Lecture22222

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Lecture22222

  1. 1. Kinetics: the Rate Law and Effect of Concentration Lecture 22
  2. 2. The rate law for a chemical reaction is an equation which links the reaction rate with concentrations or pressures of reactants and constant parameters (normally rate coefficients and partial reaction orders).
  3. 3. For aA+bB+…  cC+dD+… Rate = k[A] m [B] n …
  4. 4. Rate = k[A] m [B] n … <ul><li>Herein k is the rate constant. It depends on temperature. </li></ul><ul><li>m and n are reaction orders. They depend on the reaction mechanism. </li></ul>
  5. 5. Rate = k[A] m [B] n … <ul><li>If the rate doubles when [A] doubles, m=1. </li></ul><ul><li>If the rate quadruples when [A] doubles, m=2. </li></ul><ul><li>If the rate does not change when [A] doubles, m=0. </li></ul>
  6. 6. Coefficients a and b in aA+bB+…  cC+dD+… may or may not be related in any way to the reaction orders m and n : Rate = k[A] m [B] n …
  7. 7. The reaction mechanism must conform to to the rate law. The rate law is based on experimental fact.
  8. 8. Components of the rate law must be found experimentally: <ul><li>Measure concentrations to find the initial rate. </li></ul><ul><li>Use initial rates from several experiments to find the reaction orders. </li></ul><ul><li>Use reaction orders to find the rate constant. </li></ul>
  9. 9. How to define the reaction orders: <ul><li>Rate=k[A] . First order overall. </li></ul><ul><li>Rate=k[A] 2 . Second order overall. </li></ul><ul><li>Rate=k[A] 0 =k(1)=k . Zero order overall. </li></ul>
  10. 10. How to define the reaction orders: <ul><li>NO (g) +O 3(g)  NO 2(g) +O 2(g) </li></ul><ul><li>Rate=k[NO][O 3 ] </li></ul><ul><li>First order with respect to NO. </li></ul><ul><li>First order with respect to O 3 . </li></ul><ul><li>Second order overall. </li></ul>
  11. 11. How to define the reaction orders: <ul><li>2NO (g) +2H 2(g)  N 2(g) +2H 2 O (g) </li></ul><ul><li>Rate=k[NO] 2 [H 2 ] </li></ul><ul><li>Second order with respect to NO. </li></ul><ul><li>First order with respect to H 2 . </li></ul><ul><li>Third order overall. </li></ul>
  12. 12. Reaction orders cannot be deduced from the balanced equation.
  13. 13. Special cases: <ul><li>If a reaction order is fractional, the rate depends on the square (cubic) root of the concentration: rate=k[A][B] 1/2 </li></ul><ul><li>If a reaction order is negative, the rate decreases when the concentration of that component increases. </li></ul>
  14. 14. A sample problem on determining rate order from rate laws.
  15. 15. It we do not know the reaction law, we have to find it from a series of experiments, starting each one with a different set of reactant concentrations and obtaining an initial rate in each case.
  16. 16. A sample problem on determining reaction order from initial rate data.
  17. 17. THE END

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