of concentrations in to find the rate constant. For continuous experiments either: 1. Plot the reactant concentration against time 2. Is it a straight line? If so then the order is zero. 3. If not order is first or second, measure and tabulate half lives. 4. Are they constant? If so , then the order is first and rate constant is k = loge2/half life 5. If not, work out the initial concentration c0 times the half life for several values of half life. 6. Are they constant? If so then the order is second

Kinetics: the factsor: 1. Plot the reactant concentration against time.2. Is it a straight line? If so then the order is zero.3. If not, plot In [reactant] against time.4.Is it a straight line? If so the order is first andthe gradient = the rate constant k.5. If not, plot the reciprocal of [reactant] against time.6.Is it a straight line? If so the order is second andthe gradient= the rate constant k.6

6 Kinetics: the facts or: 1. Plot the reactant concentration against time. 2. Is it a straight line? If so then the order is zero. 3. If not, plot ln [reactant] against time. 4.Is it a straight line? If so the order is first and the gradient = the rate constant k. 5. If not, plot the reciprocal of [reactant] against time. 6. Is it a straight line? If so the order is second and the gradient = the rate constant k

Kinetics: the factsWordsWordsandExpressionskineticsrate of reactionrateequation;rateconstantreaction order; the order with respect to A; zero order; firstorder;secondorder;overall order;overall reaction; elementary reaction; rate determining step(rds)chain reaction; heterogeneous reaction; homogeneous reaction7

7 Kinetics: the facts Words Words and Expressions kinetics rate of reaction rate equation; rate constant reaction order; the order with respect to A; zero order; first order; second order; overall order; overall reaction; elementary reaction; rate determining step (rds) chain reaction; heterogeneous reaction; homogeneous reaction

Kinetics: the theoryTHE COLLISION THEORYThis states that to react particles must collide.with enough energy to break existing bonds and with the correct orientation to bring reactive sites closetogetherRELATING THE THEORY TO THE FACTORS AFFECTINGRATEChanges in concentration (orSurface area changes inpressure for a gas ) changeheterogeneous reactions changethe number of particles in athe number of collisionsunitvolumeandhencethebetween the fluid phase (liquidnumber of collisions per unitorgas)andthesolidsurface.time in that volume. If theOnce again, if the number ofnumber of collisions changescollisions changes then therate8the rate will change.will also change

8 Kinetics: the theory THE COLLISION THEORY This states that to react ● particles must collide ● with enough energy to break existing bonds ● and with the correct orientation to bring reactive sites close together RELATING THE THEORY TO THE FACTORS AFFECTING RATE Changes in concentration (or pressure for a gas ) change the number of particles in a unit volume and hence the number of collisions per unit time in that volume. If the number of collisions changes the rate will change. Surface area changes in heterogeneous reactions change the number of collisions between the fluid phase (liquid or gas) and the solid surface. Once again, if the number of collisions changes then the rate will also change

Kinetics: the theoryChanges in temperature change the kinetic energy of the particlesand hence the number of successful collisions with enough energyto break existing bonds and make product particles. The minimumenergy needed for a successful collision is called the activationenergy.Increasing the temperature of the system:1. increases the range of kinetic energies;2. increases the average kinetic energy;3. increases the population of particles with more than theactivation energy (shown by the shaded areas under the graph)T.T2>T,EP12EANo.ofEparticlesReactantsProductsenergy

9 Kinetics: the theory Changes in temperature change the kinetic energy of the particles and hence the number of successful collisions with enough energy to break existing bonds and make product particles. The minimum energy needed for a successful collision is called the activation energy. Increasing the temperature of the system: 1. increases the range of kinetic energies; 2. increases the average kinetic energy; 3. increases the population of particles with more than the activation energy (shown by the shaded areas under the graph)

Kinetics: the theoryAddition of a catalyst can decrease the required activationenergy so that a greater population of particles will collidesuccessfully.A catalyst increases the rate without being used up. It does thisby providing a reaction pathway with a lower activation energythrough:10

10 Kinetics: the theory Addition of a catalyst can decrease the required activation energy so that a greater population of particles will collide successfully. A catalyst increases the rate without being used up. It does this by providing a reaction pathway with a lower activation energy through: