Chemistry - an illustrated guide to science (794128), страница 23
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The graphshows the Maxwell-Boltzmandistribution at two temperatures, T2is greater than T1.● The number of particles is constant, sothe area under the two curves is thesame. However, the average energy ofthe particles at T2 is greater. The areaof the curve to the right of theactivation en ergy line ( EA) is greaterfor T2. Therefore, at this temperature ahigher proportion of particles havesufficient energy to react.Number of particles having a given energyT1T22 Concentration● Anincrease in the concentration of achemical, or the pressure of a gas,means that there will be more particleswithin a given space, so particles willcollide more often.EAKinetic energy2 Concentration3 Rate of reactionBAAABAABAABLow concentrationAABBABABBAABABABABBBBAAAABABBBAHigh concentration3 Rate of reactionRate of reaction =Change in concentrationTimerate of any reaction is the speed atwhich the reactan ts are converted toprodu cts.
This can be qualified as thechange of concentration of reactantsor products.● Changes in concentration can bemeasured by:1. appearance or disappearance ofcolor in reactants or products2. volume of gas evolved3. changes in pH4. heat produced5. changes in pressure.© Diagram Visual Information Ltd.● The134CHEMICAL REACTIONSKey wordsconcentrationrate of reactionRates of reaction:concentration over timeGraph to show the variation of bromine concentration with time in the reactionbetween methanoic acid and bromine0.010Concentration over time● Brominereacts with an excess ofmethanoic acid in aqueous solutionaccording to the following equation.The reaction is catalyzed by acid:0.009H+Br 2(aq) + HCOOH(aq) ➞2Br - (aq) + 2H+ (aq) + CO2(g)● The- d[ Br 2]dtrate of change of bromineconcentration is negative because thebromine is being used up.
Thenegative sign in the expression isnecessary to give the rate of reaction apositive value.● In order to obtain the rate of reactionat any given time, a tangent to thecurve must be drawn at that particulartime and the gradient measured. Theconcentration of bromine after 300seconds (s) is 0.0035 mol dm -3. Therate of reaction at this time is 1.2 x 10-5mol dm -3 s-1.© Diagram Visual Information Ltd.● The0.0070.006[Br2] mole dm–3reaction can be followed bymeasuring the intensity of the redbrown at different time intervals andrelating this to the con cen tration ofbromine.● The concentration of bromine, [ Br 2 ] ,falls during the reaction, so the rate ofthe reaction can be expressed in termsof the rate at which the bromineconcentration changes.● The rate of reaction =- rate of change of bromineconcentration =0.0080.0050.0040.0030.0020.001Tangent attime = 300s00100200300400Time (seconds)500600700135Rate of reaction vs.concentrationCHEMICAL REACTIONSKey wordsconcentrationrate of reactionGraph to show the variation of reaction rate with bromine concentration3.0Rate vs.
concentration● Inorder to draw a graph showing howthe rate of reaction varies withbromine con cen tration , it is necessaryto find the rate of reaction at differenttimes and, therefore, differentbromine concentrations.● The graph shows that the rate ofreaction is directly proportional to thebromine concentration.Reaction rate ![ Br 2] , therefore,Rate of reaction = k[ Br 2] where k is aconstant, known as the rate constantor the velocity constant for thereaction.● This reaction is said to be first ratewith respect to bromine sincedoubling the concentration of brominedoubles the rate of the reaction.● Since rate of reaction = k[ Br 2 ] , then tofind the units of k :2.72.41.8k = rate of reaction =[ Br 2]mol dm -3 s-1 = s-1mol dm -31.5dtThe unit of the rate constant, k , forfirst order reactions is s-1.1.20.90.60.3000.0 010.0 020.0 030.0 0 40.0 05[Br2 ] / mole dm–30.0 0 60.0 070.0 0 8© Diagram Visual Information Ltd.–d[Br2 ] × 10 5/ mole dm–3s–12.1136CHEMICAL REACTIONSKey wordsconcentrationproductrate of reactionreactantVariation of reaction rate1 Clock technique for measuring reaction rates1231001 Clock technique80% of reaction completedof reaction =change in concentration of a substancetime● In order to monitor the progress of areaction, we could measure thecon cen tration of a reactan t or aprodu ct at regular time intervals, sayevery 10 seconds.● Strictly speaking, this would give usthe average reaction rate during the 10second period.
By measuring thechange in concentration over shorterand shorter time periods, we wouldobtain an increasingly more accurateestimate of the rate of reaction at anyparticular moment.● Using a clock technique, the rate isobtained as the inverse of the time fora certain proportion of the reaction tooccur. Provided the reaction has onlygone a small way toward completion,the error is very small, but the errorincreases as the reaction movesfurther to completion.90● Rate70601 true initial rate502average reaction rate for 10 % completion3average reaction rate for 50 % completion403020100Time/ s2 Increasing concentrationThe variation of reaction rate with concentration for reactions whichare zero, first, and second order2 Increasing concentration● Ifdoubling the concentration of areactant has no effect on the rate of areaction, then the reaction is said tobe zero order with respect to thereactant.
