tying up the loose ends in chemical kinetics. reaction mechanisms here is a sample reaction...

Mechanisms, CatalystsIntermediates and k

Tying up the loose ends in chemical kinetics

Reaction mechanismsHere is a sample reaction mechanism

Step 1 ClO- + H2O HOCl + OH-

Step 2 Br- + HOCl HOBr + Cl-

Step 3 OH- + HOBr H2O + BrO-

There are several questions they will ask you about these reaction mechanisms1.What is the overall reaction2. Identify “intermediates” and “catalysts”

Reaction mechanismsThe overall reaction




This is easy, just add up the reactions like you did when you were working on Hess’s law. Cross out all the chemicals that appear on both sides and then add up.

Reaction mechanismsThe overall reaction




ClO- + Br- Cl- + BrO-This is the overall reaction. Not so bad. You don’t even need to flip any equations, or multiply the coefficients. All you do is cancel and add.

Reaction mechanisms“Intermediates” and “catalysts”




ClO- + Br- Cl- + BrO-The “intermediates” are crossed out in black.

They are all the chemicals that cancel out that are not present in on the reactant side of the first step or the product side of the last step.





ClO- + Br- Cl- + BrO-In the reaction above H2O is a “catalyst”. Water

is present on the reactant side of the first step and on the product side of the last step. This makes H2O a catalyst.


Step 1 H2O2 + I- OI- +H2O

Step 2 H2O2 + OI- I- + H2O + O2

1. Write the overall reaction2. Indentify both the catalyst and the

intermediate.

Free energy diagram

As we discussed earlier in thermodynamics, not all spontaneous reaction occur at observable rates.

A reaction that occurs slowly is likely to have a high activation energy (Ea).

What is this called again?

Is it positive or negative?

Free energy diagram

What is the effect of a catalyst?

A catalyst lowers the activation energy!

In this class (i.e. for the AP exam) there are two ways to increase the rate of a reaction.

1. Add a catalyst2. Increase the

temperature.

How to make it fasterIf the temperature of a non-reversible

reaction is increased then the reaction rate will increase.The higher temperature increases the average kinetic energy of the molecules.

Therefore more molecules have sufficient energy to overcome the activation energy (transition state energy).

Catalyst

A catalyst lowers the activation energy!

If the activation energy is lowered then less energy is required to go from reactants to products.

Therefore the reaction will proceed more quickly.

What you will be askedThey will try to trick you!!! (the AP people

that is)Here are the tricks1) The will get you to say ΔG is negative and

that the reaction is spontaneous. They will then ask you to comment on the speed of the reaction.

ANSWER: Rate of reaction is only affected by temperature and activation energy. Therefore the rate of the reaction is completely unaffected by any thermodynamic data you could possibly present me.

What you will be askedHere’s trick number 22) You will be told that concentration,

pressure, or volume has been change or that an inert extra chemical has been added to your system.

ANSWER: Rate of reaction is only affected by temperature and activation energy. Therefore the rate of the reaction is completely unaffected by anything else.

What you will be askedHere’s trick number 33) You will be told that anything other than a

temperature change or the addition of a catalyst has occured.

ANSWER: Rate of reaction is only affected by temperature and activation energy. Therefore the rate of the reaction is completely unaffected. If temperature goes ↑ then k goes ↑, therefore rate ↑. If temperature goes down then k goes down, therefore rate decrease.If a catalyst is added then activation energy goes down, therefore the rate of reaction increases

Order of reaction from mechanismThis is really not hard at all. I’ll show you.

Mechanism

Step 1 H2O2 + I- HOI + OH- (slow)

Step 2 HOI + I- I2 + OH- (fast)

Step 3 2OH- + H3O+ 4H2O (fast)

First, write the overall reaction

H2O2 + 2I- + 2H3O+ I2 + 4H2O


Mechanism




Next, draw a line under the slow step

H2O2 + 2I- + 2H3O+ I2 + 4H2O


Mechanism




Last, add up all the reactants up to the slow step only

H2O2 + I-

This makes: Rate = k [H2O2][I-]

Order of reaction from mechanismWhat if the location of the slow step changes

Mechanism

Step 1 H2O2 + I- HOI + OH- (fast)

Step 2 HOI + I- I2 + OH- (slow)


First, write the overall reaction.

H2O2 + 2I- + 2H3O+ I2 + 4H2O


Mechanism


Step 2 HOI + I- I2 + OH- (slow)


Next, draw a line under the slow step

H2O2 + 2I- + 2H3O+ I2 + 4H2O


Mechanism




Last, add up all the reactants up to the slow step only

H2O2 + 2I-

This makes: Rate = k [H2O2][I-]2

The coefficients become the exponents

You work itWhat if the location of the slow step changes

Mechanism



Step 3 2OH- + H3O+ 4H2O (slow)

Limitless practice, just click the circle below

http://chemistry2.csudh.edu/lecture_help/mechanisms.html

tying up the loose ends in chemical kinetics. reaction mechanisms here is a sample reaction...

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