Topic 6: Rate of Reaction Chemistry 1 (CEM 3114) 6.1 Experimental determination of a rate law • Define rate of reaction, rate equation, order of reaction and rate constant • Calculate the rate constant from initial rates • Deduce the order of a reaction (zero-, first- and second-) and the rate constant by the initial rates method. • Predict an initial rate from rate equations and experimental data 6.2 Half-life of a first-order reaction • Define half-life of a first-order reaction • Use the half-life (t½) of a first-order reaction in calculations 6.3 6.4 Theoretical Models for Chemical Kinetics • Explain collision theory, • Explain Transition state Theory (activation energy and catalyst) Reaction Mechanisms • Define mechanism of reaction • Verify that a suggested reaction mechanism is consistent with the observed kinetics • Define rate of reaction, rate equation, order of reaction and rate constant • Calculate the rate constant from initial rates • Deduce the order of a reaction (zero-, first- and second-) and the rate constant by the initial rates method. • Predict an initial rate from rate equations and experimental data A reactant 01 Definition Defined as the change of concentration of a reactant or a product per unit time. B product 02 Formula 03 Formation of Product The rate of reaction depends on how fast reactant A is used up or how fast the product B is formed. 04 How to measure the rate of reaction • Based on the reactant • Based on the product Based on product π ππ‘π = • πΆβππππ ππ π‘βπ πππππππ‘πππ‘πππ ππ πππππ’ππ‘ π΅ πΆβππππ ππ π‘πππ A B π ππ‘π = (-) indicates that concentration of A decreases with time dt = change in time • d[A] = change in concentration of product A • reactant πΆβππππ ππ π‘βπ πππππππ‘πππ‘πππ ππ πππππ’ππ‘ π΄ πΆβππππ ππ π‘πππ d[B] = change in concentration of product B • Based on reactant dt = change in time product Cont.. Cont.. N2 (g) + 3H2 (g) → 2NH3 (g) Answer Ms-1. Based on the equation above: a) Express the rate in terms of changes in [N2], [H2] and [NH3] with time. b) Identify the rate of change of NH3 and H2. Question The rate of formation of N2 is 0.0156 We Create Quality Professional k = rate PPT Presentation rate = kcx constant at certain temperature c = concentration of the reactant at certain temperature (mol L-1) x = order of reaction, x=0,1 and 2 Unit of rate = mol dm-3 s-1 or mol dm-3 min-1 Rate Equation = Rate Law Equation used to calculate the rate of reactions by using the constant and concentration of reactants. 1 2 3 Is the exponent of the reactant concentration in the rate equation It can only be found experimentally. Overall order of reaction is the sum of the order of reaction. aA + bB → cC + dD • Overall order = (n + m) • n and m are obtained through experiments only and not from the number of moles given from the stoichiometric equations. The rate equation for the oxidation of iodide ions by hydrogen peroxide is as follows: a) Identify the order of reaction with respect to i. H2O2 ii. Iiii. H+ b) Identify the overall order of the reaction? Question Rate = k [H2O2] [I-] [H+]0 Answer 1 Rate constant, k, is a constant of proportionality in the rate equation. 2 The units of k depend on the overall order of reaction. Rate = k [A][B] k= 3 4 ππππ π¨ [π©] rate constant does not depend on concentrations Rate ≠ rate constant, k 2A + B + 3C → products Answer rate = k [A] [B]2 a) Identify the overall order of reaction? b) Identify the order of reaction with respect to substance C? c) Identify the unit for the rate constant, k? Question The rate equation is given by: 3P + Y → P3Y Experiment Initial concentration (M) Initial rate P Y 1 0.15 0.75 (M min-1) 4.7 x 10-3 2 0.15 2.25 4.2 x 10-2 3 0.60 0.75 1.9 x 10-2 • The method of initial rates is a commonly used technique for deriving rate laws. • The measurement is repeated for several sets of initial concentration conditions to see how the reaction rate varies. Method to Determine Order of Reaction Step 1 Identify [A] is changed but [B] and [C] are kept constant Step 2 Write down the rate of equation Step 3 Assigning equation 1 and equation 2 Step 4 Dividing equation 1 and equation 2 The following rate data were following reaction. oC for the Identify the rate-law expression and the rateconstant for this reaction? Question 1 obtained at 25 2 A(g) + B(g) → 3 C(g) Cont.. Cont.. Answer Below is the results of an reactants used in the experiment are all unknown labelled as A, B and C. State the order of reaction for A, B and C and Question 2 experiment from a student, the predict the rate equation of the reaction. Cont.. Cont.. Answer Based on the Table given: a) Calculate the order of reaction of b) Write the rate equation. c) Determine the rate constant of the reaction, with its unit. Leave your answer in three significant figures. d) Question 3 substance A, B, and C. Determine the value of X with its correct unit. Leave your answer in three significant figures. Cont.. Cont.. Answer We Create Qualitysulphide Professional Hydrogen is removed from industrial pollution by reacting with chlorine PPT Presentation as follows: H2S (aq) + Cl2 (aq) → S(s) + 2HCl(aq) The reaction above is first order with respect to each reactant a) Write the rate equation for the reaction. What is the overall order of reaction? b) Calculate the rate of loss of hydrogen sulphide and the rate of formation of hydrochloric acid at 28 oC if the rate constant is 5.6 x 10-4 dm3 mol-1 min-1 when the concentration of H2S and Cl2 respectively are 2.5 x 10-4 M and 4.8 x 10-2 M. • Define half-life of a first-order reaction • Use the half-life (t½) of a first-order reaction in calculations Definition Example The half-life of a reaction, symbolized by t1/2 is the time required for the reactant concentration to drop to one-half of its initial value. 