Presentation on theme: "Principles of Reactivity: Chemical Kinetics"— Presentation transcript:
1Principles of Reactivity: Chemical Kinetics Dr Abd El Rahman Salah Khder
2KineticsChemical kinetics is the study of the rates of chemical reactions.Macroscopic level: rates of reactions, reaction rate, factors affecting ratesParticulate level: reaction mechanism (the detailed pathway taken by atoms and molecules as a reaction proceeds)
3Rates of Chemical Reactions reaction rate is a measure of the speed of a reactionrate of a chemical reaction refers to the change in concentration of substance per unit of timeamounts of reactants decrease and amounts of products increaseoften in units of M/s
4Rates of Chemical Reactions 2N2O5 4NO2 + O2rate of rxn = - Δ[N2O5]/ΔtOrrate of rxn = Δ[NO2]/Δtrate is expressed as a positive value; the sign on the rate indicates a decrease (reactant concentration) or increase (product concentration)
6Rates of Chemical Reactions 2N2O5 4NO2 + O2The graph of concentration vs. time is not straight because the rate of reaction changes during the course of the reaction. (largest at time = 0)Over a time period, the average rate can be calculated. For a single point in time, an instantaneous rate can also be calculated from the slope of the tangent line.
7C4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq) Reaction RatesC4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq)A plot of concentration vs. time for this reaction yields a curve like this.The slope of a line tangent to the curve at any point is the instantaneous rate at that time.
8C4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq) Reaction RatesC4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq)The reaction slows down with time because the concentration of the reactants decreases.
9Reaction Rates and Stoichiometry To generalize, for the reactionaA + bBcC + dDReactants (decrease)Products (increase)
10REACTION STOICHIOMETRY For the decomposition of HBr 2HBr(g) → H2(g) + Br2(g)If HBr concentration is decreasing at a rate of 0.52 M/sWhat is the rate of the reaction?What is the rate of appearance of H2 and Br2?
11Practice ProblemCompare the rates of appearance or disappearance of each product and reactant, in the decomposition of nitrosyl chloride, NOCl.2NOCl(g) 2NO(g) + Cl2(g)
12Reaction Conditions and Rate Reaction rate increases with more collisions and more energy. Factors affecting the speed of reaction:concentration of reactants – increasing [reactant] causes more collisions therefore increasing reaction ratesurface area – more surface area results in more collisionsphysical state of reactants – homogeneous mixtures of liquids and gases tend to react faster (tied to surface area)temperature – higher temp increases KE therefore increases collisionscatalysts – accelerate reaction but are not transformed themselves; changes reaction mechanism
13Effect of Concentration on Rate the rate of reaction is proportional to the reactant concentrationmay be proportional to the concentrations of more than one reactantcatalyst can affect the rate
14Rate EquationsA rate equation or rate law expresses the relationship between reactant concentration and rate
15Rate Equations For a reaction such as aA + bB xX the rate equation israte = k[A]m[B]nwhere m and n are experimentally determined exponents NOT necessarily the stoichiometric coefficients (usually positive whole numbers)
16Concentration and Rate Each reaction has its own equation that gives its rate as a function of reactant concentrations.this is called its Rate LawTo determine the rate law we measure the rate at different starting concentrations.
17Concentration and Rate Compare Experiments 1 and 2: when [NH4+] doubles, the initial rate doubles.
18Concentration and Rate Likewise, compare Experiments 5 and 6: when [NO2-] doubles, the initial rate doubles.
19Concentration and Rate This equation is called the rate law, and k is the rate constant.
20Rate LawsA rate law shows the relationship between the reaction rate and the concentrations of reactants.For gas-phase reactants use PA instead of [A].k is a constant that has a specific value for each reaction.The value of k is determined experimentally.“Constant” is relative here-k is unique for each rxn
21Reaction orderExponents tell the order of the reaction with respect to each reactant.This reaction isFirst-order in [NH4+]First-order in [NO2−]The overall reaction order can be found by adding the exponents on the reactants in the rate law.This reaction is second-order overall.
22Integrated Rate Laws Differential form: Consider a simple 1st order rxn: A BDifferential form:How much A is left after time t? Integrate:
23Integrated Rate Laws The integrated form of first order rate law: Can be rearranged to give:[A]0 is the initial concentration of A (t=0).[A]t is the concentration of A at some time, t, during the course of the reaction.
24Integrated Rate Laws Manipulating this equation produces… y = mx + b …which is in the formy = mx + b
25First-Order Processes If a reaction is first-order, a plot of ln [A]t vs. t will yield a straight line with a slope of -k.So, use graphs to determine rxn order.
26First-Order Processes Consider the process in which methyl isonitrile is converted to acetonitrile.CH3NCCH3CNHow do we know this is a first order rxn?
