ch13---rates-of-chemical-reactions.ppt

5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 21Chapter 13Rates of Chemical Reactions13.1 Rates of Chemical Reactions13.2 Expressions of Reaction Rates in Terms of Rates of Changes in Concentrations of Reactants or Products13.3 Methods of Measuring Reaction Rates13.4 Factors Affecting Reaction Rates5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 22This highly exothermic reaction does NOT take place at room temperature.Why?Although the reaction rate at room conditions is negligible,it is vigorously fast at high temperature.Chemical kinetics is the study of reaction rates,factors that affect rates and the theory of reaction rate in terms of reaction mechanisms.Chapter 13 Rates of Chemical Reactions(SB p.2)Rates of Chemical ReactionsC12H22O11(s)+12O2(g)12CO2(g)+11H2O(l)Hc C12H22O11(s)=-5644 kJ mol-15/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 23Mechanism of chemical reactionsMechanism=information on how old bond are broken and how new bonds are formed in a chemical reaction.Why the study of chemical kinetics is important?Chemists or engineers can make industrial products as economically as possible.Chapter 13 Rates of Chemical Reactions(SB p.2)How to determine the reaction rates?-from experimentsStudy of reaction rates5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 24Rates of Chemical Reactions13.1 Rates of Chemical Reactions(SB p.3)Chemical reactionReaction rate1.Reaction between hydrogen and oxygen2.Reaction between potassium and water3.Precipitation of insoluble metal hydroxides4.Displacement reaction of copper and silver nitrate solution5.Fermentation of sucrose6.Rusting of iron7.Weathering of marbles by acid rainExplosively fastVigorous,takes a fraction of a secondInstantaneousTakes a several minutesTakes a few weeksTakes a few monthsSlow,take centuries5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 25 How to describe the rate of reaction quantitatively?Average rate of a chemical reaction is a measure of the change in the amount or concentration of a reactant or a product in a given period of time.13.1 Rates of Chemical Reactions(SB p.4)Change in amount or concentration of a reactant or a product TimeRate=5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 26Do you think the rate is uniform throughout the reaction?If not,how does it change during the course of the reaction?13.1 Rates of Chemical Reactions(SB p.4)Instantaneous Rate and Initial Rate5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 27Consider a reaction A B (reactant)(product)13.1 Rates of Chemical Reactions(SB p.4)5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 28Consider a reaction A B (reactant)(product)13.1 Rates of Chemical Reactions(SB p.5)Initial reaction rate=rate at start=slope of tangent line at t=05/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 29Consider a reaction A B (reactant)(product)13.1 Rates of Chemical Reactions(SB p.5)Instantaneous reaction rate=rate at a given time t1=slope of tangent line at t=t15/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 210Consider the reaction:A +B 2C13.2 Expressions of Reactions Rates in Terms of Rates of Changes in Concentration of Reactants or Products(SB p.8)Rate EquationWhere dA and dB represent the decreases in concentrations of reactants A and B respectively,dC represents the increase in concentration of product C,dt represents a very short time intervalRate=-=-=x5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 211In general:aA +bB cC +dD13.2 Expressions of Reactions Rates in Terms of Rates of Changes in Concentration of Reactants or Products(SB p.8)Where A and B represent reactants in the reaction,C and D represent the products in the reaction,a,b,c and d represent the stoichiometric coefficients in a balanced chemical equation.Rate=5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 212The rate of a reaction can be determined experimentally by measuring either:(1)the decrease in the concentrations of the reactants/the amounts of reactants used up(2)the increase in the concentrations of the products/the amounts of products formedwithin a certain time interval.or13.3 Methods of Measuring Reaction Rates(SB p.9)Reaction Rates5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 213Techniques used:(1)chemical method:titration(2)physical method:volume measurement/colorimetric measurementReaction Rates13.3 Methods of Measuring Reaction Rates(SB p.9)5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 214Chemical Method-TitrationReactants are mixed and time is measuredReaction carried out in a constant temp.environmentSmall samples of reaction mixture are taken out for chemical analysis at certain time intervals.Quenched by:1.rapidly cooling in