Catalyst

Diet Coke and Mentos http://www.youtube.com/watch?v=LjbJELjLgZg

Justify – TPS Why do smooth coated Mentos have a much slower reaction than rough coated Mentos?

Lecture 6.6 – Molecularity, Mechanisms, Rate Limiting Steps, and Catalysts

Today’s Learning Targets LT 6.16 – I can hypothesize about the moleculariry of a elementary reaction. LT 6.17 – I can determine the rate law for a multistep reaction and discuss the relationship between the fast and slow steps for the reaction. LT 6.18 – I can compare and contrast homogeneous and heterogeneous catalysts. Furthermore, I can identify a catalyst for a multi-step reaction.

Reaction Mechanisms The steps that a reaction takes in order to occur is the reaction mechanism These are the steps of bonds breaking, atoms rearranging, and new bonds forming

Elementary Reactions Reactions that occur in a single step are elementary reactions The number of molecules that participate in elementary reactions is known as the molecularity of the reaction. Unimolecular – Single molecule involved Bimolecular – Collision of two molecules required Termolecular – Collision of three molecules required Molecularity is never higher than 3 because the odds of 4 molecules colliding all at the same time and in the correct orientation is highly unlikely.

Multistep Mechanisms Most chemical reactions require multiple steps in order to occur For example the reaction: NO2 + CO  NO + CO2 Requires 2 steps in order to occur: NO2 + NO2  NO3 + NO NO3 + CO  NO2 + CO2 They must create an intermediate, those molecules that are produces and then consumed before reaction completion, before producing the final product Insert Figure 14.20

Rate Laws and elementary Reactions Every reaction is made up of one or more elementary reaction If the reaction is elementary, then its rate law is based on molecularity For example, if the reaction is elementary and it is of the form: A  Products Then the rate law is simply: Rate = k [A] Show for different types of reactions!

The Big Football Game! Super Bowl This Way!

Rate Limiting Steps The majority of reactions require 2 or more elementary reactions. Each step has its own rate constant and activation energy The overall rate of the reaction can not exceed the rate of the slowest elementary reaction. The slow step is the rate – determining step The rate – limiting step greatly inhibits mechanism and rate law.

Mechanisms with Slow Initial Step For the multistep reaction previously discussed: The first step is the rate limiting step, so k2 >> k1 Therefore, we can only use the rate constant and reaction from the slow first step. Therefore, the rete law is:

Table Talk The decomposition of nitrous oxide, N2O, is believed to occur by a two step mechanism. What is the rate law using the mechanism below?

Mechanisms with Fast Initial Step Sometimes the first step is not the slow step. Consider the following reaction: NO + NO + Br2  2 NOBr This occurs through two steps where the second step is the slow step k-1 Because the second step is the rate limiting step, the first step achieves equilibrium. There is a rate of the forward reaction and a rate of the reverse reaction. The second step has a rate law of: Rate = k2[NoBr2][NO] NOBr2 is an intermediate, so it is useless. We need to do something else… We know that rate of forward = rate of reverse, so the rate law of the forward and reverse are: Rate = k-1[NOBr2] Rate = k1[NO][Br2] At equilibrium: k1[NO][Br2] = k-1[NOBr2] [NOBr2] = (k1/k-1)[NO][Br2] This can be inserted into the rate law: Rate = k2(k1/k-1)[NO][Br2][NO]= k[NO]2[Br2]

Table Talk You run the following reaction: NO + O2  NO2 Based on the data the following mechanism is proposed. Determine the rate law using the mechanism below. k-1

Catalysts A catalyst is a substance that speeds up a chemical reaction by lowering the activation energy It provides an alterative pathway by which the reaction can occur ΔG is not impacted A homogeneous catalyst is a catalyst whose phase is the same as the reactants A heterogeneous catalyst is a catalyst whose phase is different from that of the reactants.

White Board Problems

White Board Problems Complete the following table: Determine the rate law for the following mechanism:

White Board Problems 3. What is the intermediate for this mechanism? 4. For the Diet Coke and Mentos experiment, are the Mentos a heterogeneous or homogeneous catalyst? 5. Based on the following diagram, how many intermediates are formed when a catalyst is used? Is it exothermic or endothermic?

Rate Yourself Using your learning target log, rate yourself 1 – 4 on 6.1 and 6.18

Stations Review Station 1 – Station 2 – Station 3 – Station 4 –

Rate Yourself Using your learning target log, rate yourself 1 – 4 on 6.1 and 6.18

Closing Time Read 14.6 and 14.7 Homework: Reactions Mechanisms and Catalysts due Monday/Tuesday Saturday School 11 – 2 this Saturday! Unit 6 Exam next Monday/Tuesday!