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Chemical Kinetics

Chemical Kinetics. For a rxn to occur we need to know two things. Our reactants and products If our reaction is going to occur!!!. SPONTANEOUS. Inherent tendency for the process to occur DOES NOT MEAN FAST!!!!!. Spontaneous Rxn. Spontaneous reactions can take weeks to occur

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Chemical Kinetics

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  1. Chemical Kinetics

  2. For a rxn to occur we need to know two things • Our reactants and products • If our reaction is going to occur!!!

  3. SPONTANEOUS • Inherent tendency for the process to occur • DOES NOT MEAN FAST!!!!!

  4. Spontaneous Rxn • Spontaneous reactions can take weeks to occur • Example: graphite turns to diamond spontaneously!

  5. To be useful • Rxns must occur at a reasonable rate • Must understand: • stoichiometry • thermodynamics and...

  6. Chemical Kinetics • What governs the rate of reactions

  7. Reaction mechanism • The series of steps by which a reaction takes place

  8. 2NO2(g) -->2NO(g) + O2(g) • Decomposition of NO2 (the gas in air pollution) • Start with flask of NO2 and measure the concentration of 3 gases as NO2 decomposes

  9. What will happen? • NO will increase • O2 will increase • NO2 will decrease • we need to look at speed at which this occurs

  10. Chemical Kinetics • Rate = change in given quantity/time • For chem rxns- the change is the amt or concentration of a reactant or product

  11. For reaction • Rate = (conc of A at time t2- conc of A at time t1) (t2- t1) Rate =[A] t

  12. 2NO2(g) -->2NO(g) + O2(g) • NO will have negative slope of NO2 • Rate O2 is half as fast as NO

  13. Rate Laws... • Rxns are reversible! • NO2 in empty container • Forward rxn dominant • After time, there are products and reverse rxn is important

  14. Now... • [NO2] depends on the difference in the rates of the forward and reverse rxns

  15. To eliminate problems • Study the problem where reverse rxn is negligible (right after start of rxn) • Choose conditions where reverse rxn is neglected

  16. Rate = k[NO2]n • Rate depends on • conc • k = rate constant • n = order of reactant (+,-, integer or fraction) • Determined experimentally!!

  17. Types of Rate Laws • Rate is a fcn of concentration • Rate = -[NO2]/ t = k[NO2]n

  18. Differential Rate Law • Rate law that expresses how rate depends on the concentration (usually called rate law)

  19. Why bother with rate laws? • To find steps that make rxn occur (series) • speed up rxns if we know slowest step

  20. Homework

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