H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 1 Chapter 13 Rates of Chemical Reactions 13.1Rates of Chemical Reactions 13.2Expressions of Reaction Rates in Terms of Rates of Changes in Terms of Rates of Changes in Concentrations of Reactants or Products Concentrations of Reactants or Products 13.3Methods of Measuring Reaction Rates 13.4Factors Affecting Reaction Rates

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 2 This 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 Reactions C 12 H 22 O 11 (s) + 12O 2 (g) 12CO 2 (g) + 11H 2 O(l) ΔH c [C 12 H 22 O 11 (s)] = kJ mol -1 ø

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 3 Mechanism of chemical reactions Mechanism = 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 experiments Study of reaction rates

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 4 Rates of Chemical Reactions 13.1 Rates of Chemical Reactions (SB p.3) Chemical reactionReaction rate 1.Reaction between hydrogen and oxygen 2.Reaction between potassium and water 3.Precipitation of insoluble metal hydroxides 4.Displacement reaction of copper and silver nitrate solution 5.Fermentation of sucrose 6.Rusting of iron 7.Weathering of marbles by acid rain Explosively fast Vigorous, takes a fraction of a second Instantaneous Takes a several minutes Takes a few weeks Takes a few months Slow, take centuries

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 5 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 Rates of Chemical Reactions (SB p.4) Change in amount or concentration of a reactant or a product Time Rate =

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 6 Do 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 Rate

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 7 Consider a reaction A  B (reactant) (product) 13.1 Rates of Chemical Reactions (SB p.4)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 8 Consider 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=0

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 9 Consider a reaction A  B (reactant) (product) 13.1 Rates of Chemical Reactions (SB p.5) Instantaneous reaction rate = rate at a given time t 1 = slope of tangent line at t= t 1

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 10 Consider the reaction: A + B  2C 13.2 Expressions of Reactions Rates in Terms of Rates of Changes in Concentration of Reactants or Products (SB p.8) Rate Equation Where –d[A] and –d[B] represent the decreases in concentrations of reactants A and B respectively, d[C] represents the increase in concentration of product C, dt represents a very short time interval Rate = - = - = x

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 11 In general: aA + bB  cC + dD 13.2 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 =

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 12 The 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 formed within a certain time interval. or 13.3 Methods of Measuring Reaction Rates (SB p.9) Reaction Rates

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 13 Techniques used: (1) chemical method : titration (2) physical method : volume measurement / colorimetric measurement (2) physical method : volume measurement / colorimetric measurement Reaction Rates 13.3 Methods of Measuring Reaction Rates (SB p.9)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 14 Chemical Method --- Titration Reactants are mixed and time is measured Reaction carried out in a constant temp. environment Small samples of reaction mixture are taken out for chemical analysis at certain time intervals. Quenched by: 1. rapidly cooling in ice 2. removing the catalyst 3. removing one of the reactant by another chemical reaction 4. diluting with a large vol. of cold water before analysis 13.3 Methods of Measuring Reaction Rates (SB p.9)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 15 Chemical Method --- Titration Time (min) Conc. of product 13.3 Methods of Measuring Reaction Rates (SB p.9)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 16 Physical Method --- Determining the Volume of Gas Formed Rates of reactions which give gaseous products can be followed by recording the volume of the gas given out at various time Methods of Measuring Reaction Rates (SB p.11)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 17 Physical Method --- Colorimetric Measurement In some reactions, the reaction mixture may have change in colour or change in colour intensity, e.g 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.

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 18 Physical Method --- Colorimetric Measurment The change in colour intensity at different time can be followed by using a colorimeter Methods of Measuring Reaction Rates (SB p.13)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 19 Factors Influencing Reaction Rate concentration temperature pressure surface area catalyst light 13.4 Factors affecting Reaction Rates (SB p.14)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 20 Concentration of Reactants Expt. run 3 times: (a) 2.0M HCl (b) 1.0M HCl (c) 0.5M HCl 13.4 Factors affecting Reaction Rates (SB p.14) Mg(s) + 2HCl(aq) MgCl 2 (aq) + H 2 (g)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 21 as 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: t 1 < t 2 < t 3

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 22 as reflected by:  Time for reaction to complete: t 1 < t 2 < t 3  Slope of a’ > b’ > c’ (average reaction rate)  Slope of a’’ > b’’ > c’’ (initial reaction rate) 13.4 Factors affecting Reaction Rates (SB p.15) a’’ b’’ c’’ Reaction rate: (a) > (b) > (c)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book Chemical reactions are due to the collision of reactant particles. 2. Effective collisions = collisions with high energies & correct orientation 3. Increase in conc.  increase in the no. of effective collisions per unit time 13.4 Factors affecting Reaction Rates (SB p.15) Concentration of Reactants

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 24 Pressure 1. For reactions involving gaseous reactants, a higher pressure  a higher conc.  a higher reaction rate 2. As solids and liquids are incompressible, pressure change may not have significant effect on rate of reactions which just involve them Factors affecting Reaction Rates (SB p.16)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 25 Temperature t = time taken for the disappearance of the cross rate  1/t t 2 a) 13.4 Factors affecting Reaction Rates (SB p.17) Na 2 S 2 O 3 (aq) + 2HCl(aq) 2NaCl(aq) + SO 2 (g) + H 2 O(l) + S(s)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book Increase in temp.  increases the kinetic energies of reactant particles 2. 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) Temperature

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 27 Surface Area The smaller the size of a solid reactant, the greater is the surface area of contact between the reactants Factors affecting Reaction Rates (SB p.18)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 28 Slope: a > b (initial rates) Rate involving powdered solid reactant being higher Reason: higher chance of contact between reactant particles 13.4 Factors affecting Reaction Rates (SB p.18) CaCO 3 (s) + 2HCl(aq) CaCl 2 (aq) + H 2 O(l) + CO 2 (g) b a

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 29 A 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 months MnO 2 as catalyst: rate significantly increased Reason: Reaction through an alternative pathway 13.4 Factors affecting Reaction Rates (SB p.20) Catalyst 2H 2 O 2 (aq) 2H 2 O(l) + O 2 (g)

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 30 light Consider the following reaction: C 6 H 14 + Br 2  C 6 H 13 Br  ……… light (substitution reactions) Light provides energy to split up Br 2 molecules into Br. radicals which are highly reactive Factors affecting Reaction Rates (SB p.20) Br 2 2Br · Reddish brown UV

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 31 The END