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