ice2.removing the catalyst3.removing one of the reactant by another chemical reaction4.diluting with a large vol.of cold water before analysis13.3 Methods of Measuring Reaction Rates(SB p.9)5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 215Chemical Method-TitrationTime(min)Conc.of product13.3 Methods of Measuring Reaction Rates(SB p.9)5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 216Physical Method-Determining the Volume of Gas FormedRates of reactions which give gaseous products can be followed by recording the volume of the gas given out at various time.13.3 Methods of Measuring Reaction Rates(SB p.11)5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 217Physical Method-Colorimetric MeasurementIn some reactions,the reaction mixture may have change in colour or change in colour intensity,e.g.13.3 Methods of Measuring Reaction Rates(SB p.13)1.In the oxidation of methanoic acid by bromine,the orange colour of bromine will disappear as the reaction proceeds.2.In the oxidation of oxalate ion by manganate(VII)ion,the violet colour of the manganate(VII)ion will disappear as the reaction proceeds.5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 218Physical Method-Colorimetric MeasurmentThe change in colour intensity at different time can be followed by using a colorimeter.13.3 Methods of Measuring Reaction Rates(SB p.13)5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 219Factors Influencing Reaction Rateconcentrationtemperaturepressuresurface areacatalystlight13.4 Factors affecting Reaction Rates(SB p.14)5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 220Concentration of ReactantsExpt.run 3 times:(a)2.0M HCl(b)1.0M HCl(c)0.5M HCl13.4 Factors affecting Reaction Rates(SB p.14)Mg(s)+2HCl(aq)MgCl2(aq)+H2(g)5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 221as reflected by:Slope of a b c(average reaction rate)13.4 Factors affecting Reaction Rates(SB p.15)Reaction rate:(a)(b)(c)Time for reaction to complete:t1 t2 t35/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 222as reflected by:Time for reaction to complete:t1 t2 b c(average reaction rate)Slope of a b c(initial reaction rate)13.4 Factors affecting Reaction Rates(SB p.15)abcReaction rate:(a)(b)(c)5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 2231.Chemical reactions are due to the collision of reactant particles.2.Effective collisions=collisions with high energies&correct orientation3.Increase in conc.increase in the no.of effective collisions per unit time13.4 Factors affecting Reaction Rates(SB p.15)Concentration of Reactants5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 224Pressure1.For reactions involving gaseous reactants,a higher pressure a higher conc.a higher reaction rate2.As solids and liquids are incompressible,pressure change may not have significant effect on rate of reactions which just involve them.13.4 Factors affecting Reaction Rates(SB p.16)5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 225Temperaturet=time taken for the disappearance of the crossrate 1/tt2 a)13.4 Factors affecting Reaction Rates(SB p.17)Na2S2O3(aq)+2HCl(aq)2NaCl(aq)+SO2(g)+H2O(l)+S(s)5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 2261.Increase in temp.increases the kinetic energies of reactant particles2.Increase in K.E.of particles increases the no.of effective collisions per unit time(collision frequency&proportion of high energy collisions)13.4 Factors affecting Reaction Rates(SB p.17)Temperature5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 227Surface AreaThe smaller the size of a solid reactant,the greater is the surface area of contact between the reactants.13.4 Factors affecting Reaction Rates(SB p.18)5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 228Slope:a b(initial rates)Rate involvingpowdered solid reactant being higherReason:higher chance of contact between reactant particles13.4 Factors affecting Reaction Rates(SB p.18)CaCO3(s)+2HCl(aq)CaCl2(aq)+H2O(l)+CO2(g)ba5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 229A catalyst is a substance that alters the rate of a chemical reaction and remains chemically unchanged at the end of the reaction.Uncatalysed reaction not completed after monthsMnO2 as catalyst:ratesignificantly increasedReason:Reaction through an alternative pathway13.4 Factors affecting Reaction Rates(SB p.20)Catalyst2H2O2(aq)2H2O(l)+O2(g)5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 230lightConsider the following reaction:C6H14 +Br2 C6H13Br light(substitution reactions)Light provides energy to split up Br2 molecules into Br.radicals which are highly reactive.13.4 Factors affecting Reaction Rates(SB p.20)Br2 2BrReddish brownUV5/21/20245/21/20245/21/20245/21/2024H+H+H+OH-OH-OH-New Way Chemistry for Hong Kong A-Level Book 231