•Given the rxn: 2NO(g) + O2(g) -> 2NO2(g)

•Express the reaction rate in terms of O2 and NO2.•If O2 is reacting at a rate of 0.066 M/s, at what rate is NO2 being formed?•What can we do to speed up or slow down this reaction?

Reaction Ratecan 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: Br2 + HCOOH -> 2Br- + 2H+ + CO2

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 rateWe 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 RatesRxn: A + 2 B -> 2 C

Rate Law: Rate = k[A]x[B]y

k is the rate law constantWhat is factored into the constant?

Exp Initial [A] Initial [B] Initial Rate

1 0.100 M 0.100 M 2.73 M/s

2 0.150 M 0.100 M 6.14 M/s

3 0.100 M 0.200 M 5.42 M/s

The rate law for the reaction of A + 2B -> C is

rate = 2730/M2s[A]o2[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] = kt

A= -kt + Ao

What is half life?t1/2 = Time for Ao to be halvedt1/2 = [A]o/2

For rxn: A-> products

For rxn: A-> products

1. At 1000oC, cyclobutane decomposes in a first order reaction, with the very high rate constant of 87/s, to two molecules of ethylene (C2H4)a. The initial concentration of C4H8 is 2.00 M. What is the concentration after 0.010 s?b. What fraction of C4H8 has decomposed in this time?2. At 25oC, HI breaks down very slowly to form hydrogen and iodine: rate =k[HI]2. The rate constant at 25oC is 2.4x10-21/Ms. If 0.0100 mole of HI is placed in a 1.0 L container, how long will it take for 10% to react?3. H2O2 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

k = Ae-Ea/RT

To relate k to T, take ln of both sides of equation:

Two point eqn:ln(k2/k1) =Ea/R (1/T1-1/T2)

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:

Cl2(g) <-> 2Cl(g)

Cl(g) + CHCl3(g) -> HCl(g) + CCl3(g)

Cl(g) + CCl3(g) -> CCl4(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.

Cl2(g) <-> 2Cl(g) (fast)

Cl(g) + CHCl3(g) -> HCl(g) + CCl3(g) (slow)

Cl(g) + CCl3(g) -> CCl4(g) (fast)

Cl2(g) + CHCl3(g) -> HCl(g) + CCl4(g) Overall Reaction

Which step will limit the rate of the reaction, rate-determining step?

Observed rate law is: Rate = k [Cl2]1/2[CHCl3]

#3. Is this mechanism consistent with the observed rate law?

Cl2(g) <-> 2Cl(g) (fast)

Cl(g) + CHCl3(g) -> HCl(g) + CCl3(g) (slow)

Cl(g) + CCl3(g) -> CCl4(g) (slow)

Cl2(g) + CHCl3(g) -> HCl(g) + CCl4(g) Overall Reaction

The proposed mechanism for a reaction is1. 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