27First-Order Processes CH3NCCH3CNThis data was collected for this reaction at 198.9°C.Doesrate=k[CH3NC]for all time intervals?
28First-Order Processes When ln P is plotted as a function of time, a straight line results.The process is first-order.k is the negative slope: 5.1 10-5 s-1.
29Order of a ReactionThe order with respect to a reactant is the exponent of its concentration term in the rate expression.Total reaction order is the sum of the exponents in the concentration terms.
30Order of a ReactionA reactant is first order if doubling it (while holding the other reactants constant) doubles the rate.A reactant is second order if doubling it quadruples the rate.A reactant is third order if doubling it increases the rate by 8.A reactant is zero order if doubling it has no effect on the rate.
31Order of a Reaction 2NO(g) + Cl2(g) 2NOCl(g) Rate = k[NO]2[Cl2] This reaction is second order in NO, first order in Cl2, and third order overall.Compare experimental evidence to figure out the order respective to reactants.Exp.[NO] mol/L[Cl2] mol/LRate mol/L∙s10.2501.43e-620.5005.72e-632.86e-6411.4e-6
32Rate Constant, kThe rate constant, k, is a proportionality constant relating rate and concentrations at a given temp.Units of the rate constant have to match; depends on order of reaction.0 order = mol/L∙time1st order = time-12nd order = L/mol∙time3rd order = L2/mol2∙time
33Determining a Rate Equation A rate equation must be determined experimentally.“method of initial rates”initial rate is the instantaneous reaction rate at the start of the reaction (at t = 0)
34Practice ProblemThe initial rate of the reaction of nitrogen monoxide and oxygen2NO(g) + O2(g) 2NO2(g)was measured at 25oC for various initial concentrations of NO and O2. Data are collected in the table on p Determine the rate equation from these data. What is the value of the rate constant and what are the appropriate units of k?
35Practice Problem The rate constant k is 0.090 h-1 for the reaction Pt(NH3)2Cl2(aq) + H2O(l) [Pt(NH3)2(H2O)Cl]+(aq) + Cl-(aq)and the rate equation israte = k[Pt(NH3)2Cl2]Calculate the rate of reaction when the concentration of Pt(NH3)2Cl2 is 0.020M. What is the rate of change in the concentration of Cl- under those conditions?
36HomeworkAfter reading section 15.3, you should be able to do the following…P. 713 (9-13)
3715.4 Concentration-Time Relationships Integrated rate equation:ln([R]t/[R]o) = -kt[R]o and [R]t are concentrations of reactant at time t = 0 and at a later time, t. The ratio of concentrations is the fraction of reactant that remains after a given time has elapsed.
38First Order Reactions ln[R]t/[R]o = -kt For a first order reaction, k has units of time -1, which means that you can choose any unit for [R] such as M, moles, grams, atoms, molecules, or pressure.
39Practice ProblemSucrose, a sugar, decomposes in acid solution to glucose and fructose. The reaction is first order in sucrose, and the rate constant at 25oC is k=0.21h-1. If the initial concentration of sucrose is M, what is its concentration after 5.0 h.
40Practice ProblemGaseous NO2 decomposes when heated 2NO2(g) 2NO(g) + O2(g)The disappearance of NO2 is a first order reaction with k = 3.6x10-3 s-1 at 300oC.A sample of gaseous NO2 is placed in a flask and heated at 300 oC for 150 s. What fraction of the initial sample remains after this time?How long must a sample be heated so that 99% of the sample has decomposed?
41Second-Order Reaction For second-order reactions, the rate equation becomes1/[R]t – 1/[R]o = ktsame symbolism applies and k has units of L/mol∙time.
42Practice ProblemUsing the rate constant for HI as k = 30. L/mol∙min, calculate the concentration of HI after 12 min if [HI]0 = mol/L.
43Zero-Order ReactionsFor a zero-order reaction, the integrated rate equation is[R]0 – [R]t = ktwhere the units of k are mol/(L∙time).
44Graphical MethodsEach of the integrated rate laws can be rearranged to take the form of a straight line y = mx + b where m is the slope and b is the y-intercept.
45Graphical Methods Zero Order First Order Second Order [R]t = -kt + [R]oFirst Orderln[R]t = -kt + ln[R]oSecond Order1/[R]t = kt + 1/[R]o
46Half-Life and First-Order Reactions The half-life, t1/2, of a reaction is the time required for the concentration of a reactant to decrease to one-half its initial value.Longer half-life equals slower reaction. For first order reactions:t1/2 = 0.693/khalf life is independent of concentration!
47Half-Life and Other Reactions Zero Ordert1/2 = [R]o/2kSecond Ordert1/2 = 1/(k[R]o)
48Practice ProblemAmericium is used in smoke detectors and in medicine for the treatment of certain malignancies. One isotope of americium, 241Am, has a rate constant, k, for radioactive decay ofyear -1. In contrast, radioactive iodine-125, which is used for studies of thyroid functioning, has a rate constant for decay of day-1.a. What are the half-lives of these isotopes?b. Which element decays faster?c. If you begin a treatment with iodine-125, and have 1.6e15 atoms, how many remain after 2.0 days?
