Chemical Kinetics “Rates of Reactions”

1. Chemical Kinetics“Rates of Reactions”

2. Reaction Rates Average rate: Change of reactant or product concentrations over a specific time interval Initial rate: Rate at the beginning of a reaction.

4. Question? For the reaction: The dependence of the concentration of H2 on time is shown below. Is the reaction rate faster at point A or point B? A, B, both rates are the same

5. Rate Determining Step • slowest step in a multi-step mechanism • the step which determines the overall rate of the reaction

6. Rate Law • an expression which relates the rate to the concentrations and a specific rate constant

7. Rate Law Reaction rate = k [A]m [B]n where m => order with respect to A n => order with respect to B overall order = m + n

8. Question? The empirical rate law for the reaction is Rate = k[NO2][F2]. Which of the following mechanisms is consistent with this rate law? A) NO2 (g) + F2 (g) <=> NO2F(g) + F(g) fast NO2(g) + F(g) --->NO2F(g) slow B) NO2 (g) + F2(g) <==> NO2F(g) + F(g) slow NO2(g) + F(g) ---> NO2F(g) fast C) F2(g) <==> F(g) + F(g) slow 2NO2(g) + 2F(g) ---> 2NO2F(g) fast A, B, or C

9. Order of Reaction • The exponent of the concentration for a reactant implies the number of molecules of that species involved in the rate determining step • first order, exponent equals one • second order, exponent equals two

10. Integrated Rate Laws A ---> products rate = - (D[A]/Dt) = k[A]m average rate rate = - (d[A]/dt) = k[A]m instantaneous rate

12. This plot of ln[H2O2] vs. time produces a straight line, suggesting that the reaction is first order.

13. Elementary Reactions • Molecularity is the number of particles coming together in a reaction step. • unimolecular - rearrangement of a molecule • bimolecular - reaction involving the collision of two particles • termolecular - reaction involving the collision of three particles (statistically unlikely)

14. Unimolecular Reaction

15. Bimolecular Reaction

16. Collision Rate Model Three conditions must be met at the molecular level if a reaction is to occur: • the molecules must collide; • they must be positioned so that the reacting groups are together in a transition state between reactants and products; • and the collision must have enough energy to break bonds, form the transition state and convert it into products.

18. Transition State: Activated Complex or Reaction Intermediates • an unstable arrangement of atoms that has the highest energy reached during the rearrangement of the reactant atoms to give products of a reaction

19. Activation Energy the minimum energy required to start a reaction

22. Temperature & Reaction Rate

23. Catalyst • A substance which speeds up the rate of a reaction while not being consumed Homogeneous Catalysis - a catalyst which is in the same phase as the reactants Heterogeneous Catalysis - a catalyst which is in the different phase as the reactants catalytic converter • solid catalyst working on gaseous materials

25. Question? Which curve illustrates the effect of a catalyst on the reaction diagram, given that it speeds up the rate of a reaction? A, B, C, D

26. Reaction Mechanism • A set of elementary reactions which represent the overall reaction

27. Car Catalytic Converter catalyst H2O(g) + HCs -------> CO(g) + H2(g) (unbalanced) catalyst 2 H2(g) + 2 NO(g) ------> N2(g) + 2 H2O(g) catalyst HCs + O2(g) --------> CO2(g) + H2O(g) (unbalanced) catalyst CO(g) + O2(g) --------> CO2(g)(unbalanced) catalyst = Pt-NiO HCs = unburned hydrocarbons

28. Chain Mechanisms chain initiating step - the step of a mechanism which starts the chain chain propagating step(s) - the step or steps which keeps the chain going chain terminating step(s) - the step or steps which break the chain

29. Chain Mechanisms combustion of gasoline in an internal combustion engine chain initiating step - additives which generate free radicals, particles with unpaired electrons chain propagating step(s) - steps which generate new free radicals chain terminating step(s) - steps which do not generate new free radicals

30. Enzymes Any one of many specialized organic substances, composed of polymers of amino acids, that act as catalysts to regulate the speed of the many chemical reactions involved in the metabolism of living organisms.