The rate equation is:3rate = k[ reactant] 0 = k1● Ifrate = k[ reactant]© Diagram Visual Information Ltd.● Ifdoubling the concentration of areactant quadruples the rate of areaction, then the reaction is said tobe second order with respect to thereactant. The rate equation is:rate =Reaction ratedoubling the concentration of areactant doubles the rate of a reaction,then the reaction is said to be firstorder with respect to the reactant. Therate equation is:21 zero order2first order3second orderk[ reactant] 2Concentration of X137Rates of reaction: effectof temperature 1The effect of temperature on different reactions1 Most reactionsCHEMICAL REACTIONSKey wordseffective collisionenzymekinetic energypolymerrate of reactionEffect of temperaturea substance is heated, itsparticles gain kin etic en ergy and movearound more quickly.
The frequency ofcollisions increases, and because theparticles have a greater momentum,the frequency of effective collision salso increases. The result is an increasein the rate of reaction .Reaction rate● When1 Most reactions● InTemperature2 Enzyme-catalyzed reactionsmost chemical reactions, the rate ofreaction increases steadily with risingtemperature. It is for this reason thatchemical reactions are often heated.2 Enzyme-catalyzedreactionsReaction rate● En zym esTemperature3 Explosive reactionscatalyze chemical reactionswith a high degree of specificity andefficiency.
An enzyme molecule is apolym er composed of a long chain ofamino acids that folds over on itself,giving it a particular shape. Reactingmolecules, called the substrate, fit intothis shape rather like a key in a lock.● Up to a point, the rate of an enzymecatalyzed reaction increases with risingtemperature in the same way as mostother reactions. However, afterreaching an optimum temperature atwhich the activity of the enzyme isgreatest, the reaction rate rapidly falls.● Heating an enzyme causes its shape tochange, and thus the enzyme ceases tobe able to catalyze the reaction.
It issaid to be denatured.Reaction rate3 Explosive reactionsan explosive reaction, the reactionrate increases with rising temperatureup to some point where the reactionrate suddenly rises sharply.Temperature© Diagram Visual Information Ltd.● In138CHEMICAL REACTIONSKey wordsactivation energyRates of reaction: effectof temperature 21 Rate constant for reactionk = Ae–Ea/ RT1 Rate constant forreactiona general rule of thumb, the rate ofa reaction doubles for every 10 K risein temperature.
This would seem tosuggest that there is an exponentialrelationship between rate andtemperature.● The exact relationship was proposedby the Swedish chemist SvanteArrhenius in 1889. The Arrheniusequation relates the rate constant (notthe rate of reaction) to temperature.● The equation can be expressed in alogarithmic form and in terms of log tothe base 10. The latter form of theequation is the most useful forcalculation purposes.In k = ln A – Ea/ RT● As2 Plotting the Arrheniusconstantk = rate constant for the reactionA = constant for the reaction (Arrhenius constant)Ea = activation energyR = gas constantT = absolute temperature● The© Diagram Visual Information Ltd.Ea = 2.30 3 x 0 .00 8314 xslope kJ mol -12 Plotting the Arrhenius constantLog (k/ s–1)constants A and Ea for a givenreaction can be obtained by plottinglog k against 1/ T: the temperature, T,must be expressed in kelvin.The slope of the graph is equal toEa / 2.303R.● The Arrhenius constant, A , can beobtained by substituting values for theslope ( Ea / 2.30 3R), log k and T in theArrhenius equation.● The activation en ergy, Ea, can also befound from the slope of the graph.Slope = - Ea / 2.30 3 RThe slope of the graph is negative, andits unit is K thereforeEa = 2.30 3 x R x slopeThe gas constant, R, = 0.0 08314 kJK-1 mol -1 thereforelog k = log A – Ea/ 2.303RTyslope = xyxT–1/ k–1139Exothermic andendothermic reactionsCHEMICAL REACTIONSKey wordsendothermicenthalpyexothermicproductreactant1 Exothermic1 Exothermic● InEAEnergyreactants∆Hproductsan exotherm ic reaction, energy isgiven out, and the temperature of thereaction mixture increases as thereaction proceeds.
The produ cts are ata lower energy than the reactan ts.● The energy released is due to adecrease in the en thalpy, !H, of thesystem. Enthalpy is a measure of thestored heat energy of a substance.Therefore, !H is negative for anexothermic reaction.● The following equation represents thecombustion of methane in a goodsupply of air:CH4 (g) + 2O2(g) ➞ CO2(g) + 2H2O(g)!H= -890 kJ mol -1This is an exothermic reaction.
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