49HomeworkAfter reading section 15.4, you should be able to do the following…P. 714 (19-23)
5015.5 Particulate View of Reaction Rates For a reaction to occur:Collision TheoryReacting molecules must collide with each other.Reacting molecules must collide with sufficient energy.Molecules must collide in an orientation that can lead to rearrangement of the atoms.
51Collision TheoryIncreasing the concentration of the reactant particles will increase the number of collisions.The number of collisions between the reactant molecules is directly proportional to the concentrations of each reactant.
52Activation EnergyAn “energy barrier” must be overcome for a reaction to occur.The energy required to overcome this barrier is called activation energy, Ea.Fast reactions usually have low activation energies, and slow reactions have high activation energies.
53Activation EnergyAs a reaction passes over the activation energy barrier, the transition state (also called an activated complex) is the arrangement of reactant molecules and atoms at the maximum point in the reaction coordinate diagram. (see page 625)Must be inferred!
54Effect of Temperature Increase Raising temperature increases the reaction rate by increasing the fraction of molecules with enough energy to surmount the activation energy barrier.
55Effect of Molecular Orientation “steric factor”The lower the probability of achieving the proper alignment, the lower the value of k and the slower the reaction.
56Arrhenius EquationThe observation that reaction rates depend on the energy and frequency of collisions between reacting molecules, on the temperature, and on whether the collisions have the correct geometry is summarized by the Arrhenius equation:k = Ae-Ea/RT
57Arrhenius Equation k = Ae-Ea/RT where R is the gas constant ( e-3 kJ/Kmol) and T is the temperature in kelvins. The parameter A is called the frequency factor and is related to the number of collisions that have correct geometry. The other factor refers to the fraction of molecules having at least the minimum energy required for reaction.
58Arrhenius EquationCan be used to calculate the value of activation energy from the temperature dependence of the rate constant AND calculate the rate constant for a given temperature if the activation energy and A are known.
59Arrenius EquationCan be rearranged to form an equation for a straight line relating ln k to 1/T.k = Ae-Ea/RTln k = ln A – (Ea/R)(1/T)y = mx + b
60Arrhenius EquationWe can also solve for Ea by comparing k at two different temperatures…ln k2/k1 = - Ea/R[1/T2 – 1/T1]
61Practice ProblemThe colorless gas N2O4 decomposes to the brown gas NO2 in a first-order reaction: N2O4(g) 2NO2(g)The rate constant k = 4.5 x 103 s-1 at 274K and 1.00 x 104 s-1 at 283K. What is the energy of activation?
62Effect of CatalystsCatalysts speed up the rate of a chemical reaction but are not consumed in the reaction.Reaction intermediates are species formed in one step of a reaction that are then consumed in a later step.
63HomeworkAfter reading section 15.5, you should be able to do the following…P. 715 (32-36)
6415.6 Reaction MechanismsReaction mechanisms refer to the sequence of bond-making and bond-breaking steps that occurs during the conversion of reactants to products.
65Reaction Mechanisms Step 1: Br2 + NO Br2NO Step 2: Br2NO + NO 2BrNOOverall: Br2 + 2NO 2BrNOEach step is an elementary step. Each step has its own Ea and k.Intermediates are substances formed in one step and then consumed in another (don’t appear in overall reaction).
66Elementary StepsAlthough the rate constant for a reaction MUST be determined experimentally, for an elementary step it is determined stoichiometrically.The molecularity (unimolecular, bimolecular, termolecular) is the same as its order. Classified by the number of reactant molecules that come together.NOT true for the overall reaction!
67Practice ProblemNitric oxide is reduced by hydrogen to give nitrogen and water,2NO(g) + 2H2(g) N2(g) + 2H2O(g)and one possible mechanism is2NO(g) N2O2(g)N2O2(g) + H2(g) N2O(g) + H2O(g)N2O(g) + H2(g) N2(g) + H2O(g)What is the molecularity of each of the three steps? What is the rate equation for the third step? Show that the sum of these elementary steps gives the net equation.
68Rate-Determining Steps Products can never be produced faster than the rate of the slowest elementary step, which is therefore called the rate-determining step.
69Practice ProblemThe Raschig reaction produces the industrially important reducing agent hydrazine, N2H4, from NH3 and OCl- in basic, aqueous solution. A proposed mechanism is given on page 638.What is the overall stoichiometric equation?Which step is rate-determining?Write the rate equation for the rate-determining step.What reaction intermediates are involved?
70HomeworkAfter reading section 15.6, you should be able to do the following…P. 716 (39-43)