Notes On LeChatlier's Principle

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Definition: If you apply a stress to an equilibrium problem, the system will try to get rid of that stress in order to go back to the original equilibrium.

Mr. McLeod addition to definition:

a. System can only make more reactant (run the backward reaction) or more product (run the forward reaction), nothing else.
b. System will never get back to original equilibrium (since you add something in that has matter and energy) but comes to a new equilibrium. This is what the shift means, compare the original equilibrium material (ratio of reactant to product) compared to the new equilibrium material (ratio of reactant to product)

How you answer LeChatlier's Principle problems for Mr. McLeod (see blank answer form below)

1. What is stress
2. When you are reducing stress, do you make more reactant or product (for pressure you need to indicate # of total reactant particles and product particles)
3. Favors the forward or backward reaction (for change in temp must add endo or exo)
4. Shift equil to the Right or Left.
a. Shift equilibrium to the Left means that the new equilibrium position has a higher ratio of reactants to product than in the original equilibrium position.
b. Shift equilibrium to the Right means that the new equilibrium position has a higher ratio of product to reactant than in the original equilibrium.

For AP students (how actual amount of material change or is it actual concentrations of reactant to product or the ratio of reactant to product for original to new equilibrium position??):

Here is a blank answer form you will fill out for each LeChatelier's Principle problem.

Stress: _______________________________________________________________

How rxn tries to remove stress: _________________________________________________

Circle: Make more product or more reactant (fill in for pressure changes)__________________

Circle: Favoring: Forward rxn or Backward rxn: (fill in for temperature changes) _____________

Circle: Shift Equilibrium to: Left(more reactant) or Right (more product)

There are 4 Types of Stresses

a. Adding more reactant/products
b. Changing Temperature (really means increase Energy).
c. Pressure change (there are more than one way to do this)
d. Add a catalysts

FYI, only substances or attributes that are found in the reversible reaction can stress the equilibrium system (shift the equilibrium).

Here is a link to a Flash Animation on LeChatelier's Principle,[LeChatlier's Principle Flash Animation]

I will use the following chemical reaction (called the Habor process, 1st process to fix nitrogen to ammonia industrially) for examples of LeChatelier's Principle

1 N_{2 (g)} + 3 H_{2 (g)} <==> 2 NH_{3 (g)} + Energy (i.e. exothermic rxn)

I will use the above blank answer form but will also include explanations of why. When you do your homework , you do not have to include explanations (I will italicize the words) and you will circle some answer that I bolded below (can't circle with computer).

a. Adding / Removing reactant/products:
Stress: Adding product (increasing concentration of products)

How rxn tries to remove stress: Consume or decreasing concentration of product

Comment: Since the stress is added product, the system will try to consume the product to get back to original equilibrium (amount of product)

Circle: Make more product or more reactant (fill in for pressure changes)__________________

Comment: since I am consuming product, I am making more reactant

Circle: Favoring: Forward rxn or Backward rxn: (fill in for temperature changes) _____________

Comment: Favoring reaction means that overall that reaction is used more than the other reaction. Remember since this is equilibrium, both the forward and backward reaction occurs, but in this case overall the backward reaction was done more than the forward reaction

Circle: Shift Equilibrium to: Left(more reactant) or Right (more product)

Comment: Since the new equilibrium position has more reactant than the original equilibrium position, we say the equilibrium is shifted to the Left (side pointing to the reactant).

b. Changing Temperature (since Temperature is not in the reactions, but Energy is so, we will use Energy to represent Temperature).

Stress: Decreasing Temperature (decreasing, consuming or removing Energy

How rxn tries to remove stress:Making Energy (Increasing Temperature)

Comment: Since the stress is decreasing Temperature or removing Energy, the reaction will want to make Energy (to increase Temperature) so the Energy should be the "product".

Circle: Make more product or more reactant (fill in for pressure changes)__________________

Comment: Since reaction wants to make Energy, Energy should be a "product" so it is the product in the forward reaction which we call the product.

Circle: Favoring: Forward rxn or Backward rxn: (fill in for temperature changes): Forward reaction is the Exothermic reaction

<i>Comment: Since we want to make Energy, the forward reaction (which is Exothermic) will occur (overall) more than the reverse reaction.

Circle: Shift Equilibrium to: Left(more reactant) or Right (more product)

Comment: Since we want to make more Energy in the new equilibrium position than the original equilibrium position, the other products will be in greater amount also.

c. Pressure change (Pressure = Number of gas particles per volume)

Comment: In this concept, you can equate pressure of gas (does not work for liquid or solids since they are incompressible) to number of particles per volume. So if you decrease pressure, you are reducing the number of particles per volume (i.e. removing particles).

Reason to Pressure = Number of particles: As you know, only attributes or substance in the reversible reaction shift equilibrium and there is no pressure term in the reaction. However, there are number of particles of each species of reactant and product in the form of coefficients.

Stress: Decreasing Pressure (Decreasing/removing/consuming number of particle

How rxn tries to remove stress: Increase Pressure (Increase/make particle

Circle: Make more product or more reactant (fill in for pressure changes): 2 product particles --> 4 reactant particles

Comment: Remember that the system can only 2 things to get rid of the stress, run the forward reaction or run the backward reaction. (Of course, you know that both reaction will occur before you reach equilibrium but "run" means overall which reaction occurs more). In this reaction, there are 4 reactant particles and 2 product particle (you only consider substances in gas phase). So the forward reaction decreases number of particle since 4 reactant particles become 2 product particles and the backward reaction increases number of particles since 2 product particles become 4 reactant particles. In this case, the system wants to increase particles so you write, "2 product particles --> 4 reactant particles" thereby making more reactants.

Circle: Favoring: Forward rxn or Backward rxn: (fill in for temperature changes) _____________

Circle: Shift Equilibrium to: Left(more reactant) or Right (more product)

Note: What would happen to the above if the number of reactant particles and number of product particles are the same.
Answer: No affect on equilibrium.
So you fill out stress portion and then explain in How rxn tries to remove stress portion that #

d. Add a catalysts

<i>Comment: Two major attributes of a Catalysts are: 1) Changes the rate of reaction (how fast products can be made (by changing the activation energy (E_ac) via changing reaction mechanism) and 2) Does not get consumed in overall chemical reaction.

A Major attributes of Equilibrium is that it does not care about time ( it is time independent). So Catalysts DO NOT SHIFT THE EQUILIBRIUM.

Stress: Adding a catalyst

Comment: For Habor process, there have been several and most are iron containing compound

How rxn tries to remove stress:Catalysts do not affect Equilibrium

Circle: Make more product or more reactant (fill in for pressure changes): N/A

Circle: Favoring: Forward rxn or Backward rxn: (fill in for temperature changes): N/A

Circle: Shift Equilibrium to: Left(more reactant) or Right (more product): N/A

Comment: So after stating in the "How rxn tries to remove stress" line that catalysts do not affect equilibrium, the rest of the "answers" you fill in as N/A (even the one you are circling).