What is this?

1. What is this?

2. Kinetics Reaction Rates: How fast reactions occur

3. How do we measure rxn rates? • Rates must be measured by experiment • Indicators that a reaction is happening • Color change • Gas evolution • Precipitate formation • Heat and light • Many ways to measure the rate • Volume / time • Concentration / time • Mass / time • Pressure / time

4. How do we measure rxn rate? A  B • How fast product appears • How fast reactant disappears

5. Forward vs Reverse Rxn • Some rxns are reversible • After a sufficient amount of product is made, the products begin to collide and form the reactants • We will deal only w/ rxns for which reverse rxn is insignificant 2 N2O5(aq)  4 NO2(aq) + O2 (g) • Why is reverse rxn not important here?

6. Rate Law • Math equation that tells how reaction rate depends on concentration of reactants and products • Rates = k[A]n • K = rate constant / proportionality constant • n = order of reaction • Tells how reaction depends on concentration • Does rate double when concentration doubles? • Does rate quadruple when concentration doubles?

7. 2 kinds of rate laws • Both determined by experiment • Differential Rate Law • How rate depends on [ ] • Integrated Rate Law • How rate depends on time

8. Differential Rate Law • 2 methods • Graphical analysis • Method of initial rates

9. Graphical Analysis • Graph [ ] vs. time • Take slope at various pts • Evaluate rate for various concentrations

10. Graphical Analysis • When concentration is halved… • Rate is halved • Order = 1 • Rate = k[N2O5]1

11. Graphical Analysis • When concentration is doubled… • Rate is quadrupled • Order = 2 • Rate = k[N2O5]2

12. Method of Initial Rates • Initial rate calculated right after rxn begins for various initial concentrations • NH4+(aq) + NO2-(aq)  N2(g) + 2H2O(l) • Rate = k [NH4+]n[NO2-]m

13. When [NO2] doubles, rate doubles, First order with respect to (wrt) NO2 n = 1 When [NO2] doubles, rate doubles, First order with respect to (wrt) NO2 m = 1 Rate = k[NH4+] [NO2-]

14. Try this one: Rate = k [NO2-]2 Calculate k, using any of the trials, you should get the same value

15. Integrated Rate Law • Tells how rate changes with time • Laws are different depending on order • Overall reaction order is sum of exponents • Rate = k  zero order • Rate = k[A]  first order • Rate = k[A]2  second order • Rate= k[A][B]  second order

16. First order integrated rate law • Rearrange and use some calculus to get: • This is y = mx + b form • A plot of ln[A] vs time will give a straight line • If k and [A]0 (initial concentration) known, then you know the concentration at any time

17. Second order integrated rate law • Rearrange and use some calculus to get: • This is y = mx + b form • A plot of 1/[A] vs time will give a straight line • If k and [A]0 (initial concentration) known, then you can now the concentration at any time

18. Zero order integrated rate law • Rearrange and use some calculus to get: • This is y = mx + b form • A plot of [A] vs time will give a straight line • If k and [A]0 (initial concentration) known, then you can now the concentration at any time

19. Graphs give order of rxn • Use graphs to determine order • If [A] vs time = zero order • If ln [A] vs time = first order • If 1/ [A] vs time = second order

20. Half-life • Def’n: time it takes for concentration to halve • Depends on order of rxn • At t1/2 [A]=[A]0/2

21. Half-life: First order

22. Half-Life • First order • Second order • Zero Order