1 / 20

Kinetics

Kinetics. Ways to Express Rates. Relative Average Instantaneous. More commonly, rates are expressed by a rate law or rate expression – two methods: Differential – expresses how rate depends on initial concentration Integrated – expresses how concentration depends on time.

devon
Download Presentation

Kinetics

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Kinetics

  2. Ways to Express Rates • Relative • Average • Instantaneous • More commonly, rates are expressed by a rate law or rate expression– two methods: • Differential – expresses how rate depends on initial concentration • Integrated – expresses how concentration depends on time

  3. Initial rxn rate = k[A]om[B]on k = rate constant [A] = initial concentration of reactant A [B] = initial concentration of reactant B m = order of reaction for reactant A n = order of reaction for reactant B

  4. Exponents (orders) can be zero, whole numbers, or fractions -- AND MUST BE DETERMINED BY EXPERIMENTATION!!

  5. THE RATE CONSTANT, k Is temperature dependent & must be evaluated by experiment. The units of k depend on the rate law and the units in which the data was collected

  6. Differential • Experiment and rate change

  7. Zero order • The change in concentration of reactant has no effect on the rate. • These are not very common. • General form of rate equation: Rate = k[A]0 = k

  8. First order • Rate is directly proportional to the reactants concentration; doubling [rxt], doubles rate. These are very common! Nuclear decay reactions usually fit into this category. • General form of rate equation: Rate = k [A]1 = k[A]

  9. Second order • Rate is quadrupled when [rxt] is doubled and increases by a factor of 9 when [rxt] is tripled, etc. These are common, particularly in gas-phase reactions. • General form of rate equation: Rate = k [A]2

  10. A + B  C

  11. 12.4 INTEGRATED RATE LAW -CONCENTRATION/TIME RELATIONSHIPS When we wish to know how longa reaction must proceed to reach a predetermined concentration of some reagent, we can construct curves or derive an equation that relates concentration and time.

  12. Graphical methods fordistinguishing FIRST, SECOND and ZERO order reactions First order: ln[A] = -kt + ln[A]o (y = ax + b) ln[reactant] vs. time straight line for first order in that reactant & since a = -k the slope of the line is negative. ln[A] t

  13. Second order: 1/[A] = kt + 1/[A]o (y = ax + b) 1/[reactant] vs. time straight line for second order in that reactant since a = k the slope is positive. 1 [A] t

  14. Zero order: [A] = -kt + [A]o (y = ax + b) [A] vs. time straight line for zero order in that reactant & since a = -k the slope of the line is negative [A] t

  15. Half-life and reaction rate for FIRST-ORDER REACTIONS, t1/2 rate = k[A]1 integrated rate law is: ln[A] = -kt + ln[A]o k = ln2 t ½ = .693 t ½ k

  16. Mechanisms • Rate determining step is the slowest step • It will reflect the rate law

  17. 2 H2(g) + 2 NO(g) N2(g) + 2H2O(g) Possible mechanism 2 NO(g)  N2O2(g) N2O2(g) + H2(g)  N2O(g) + H2O(g) N2O(g) + H2(g)  N2(g) + H2O(g)

  18. H2O2 + Br-1 BrO-1 + H2O H2O+ BrO-1  Br-1 + H2O+ O2 Overall reaction? Catalysts? Intermediates?

More Related