CATALYST

DIET COKE AND MENTOS

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]

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

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 PROBLEMS1. Complete the following table:

2. Determine the rate law for the following mechanism:

WHITE BOARD PROBLEMS3. 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 –

• Station 6 –

• Station 7 –

• Station 8 –

• Station 9 –

• Station 10 –

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!