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Chemistry 1011

Chemistry 1011. TOPIC Rate of Reaction TEXT REFERENCE Masterton and Hurley Chapter 11. Reviewing Rates of Reaction. Reaction rates are affected by: reactant concentrations, temperature, catalysts, physical state of reactants Rate of reaction must be determined by experiment

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Chemistry 1011

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  1. Chemistry 1011 TOPIC Rate of Reaction TEXT REFERENCE Masterton and Hurley Chapter 11 Chemistry 1011 Slot 5

  2. Reviewing Rates of Reaction • Reaction rates are affected by: • reactant concentrations, • temperature, • catalysts, • physical state of reactants • Rate of reaction must be determined by experiment • Relation between the rate and the concentrations of reactants is the • rate law expression Chemistry 1011 Slot 5

  3. Reviewing Rates of Reaction • If aA + bB  xX • Then rate = k[A]m [B]n • k is the rate constant • exponents m and n are the order of reaction with respect to A and B • overall order of reaction is m + n • Collision theory postulates an energy barrier, the activation energy • Transition state theory postulates the formation of an activated complex Chemistry 1011 Slot 5

  4. Reviewing Rates of Reaction • Catalysts alter the rate of reaction without being consumed • The Arrhenius equation is derived from collision theory and links the rate constant to the activation energy k = Ae-Ea/RT • Reaction mechanisms are developed to explain experimental results. • They consist of a number of elementary steps. • The slowest step is the rate determining step • This step determines the overall rate of reaction Chemistry 1011 Slot 5

  5. Review - Reaction Mechanisms YOU ARE EXPECTED TO BE ABLE TO: • Define reaction mechanism and show how the reaction order is dependent upon the mechanism by which a reaction takes place. • For a reaction taking place in more than one step, identify the rate determining step and identify reaction intermediates. • Determine if a proposed reaction mechanism is consistent with experimental rate data. Chemistry 1011 Slot 5

  6. Review – Reaction Mechanism • A reaction mechanism is a suggested path or sequence of steps by which a reaction occurs • The suggested mechanism must explain the experimental determined rate law expression and order of reaction • The individual steps that make up a reaction pathway are called elementary steps • For elementary reactions, the rate law can be determined from the equation Chemistry 1011 Slot 5

  7. Reaction of CO(g) with NO2(g) CO(g) +NO2(g)  NO(g) + CO2(g) • At low temperatures, the experimentally determined rate law expression is: Rate = k [NO2]2 • A two step reaction mechanism is suggested Step 1 SLOW NO2(g) +NO2(g)  NO3(g) +NO(g) Step 2 FAST CO(g) +NO3(g)  CO2(g) +NO2(g) CO(g) +NO2(g)  NO(g) + CO2(g) Chemistry 1011 Slot 5

  8. Reaction of CO(g) with NO2(g) • For the elementary step #1: NO2(g) +NO2(g)  NO3(g) +NO(g) • The rate law is: Rate = k1[NO2]2 • For the elementary step #2: CO(g) +NO3(g)  CO2(g) +NO2(g) • The rate law is: Rate = k2[CO][NO3] Chemistry 1011 Slot 5

  9. Reaction of CO(g) with NO2(g) • In this mechanism, SLOW step #1 is the rate determining step Step 1 SLOW: NO2(g) +NO2(g)  NO3(g) +NO(g) • The rate law expression for this step is Rate = k1[NO2]2 • This is the rate law predicted for the overall reaction by the proposed mechanism • This is consistent with experiment Chemistry 1011 Slot 5

  10. Review - Mechanism with a Fast Initial Step • Sometimes the first step in a reaction mechanism, which results in the creation of a reaction intermediate, will be FAST, and the second step, where the reaction intermediate is a reactant, may be SLOW • The rate determining step will be the second step • The rate law expression should then include the concentration of the reaction intermediate, but this cannot be measured • The final rate law expression can only include species occurring in the balanced equation Chemistry 1011 Slot 5

  11. Reaction of NO with Cl2 Step 1: NO(g) + Cl2(g)NOCl2(g)FAST Step 2: NOCl2(g) + NO(g) 2NOCl(g)SLOW Overall: 2 NO(g) + Cl2(g) 2NOCl(g) • Rate of overall reaction = rate of step 2 Rate = k2[NOCl2][NO] • The first (fast) elementary step in the reaction is reversible; The reactants and products are in equilibrium rate forward = rate reverse Chemistry 1011 Slot 5

  12. Reaction of NO with Cl2 • For the first (fast) elementary step: rate forward = rate reverse k1[NO][Cl2] =k-1[NOCl2] [NOCl2] = k1[NO][Cl2] k-1 Substitute in overall rate law expression Rate of reaction = rate of step 2 = k2[NOCl2][NO] Rate = k2k1 [NO]2[Cl2] = kexp [NO]2[Cl2] k-1 Chemistry 1011 Slot 5

  13. Limitations of Mechanism Studies • Mechanisms are suggested in order to explain observed rate laws and orders of reaction • Often more than one mechanism can explain experimental results Chemistry 1011 Slot 5

  14. Review Problem #1 • Hydrogen peroxide decomposes: 2H2O2(aq) 2H2O(l) + O2(g) • Initial rate data: [H2O2] Initial Rate (mol/L.min) 0.200 1.04 x 10-4 0.300 1.55 x 10-4 0.500 2.59 x 10-4 • Determine the order of reaction • Write the rate law expression • Calculate the rate constant, k Chemistry 1011 Slot 5

  15. Review Problem #2 • Two mechanisms are proposed for the reaction: 2NO(g) + O2(g)  2NO2(g) Mechanism #1: NO + O2 NO3 (fast) NO3 + NO  2NO2 (slow) Mechanism #2: NO + NO N2O2 (fast) N2O2 + O2  2NO2 (slow) Show that each is consistent with the rate law: Rate = k[NO]2[O2] Chemistry 1011 Slot 5

  16. Review Problem #2 • Mechanism #1 NO + O2 NO3 (fast) NO3 + NO  2NO2 (slow) • Step 2 is rate determining Rate2 = k2 [NO3][NO] • But NO3 is an intermediate • The reactants and products in Step 1 are in equilibrium • rate forward = rate reverse k1[NO][O2] = k-1[NO3] • Substitute for [NO3] in rate law expression Rate2 = Rate overall = k2.k1/k-1[NO][O2][NO] Rate = k[NO]2[O2] Chemistry 1011 Slot 5

  17. Review Problem #2 • Mechanism #2 NO + NO N2O2 (fast) N2O2 + O2  2NO2 (slow) • Step 2 is rate determining Rate2 = k2 [N2O2][O2] • But N2O2 is an intermediate • The reactants and products in Step 1 are in equilibrium • rate forward = rate reverse k1[NO][NO] = k-1[N2O2] • Substitute for [N2O2] in rate law expression Rate2 = Rate overall = k2.k1/k-1[NO][NO][O2] Rate = k[NO]2[O2] Chemistry 1011 Slot 5

  18. Review Problem #3 • Hydrogen peroxide in basic solution oxidizes iodide ions to iodine • The proposed mechanism for the reaction is: Step 1 (SLOW): H2O2(aq) + I-(aq) HOI(aq) + OH-(aq) Step 2 (FAST): HOI(aq) + I-(aq) I2(aq) +OH-(aq) • Write the overall equation • H2O2(aq) + 2I-(aq) I2(aq)+OH-(aq) • Write an expression for the overall rate law • Rate = k [H2O2][I-] Chemistry 1011 Slot 5

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