Given the rxn: 2NO (g) + O 2(g) -> 2NO 2(g) Express the reaction rate in terms of O 2 and NO 2. If O2 is reacting at a rate of M/s, at what rate is NO2 being formed? What can we do to speed up or slow down this reaction?

Reaction Rate can be determined Initially Over some time period (average) Or at some specific time (instantaneous) Some ways it can be studied is by measuring changes in Color, pH, conductivity, pressure

For the rxn: Br 2 + HCOOH -> 2Br - + 2H + + CO 2 How is rate determined graphically? We can find the average rate. Find the average rate for the first 800s. Compare to the average rate for the first 2000 s. Try without calculator. First 800 s- First 2000 s- - If we calculate the average rate over small intervals, we can find instantaneous rate.

What will effect the rate? Why??? What happens at the molecular level in order for reactants to produce products?

What has to happen for the reaction to take place?

Reaction Profile Time Energy Reactants Products Activated Complex – Transitional structure Activation Energy

We defined and calculated Average rate Instantaneous rate (Rate) Let’s look at the Initial rate We isolate this time because the rate will depend on the starting concentrations and may be more accurate than looking at the changes in concentration and variable slope as the reaction proceeds

Rate Law… what is it? How do I find it? Mathematical expression of Reaction rate as a function of reactant concentration Experimentally determined Can use the “Method of Initial Rates” to find the initial rate

Method of Initial Rates Rxn: A + 2 B -> 2 C Rate Law: Rate = k[A] x [B] y k is the rate law constant What is factored into the constant? ExpInitial [A]Initial [B]Initial Rate M 2.73 M/s M0.100 M6.14 M/s M0.200 M5.42 M/s

The rate law for the reaction of A + 2B -> C is rate = 2730/M 2 s[A] o 2 [B] o What is the order of A? B? the overall order? Which reactant has a greater effect on the reaction rate? What happens to the rate if the concentration of A is doubled? If the concentration of B is doubled? If the concentration of A is tripled? What is the rate if [A] o =.0500 M and [B] o =.0256 M (oh and what do the little circles mean?

Expressions for rate… Rate law is a function of : Typically : zero, first or second order What if we need to know how the concentration varies with time?

Integrated Rate Law Involves taking the rate law and integrating from time 0 to t At time 0, [A] = [A]o At some time, t, later in the reaction the concentration is [A] Let’s try integrating the zero order rate law together

Zero Order Rate = k  [A] = k tt  A= -kt + A o  What is half life?  t 1/2 = Time for A o to be halved  t 1/2 = [A] o /2

For rxn: A-> products

1. At 1000 o C, cyclobutane decomposes in a first order reaction, with the very high rate constant of 87/s, to two molecules of ethylene (C 2 H 4 ) a. The initial concentration of C4H8 is 2.00 M. What is the concentration after s? b. What fraction of C4H8 has decomposed in this time? 2. At 25 o C, HI breaks down very slowly to form hydrogen and iodine: rate =k[HI] 2. The rate constant at 25 o C is 2.4x /Ms. If mole of HI is placed in a 1.0 L container, how long will it take for 10% to react? 3. H 2 O 2 decomposes to water and oxygen in a first order reaction. If 1.28M H2O2 changes to 0.85 M in 10.0 min at a given temp., what is the rate constant?

For a given rate law: rate = k[A] x, what does k represent? k = Ae -Ea/RT

To relate k to T, take ln of both sides of equation: Two point eqn: ln(k 2 /k 1 ) =Ea/R (1/T 1 -1/T 2 )

a. The decomposition of HI has rate constants of 9.51x10 -9 /Ms at 500 K and 1.10x10 -5 /Ms at 600K. Find Ea. B. What is the rate constant at 300K?

Reaction Profile Time Energy Reactants Products Activated Complex – Transitional structure Activation Energy What is the effect of a catalyst?

Reaction Mechanism Most reactions occur in a sequence of single reaction steps that sum to the overall equation. Elementary steps for proposed mechanism: Cl 2(g) 2Cl (g) Cl (g) + CHCl 3(g) -> HCl (g) + CCl 3(g) Cl (g) + CCl 3(g) -> CCl 4(g) Overall Reaction (#1. Elementary steps must add up to the overall reaction) Unimolecular reaction Bimolecular reaction

#2.Let’s write the rate law for each step: Because elementary reactions occur in one step, its rate law, unlike that for the overall reaction, can be deduced from stoichiometry. Cl 2(g) 2Cl (g) (fast) Cl (g) + CHCl 3(g) -> HCl (g) + CCl 3(g) (slow) Cl (g) + CCl 3(g) -> CCl 4(g) (fast) Cl 2(g) + CHCl 3(g) -> HCl (g) + CCl 4(g) Overall Reaction Which step will limit the rate of the reaction, rate-determining step?

Observed rate law is: Rate = k [Cl 2 ] 1/2 [CHCl 3 ] #3. Is this mechanism consistent with the observed rate law? Cl 2(g) 2Cl (g) (fast) Cl (g) + CHCl 3(g) -> HCl (g) + CCl 3(g) (slow) Cl (g) + CCl 3(g) -> CCl 4(g) (slow) Cl 2(g) + CHCl 3(g) -> HCl (g) + CCl 4(g) Overall Reaction

The proposed mechanism for a reaction is 1.A + B X (fast) 2.X + C -> Y (slow) 3.Y -> D (fast) a.What is the overall equation? b.Identify the intermediates, if any. c.What are the molecularity and rate law for each step? d.Is the mechanism consistent with the actual rate law: Rate =k[A][B][C]? e. If the following one-step mechanism equally valid? Likely? A + B + C-> D

How many elementary steps are there in this reaction? Which step is the slow (rate determining step)?

Use your text and know Homogeneous vs. heterogeneous catalyst Enzyme Active site Substrate