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Ahl kinetics


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Ahl kinetics

  1. 1. • Find a friend, and each of you crumple up a piece of paper. Sitting facing one another, with knees almost touching, toss your pieces of paper together, trying to get them to hit while still in the air. Do this 10 times - how many successful "collisions" did you record?• Now, grab another friend with their own wad of paper. This time, a successful collision will only occur when all three pieces of paper hit together simultaneously - just two out of three wont do. Out of 10 tosses, how many are successful? Add one more friend and repeat the exercise. Any successful collisions, where all four pieces of paper collide at the same instant?
  2. 2. HL Kinetics Syllabus
  3. 3. 16.1 Rate Expression• Understand – Rate constant – Overall order – Order of reaction• Use experimental data to deduce rate expression.• Solve problems using the rate expression.
  5. 5. DEFINITIONS• Rate is how fast concentration of reactant is falling ( or vice versa ) measured in – mol dm-3 s-1• Rate Constant (k) is only constant if T and Ea stays constant. A catalyst or higher T would increase k. Changes in concentration do not.
  6. 6. The Concentration Terms• Where rate = k[A]a [B]b [C]c• The powers are called the ‘order of reaction’. – If no effect on rate regardless of concentration we call it zero order. – If we double concentration of [A], and the rate doubles we say Rate = k[A] – If we double concentration of [A], and rate is quadrupled we say Rate = k[A]1 – If we double the concentration of [A}, and the rate is 8x faster we say Rate = k [A]2• Overall orders are sums of the individual orders.
  7. 7. Orders of Reaction• xA + yB + zC……  products• Rate = k [A]x [B]y [C]zthis is rate equation• How do we measure rate of reaction ? – How fast concentration of one of reactants is falling  – Units
  8. 8. Zero Order• This means the concentration of that substance does not influence the rate• Rate = k[A]0
  9. 9. First Order• Whatever you do to the concentration, you do to the rate.• Double conc = double rate• Triple conc = triple rate• Etc• Rate = k[A]
  10. 10. Second Order• Whatever you do to the concentration you square the rate.• Rate = k[A]2
  11. 11. Notes• The coeffcients in balanced chemical equation are not the orders !• You cannot predict orders they must be found experimentally.• Orders give clues to mechanisms…….later 
  12. 12. Finding Orders from Rate Experiments• Use blog to calculate experimental k and order 
  13. 13. Finding Orders of Reaction Graphically• The real world is not so straight forward 
  14. 14. Zero Order wrt [A]• Rate = constant, i.e. independent of value of [A].• If A is only substance taking part in the reaction then Rate = k• A horizontal straight line shows a zero order rxn.
  15. 15. First Order wrt [A]• Rate = k[A]• Rate constant is slope of the graph• What are units ?
  16. 16. Second Order wrt [A]• Rate = k[A]2• Plot rate against concentration of A2
  17. 17. Finding value for Rate Constant k• Rate is slope of the graph 
  18. 18. Recognising Order from Graphs• olveit/Kinetics/IntegratedRateLaws.html
  19. 19. Using Half Life• For a first order reaction, half life is constant – independent of initial concentration.• ONLY FIRST ORDER REACTIONS HAVE CONSTANT HALF LIVES.• Sums 142
  20. 20. Activation Energy• Describe qualitatively the relationship between the rate constant (k) and temperature (T).• Simple but worth restating……..The rate of reaction is affected by Temperature ( and catalyst ).
  21. 21. Arrhenius Equation
  22. 22. Qualitative description• T appears in Ea/RT• If T increases then Ea/RT gets smaller• This is a negative terms so as T increases Ea/RT gets less negative• -Ea/RT is a power so as T is raised then e-Ea/RT gets larger.• So the rate constant gets larger as T gets larger.
  23. 23. Useful Arrhenius Equation 
  24. 24. So Why is This Useful ?
  25. 25. Huh ?• For every rise in T of 10K there is a DOUBLING of reaction rate.• This is a very useful ( and oft tested ) rule of thumb 
  26. 26. Example Calculations• 146 – 7.• Lab : determination of activation energy.
  27. 27. Reaction Mechanism• Explain that reactions can occur by more than one step and that thesloweststep determines the rate of reaction• This is called the RATE DETERMINING STEP.
  28. 28. The What ?• tes/ordermech.html• Review using ChemGuide and produce a 1 side A4 substituting your own examples to illustrate the link between order and reaction mechanism.