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Kinetics

Kinetics. Until now, we have considered that reactions occur: Reactants form products and conservation of mass is used to find amounts of these Now, we investigate how fast products are formed (or how fast reactants disappear): THE RATE of REACTION

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Kinetics

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  1. Kinetics • Until now, we have considered that reactions occur: Reactants form products and conservation of mass is used to find amounts of these • Now, we investigate how fast products are formed (or how fast reactants disappear): THE RATE of REACTION • We will use differential rate laws to determine order of reaction and rate constant from experimental data 

  2. Rate of Reaction • Rate = Δ[concentration] or d [product] Δ time dt • Rate of appearance of a product = rate of disappearance of a reactant • Rate of change for any species is inversely proportional to its coefficient in a balanced equation.

  3. Rate of Reaction • Assumes nonreversible forward reaction • Rate of change for any species is inversely proportional to its coefficient in a balanced equation. • 2N2O5 4NO2 + O2 • Rate of reaction = -Δ[N2O5] = Δ[NO2] = Δ[O2] 2 Δt 4 Δt Δt where [x] is concentration of x (M) and t is time (s)

  4. Reaction of phenolphthalein in excess base • Use the data in the table to calculate the rate at which phenolphthalein reacts with the OH- ion during each of the following periods: • (a) During the first time interval, when the phenolphthalein concentration falls from 0.0050 M to 0.0045 M. • (b) During the second interval, when the concentration falls from 0.0045 M to 0.0040 M. • (c) During the third interval, when the concentration falls from 0.0040 M to 0.0035 M.

  5. Reactant Concentration by Time

  6. Finding k given time and concentration • Create a graph with time on x-axis. • Plot each vs. time to determine the graph that gives the best line: • [A] • ln[A] • 1/[A] • (Use LinReg and find the r value closest to 1) • k is detemined by the slope of best line (“a” in the linear regression equation on TI-83) • 1st order (ln[A] vs. t): k is –slope • 2nd order (1/[A] vs t: k is slope)

  7. Rate Law Expression • As concentrations of reactants change at constant temperature, the rate of reaction changes. According to this expression. Rate = k[A]x[B]y… • Where k is an experimentally determined rate constant, [ ] is concentration of product and x and y are orders related to the concentration of A and B, respectively. These are determined by looking at measured rate values to determine the order of the reaction.

  8. same 2x 2x Finding Order of a Reactant - Example2ClO2 + 2OH- ClO3- + ClO2- + H2O • Start with a table of experimental values: • To find effect of [OH-] compare change in rate to change in concentration. • When [OH-] doubles, rate doubles. Order is the power: 2x = 2. x is 1. This is 1st order for [OH-].

  9. 9x Finding Order of a Reactant - Example2ClO2 + 2OH- ClO3- + ClO2- + H2O • Start with a table of experimental values: • To find effect of [ClO2] compare change in rate to change in concentration. • When [ClO2] triples, rate increases 9 times. Order is the power: 3y = 9. y is 2. This is 2nd order for [ClO2]. same 3x

  10. Finding Order of a Reactant - Example2ClO2 + 2OH- ClO3- + ClO2- + H2O • Can use algebraic method instead. This is useful when there are not constant concentrations of one or more reactants. This example assumes you found that reaction is first order for [OH-] . 6.00 x 10-4=k(0.010)x(.030)1 1.08 x 10-2 = k (0.030)x(.060)1 0.0556 = .333x(.5) For [ClO2]x , x = 2

  11. Rate Law:2ClO2 + 2OH- ClO3- + ClO2- + H2O Rate = k[ClO2]2[OH-] To find k, substitute in any one set of experimental data from the table. For example, using the first row: k = rate/[ClO2]2[OH-] k = 6.00x10-4Ms-1 = 200 M-2s-1 [0.010M]2[0.030M] Overall reaction order is 2+1=3. Note units of k.

  12. Determining units for k given overall reaction order Rate(M/s) = k[A]x x = overall order of reaction [A] = the reactant concentration (M)

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