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Second and Zero rate orders

Second and Zero rate orders. Chapter 14 part IV. Second Order Rate Laws. Butadiene forms its dimer 2C 4 H 6 (g) - > C 8 H 12 (g). 2nd Order Data. [C 4 H 6 ] vs time. The general equation is : aA --> products The second order rate law is : Rate = -∆[A]/∆t = k[A] 2

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Second and Zero rate orders

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  1. Second and Zero rate orders Chapter 14 part IV

  2. Second Order Rate Laws • Butadiene forms its dimer • 2C4H6(g) - > C8H12(g)

  3. 2nd Order Data

  4. [C4H6] vs time

  5. The general equation is : aA --> products The second order rate law is : Rate = -∆[A]/∆t = k[A]2 The integrated rate law is : 1/[A] = kt + 1/[A]t=0 The plot of 1/[A] versus time is a straight line with slope = k Using this equation, one can calculate [A] at any time provided k and [A] at t=0 are known. Second order Rate laws

  6. Second order half life • When the half life of the second order has elapsed, [A] = [A]0/2. • So: 1/([A]0/2) = kt 1/2 + 1/[A]t=0 • And 2/[A]0 - 1/[A]0 = kt 1/2 • 1/[A]0 =kt 1/2 • The expression for second order half life is: • t 1/2 = 1/k[A]0

  7. It is important that one recognizes the difference in the half life between 1st & 2nd order reactions. For the first order reaction t1/2 is dependent only on k. For the 2nd order reaction, t1/2 is dependent on both k and [A]t=0. Note that each successive half life in a 2nd order reaction is longer than the first. In fact each successive 2nd order t1/2 is double the preceding one. Second order half life

  8. Determining Rate order

  9. 2nd order reaction using integrated law for first order

  10. 2nd order reaction and 2nd order integrated rate law

  11. Zero Order Rate laws • Most reaction are 1st or second order. • However zero order has a rate law of: • Rate = k[A]0 = k(1) = k • For zero order the rate is constant and does not depend on the [reactant]. • Zero order is concentration independent. • The integrated rate law is: • [A] = -kt + [A]t=0 • The plot of [A] versus time is a straight line, with a slope of -k.

  12. Zero order • The half life of a zero order reaction is expressed: • [A] = [A]0/2 at t=t1/2 • If [A] = -kt + [A]t=0 • Then [A]0/2 = -kt1/2 + [A]t=0 • t1/2 = [A]t=0/2k • Zero order reactions are most likely to occur in the presence of a metal surface or enzyme, some type of catalysis.

  13. Rate laws with more than one reactant • BrO3-(aq)+ 5Br-(aq) +6H+(aq) --> 3Br2(l) + 3H2O(l) • From the experimental evidence we know the rate law to be: • Rate = -∆[BrO3-]/∆t = k[BrO3-][Br-][H+]2 • All reactants are expressed in the rate law. • The overall rate order here is 4

  14. Rate Laws: Summary

  15. Identify the rate order!

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