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Rate Laws

Rate Laws. Determine the rate law and order of a chemical reaction from experimental data. Include: various reaction orders , rate versus concentration graphs. Additional KEY Terms Ratio. Rate is proportional to changes in A and B. A + B. C.

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Rate Laws

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  1. Rate Laws

  2. Determine the rate lawand order of a chemical reaction from experimental data. • Include: various reaction orders, rate versus concentration graphs. Additional KEY Terms Ratio

  3. Rate is proportional to changes in A and B A + B C Rate Law helps us calculate the rate of a REACTION not a reactant.

  4. It is an expression that shows the quantitativeeffect of concentration changes on reaction rate. Rate = k[A]x where: k rate constant [A] concentration of A x reaction order **The rate constant (k) and the order (x) can only be determined experimentally**

  5. The rate constant (k) is specific for each reaction at a specific temperature. • Temperature is the only factor to affect • the rate constant. Rate = k[A]x

  6. Reaction Order

  7. A + B Products Rate = k[A]x[B]y Each reactant can affect the rate differently The ordersof a reaction (x and y) indicate how mucheach[reactant] affects the rate of a reaction.

  8. First order reaction( x = 1) The reaction rate is directly proportional to changes in reactant concentration. [A] is doubled - rate doubles 2 = 21 [A] is tripled - rate triples 3 = 31 x Rate = k[A] 1

  9. Second order reaction (x = 2) The reaction rate is proportional to changes in reactant concentration squared. Rate = k[A]2 Doubling [A] - increase rate 4x 4=22 Tripling [A] - rate increase 9x 9=32

  10. Third order reaction (x= 3) The reaction rate is proportional to changes in reactant concentration cubed. Rate = k[A]3 • Zero order reaction(x= 0) • The rate does not depend on the [A]. • Changing [A] does NOT change the rate. Rate = k **NOT included in the rate law if determined to be zero order**

  11. Rate = k[A][B]2 Overall order of reaction is the sum of the orders: x + y = overall reaction order 1st order + 2nd order = 3rd order overall

  12. Calculating Rate Law

  13. Several ways to determine the rate law: • differential rate law - uses calculus • integrated rate law – uses graphing software • initial rates method – uses data tables Determining rate law: Measure the effect of changes in concentration of onereactanton rate, while keeping the other reactantconstant.

  14. Ratio Approach 8 = 24 2 = 12 

  15. Using ratios: A + B → products same Rate = k[A]x[B] y Rate = k[A]x[B] y Rate2 k [A]2x  [B]2y 9.0 α[3]x 1 = α Rate1k [A]1x  [B]1y 9.0 α[3]x 18.0 α0.3 x  0.2 y 2.0 0.1 0.2 x = second order

  16. Using ratios: A + B → products 2 Rate = k[A]2[B] • 1 • y Rate = k[A]x[B] y Rate3 k [A]3x  [B]3y 8.0 α[2]2 [2]y = α Rate1k [A]1x  [B]1y 8.0 α 4 [2]y 2.0 α[2]y 16.0 α0.2 2 0.4 y 2.0 0.1 0.2 y = first order

  17. Using ratios: A + B → products 2 y Rate = k[A]2[B] You might pick bad trials Rate2α [A]2x  [B]2y Can’t have multiple answers…pick new trials. 18.0 α 0.3 x  0.2 y 9.0 α [3] [1]y 2 Rate1 [A]1x  [B]1y 2.0 0.1 0.2 9.0 α 9 [1]y 1 α[1]y y = 0 or 1 or 2…..

  18. Common Sense Approach

  19. 1 1 2x 2 same same 2.0 α 1 ּ[2]y = x y 1 1 rate = k[H2O2] [HI] Rate2 k [H2O2]2x  [HI]2y 2.0 α[2]y Rate1k [H2O2]1x  [HI]1y rate = k[H2O2][HI] x = first order 0.0152 α 0.1 x  0.2 y 0.0076 0.1 0.1

  20. y x 1 Rate = k[A] [B] 8.0 α [2]x  [4]1 Rate α [A]x  [B]y 8.0 α [2]x  4 Rate3α [A]3x  [B]3y Rate2 [A]2x  [B]2y 2.0 α [2]x x = first order 5.8 -4α 0.2 x  0.048 1 7.25 -5 0.1 0.012

  21. 3 A (g) + B (g) + 2 C (g) 2 D (g) + 3 E (g) a. Write the rate law for this reaction. b. Calculate the value of the rate constant (k). c. Calculate the rate for Trial #5. d. Calculate the concentration of A in Trial #6.

  22. 2 0 1 same same same same same same a. Write the rate law for this reaction. Rate2 k [A]2x [B]2y  [C]2z Rate2 [A]2x = = Rate1k [A]1x [B]2y [C]2z Rate1[A]1x rate = k[A][B]2

  23. b. Calculate the value of the rate constant (k). rate = k[A][B]2 To find the value of k, we use that data from ANY trial. Don’t include units for k.

  24. c. Calculate the rate for Trial #5. rate = k[A][B]2 rate = (200)(0.50 mol/L)(0.40 mol/L)2 rate = 16 mol/Ls

  25. d. Calculate the concentration of A in Trial #6. rate = k[A][B]2

  26. Conclusion: Everything in the Rate Law must be determined experimentally: 1. Write a basic rate law with all reactants 2. Determine the order for each reactant (1, 2, 0) 3. Re-write the rate law with the determined order of reaction for each 4. Solve any problems

  27. CAN YOU / HAVE YOU? • Determine the rate lawand order of a chemical reaction from experimental data. • Include: various reaction orders, rate versus concentration graphs. Additional KEY Terms Ratio

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