16 g ; 1 8g; A → Products π π 4g; π π π 2g;π Rate = kοAο Formula Rate constant, k = Gradient of the graph Why the half-life of a first-order reaction is a constant? • • Each half-life represents an equal amount of time. The half-life is constant (t1 = t2 = t3) • Because it depends only on the rate constant and not on the reactant concentration. • Each successive half-life is an equal period of time in which the reactant concentration decreases by a factor of 2. Formula The half-life of a first-order reaction was found to be 10 min at a certain temperature. Identify its rate constant? Question 1 Answer If 3.0 g of substance A decomposes for 36 minutes the mass of unreacted A remaining is found to be 0.375 g. Identify the half life of this reaction if it follows first-order kinetics? Question 2 Answer Question 3 H2O2(aq), initially at a concentration of 2.32 M, is allowed to decompose. What will [H2O2] be at t = 1200s? Use k = 7.30 x 10-4 s-1 for the first-order decomposition. Question 4 Use k = 7.30 x 10-4 s-1 for the first-order decomposition of H2O2(aq) to determine the percent [H2O2] that has decomposed in the first 500.0s after the reaction begins? Consider the first-order reaction A → products, with k = 2.95 x 10-3 s-1. Identify the percent of A remains after 150 s? • Explain collision theory • Explain Transition state Theory (activation energy and catalyst) Successful / Effective Collision Unsuccessful / ineffective Collision We Create Quality Professional PPT Presentation Effective collision is collision between reactant particles with sufficient energy which reactant particles are then change into product in a correct orientation. It is a type of collision that the reactant particles are not changing into products which may be caused by the reactant particles have not enough energy or collide in a wrong orientation. • Catalysts are substances which change the rate of a reaction but is left chemically unchanged at the end of the reaction. • Catalysts work by lowering the activation energy of a reaction. Without catalyst Energy With catalyst Reactants Products Reaction coordinate Activation energy, Ea is the minimum energy that colliding particles must take for a successful collision to take place Transition state Energy Reactants Products Reaction coordinate Energy Activation Energy Minimum energy to make the reaction happen – how hard Reactants Products Reaction coordinate Energy Activated Complex or Transition State Reactants Products Reaction coordinate The potential energy profile for the onestep reaction follows. The energies are in kJ/mol relative to an arbitrary zero of energy a) What is the value of the activation energy for this reaction? b) Is the reaction exothermic or endothermic? c) Suggest a plausible structure for the transition state. Question Answer The reaction between peroxodisulphate (VI) π2 π82− and iodide ions, I- can be catalyzed by iron (III), Fe3+? a) Suggest a mechanism for the catalytic reaction. b) Sketch an energy profile for the catalysed and uncatalyzed reactions. • Define mechanism of reaction • Verify that a suggested reaction mechanism is consistent with the observed kinetics A mechanism for a reaction is a collection of 1 elementary processes (also called elementary steps or elementary reactions) that explains how the overall reaction proceeds. When an overall reaction occurs in two or more 2 elementary steps, one of the steps is often much slower than the others. This slowest step in a reaction mechanism is called 3 the rate-determining step because it acts as a bottleneck, limiting the rate at which reactants can be converted to products 4 A catalyst is consumed in one step of a reaction and is regenerated in a subsequent step 5 An intermediate is formed in one step and is consumed in a subsequent step • Each of these two reactions is called an elementary reaction. Consider the reaction of nitric oxide with hydrogen • NO3 is intermediate species because is formed in Step 1 and is consumed in Step 2 and do not appear in the overall balanced equation. Consider the iodide ion catalyzed decomposition of hydrogen peroxide to water and oxygen. • Each of these two reactions is called an elementary reaction. • The I- known as catalyst because it consumed in Step 1 and is regenerated in Step 2 and do not appear in the overall balanced equation. • IO- is intermediate species because is formed in Step 1 and is consumed in Step 2 and do not appear in the overall balanced equation. The experimental rate law for the reaction between NO2 and CO to produce NO and Answer CO2 is rate = k[NO2]2. The reaction is Step 1: NO2 + NO2 → NO3 + NO (slow) Step 2: NO3 + CO → NO2 + CO2 (fast) a) What is the equation for the overall reaction? b) Identify the intermediate species from the mechanism given above. Question 1 believed to occur via two steps: Answer The mechanism for the reaction between HX and H2O2 + X- → H2O + XO- (Step 1; Slow) H+ + XO- → HXO (Step 2; Fast) HXO + H+ + XO- → H2O2 + X2 (Step 3; Fast) a) Write the rate equation for the reaction. b) Write the overall equation for the reaction. c) Identify the catalyst of the reaction above. d) Identify the intermediate species of the reaction above. Question 2 H2O2 is shown below: The two-step mechanism has been proposed for the gas-phase decomposition of nitrous oxide (N2O): a) Write the chemical equation for the overall reaction. b) Identify any intermediate species. Explain your answer. Thank You Thank You