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Reaction Rates

Reaction Rates. What is Kinetics?. Kinetics or reaction rate is Change in a visible property Change in time The candle change in mass = m f – m i change in time t f - t i. Finding the Rate of a Reaction. aA + bB  cC + dD Rate = -[A] = -[B] = [C] = [D]

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Reaction Rates

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  1. Reaction Rates

  2. What is Kinetics? • Kinetics or reaction rate is Change in a visible property Change in time The candle change in mass = mf – mi change in time tf - ti

  3. Finding the Rate of a Reaction aA + bB  cC + dD Rate = -[A] = -[B] = [C] = [D] a t b t c  t d t The squares are meant to be delta, but my old pp program at home doesn’t know the )&(*&)^* symbol. [ ] means concentration Moles/liter –yes, this is molarity (M)

  4. Rate of Decomposition 2N2O5 4NO2 + O2 • Starting with N2O5… N2O5 (g) → 2NO2 (g) + ½O2 (g) 2NO2 ½ O2 N2O5

  5. Rate = -slope of the tangent

  6. Let’s Try Another reaction • Cool animation of rate of a reaction

  7. Cool animation of rate of reaction

  8. Measurement of rate of reaction by using color change Bleach is added to a solution of blue dye. Over time, the dye fades As it reacts with the Bleach A spectrophotometer could be used to measure The change.

  9. Notice it is all about concentration • The rate is directly related to the concentration of all of the substances involved.

  10. Why does concentration matter? • Reactions happens through collisions of reacting molecules. • Depends on two things: Speed of the molecules Orientations of molecules

  11. Kinetics simulation

  12. Collisions and Rate • Not only do collisions have to occur, they have to be effective. • How did the demonstration show effective collisions?

  13. Concentration and Pressure will effect reaction rate

  14. Note how the concentration of oxygen (increased on the right) changes the rate of the steel wool burning.

  15. Temperature Effects Rate as Well • How can we explain the glow light demonstration?

  16. Since rate is dependent upon concentration, and concentration of reactants decrease as time passes, the rate will change. • We can also graph [N2O5] versus rate

  17. K to the Rescue! • K is the relationship between rate and concentration of reactants rate = k[N2O5] This tells how the rate of the reaction depends on concentration. For the N2O5 example, the rate was directly proportional to the concentration.

  18. Order of Reaction • Some relationships are not directly proportional, it could be exponential or have a zero slope rate = k[N2O5]1 Here a reaction is shown that is first order (directly proportional) • The order can ONLY be determined by experiment

  19. The full rate formula then, involves all reactants and the ORDER rate = k[A]m[B]n[C]p… For Rate = k[A]m If m = 0, this is a zero order reaction If m = 1 a first order reaction If m = 2 a second order reaction

  20. First Order Reaction When the slope of a plot of ln[A] vs time is linear, the reaction is first order.

  21. Second Order Reaction One way to determine reaction order is to search for a linear relationship between some function of concentration and time. Because a plot of 1/[HI] vs t is linear (3rd plot), the reaction is second order.

  22. Zero Order Reaction The rate of a zero order reaction is constant, independent of concentration. These are rare. Most take place on solid surfaces. [A] vs. time will be linear. The rate constant k is numerically equal to the slope but has the opposite sign.

  23. To find the order, we compare two sets of experimental data:

  24. Let’s See Some Examples of Determining Order of a Reaction • Cool animation of an experiment about determining order of an experiment • Another animation of rate order • Half life animation

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