Dynamic Equilibrium and Le Chatelier's Principle
Part 1. Dynamic Equilibrium
The word "dynamic" means that a system is in motion. The word "equilibrium" means that the system is stable and does not change. When opposing forces or issues are balanced, a system is said to be in equilibrium. So the question becomes, "How can there be a system that's in motion and in equilibrium at the same time?" If the forward and reverse processes of a reversible process are occurring, then the system is said to be "dynamic." If the forward and reverse reactions are occurring simultaneously at the same rates, then the system is in a state of "equilibrium." Equilibrium exists when the rate of the forward reaction equals the rate of the reverse reaction.
Equilibrium Reaction:
H2 (g) + I2 (g) → 2 HI (g)
Forward Reaction:
H2(g) + I2(g) → 2 HI (g)
Reverse Reaction:
2HI(g) → H2(g) + I2(g)
VERY IMPORTANT: Equilibrium DOES NOT MEAN that there is the same amount of reactants and products. A dynamic equilibrium means that the amounts of reactant and products are staying constant even though the reaction is still going.
MODEL 1a: Dynamic Equilibrium
Acme Manufacturing has been restricted to 100 employees in the building at one time. Throughout the day, twenty employees go on break every hour as twenty other employees return from break.
Questions for Model 1a:
1. How many employees move in and out of the factory building during each hour?
2. Are the employees who move in and out of the building each hour the same people? Explain your answer.
3. Does the number of employees in the building change from hour to hour? Explain your answer.
4. Over the course of a day, the employees in the Acme Manufacturing Plant are said to be in a "dynamic equilibrium." Based on your understanding of how the staff moves in and out of the plant, explain what is meant by the term "dynamic equilibrium."
MODEL 1b: Dynamic Equilibrium
A new faster and simpler check-in/check-out process has been proposed for workers at the Acme Manufacturing Plant. Some workers have said that this new process acts like a catalyst. (A catalyst is a substance that speeds up a chemical reaction without changing the outcome of the reaction and without being used up in the process.)
Questions for Model 1b:
5. Would this new check-in/check-out process change the number of people in the building at any given time? Why or why not?
6. What would be the effect of the new check-in/check-out process on the workers at the factory? Support or refute the idea that the new check-in/check-out process is like a catalyst.
Like the Acme Manufacturing Plant, chemical reactions can also reach equilibrium. Answer the following questions about the chemical equation below by applying the insight you gained from the Acme Manufacturing Plant questions.
Chemical Equilibrium:
2H2(g) + O2(g) → 2H2O(g) + energy (heat)
7. For the given reaction above:
a) What is the forward reaction?
b) What is the reverse reaction?
8. When the reaction between hydrogen and oxygen reaches equilibrium:
a) Does the number of molecules in the reaction container change? Explain.
b) Is the reaction still proceeding in the forward direction?
c) Is the reaction still proceeding in the reverse direction?
d) Are the concentrations of the products and reactants changing?
e) Are the rates of the forward and reverse reactions the same?
f) Does the heat content of the system become constant?
Part 2: Le Chatelier's Principle
Most of the chemical reactions you have observed in the laboratory seem to have gone to completion – that is, all the reactants appear to have been used up to form the products. Actually, all chemical reactions are reversible, even though the extent of reversibility may sometimes be very small. In many reactions that you will encounter in the laboratory, you must concern yourself with their reversibility and the possibilities of controlling the extent of reversibility. In this experiment, you will study some reactions in which there is appreciable reversibility and in which the presence of the reactants and products can be readily observed by noting color changes or the formation of a precipitate.
In an aqueous solution, the chromate ion, CrO42- (aq), can be converted to the dichromate ion, Cr2O72- (aq), and, conversely, the dichromate ion, Cr2O72- (aq) ion can be converted to the chromate ion, CrO42- (aq) ion. The extent to which these reactions occur is dependent upon the concentration of the hydrogen ion, H+ (aq), in the solution. The H+(aq) concentration can be increased by adding a source of H+(aq) - hydrochloric acid, HCl. The H+ (aq) concentration can be decreased by adding a solution of sodium hydroxide, NaOH, which contains the hydroxide ion, OH- (aq). The OH- (aq) reacts with H+ (aq) to form H2O, causing a shift in the equilibrium. The net-ionic equilibrium reactions of potassium chromate and potassium dichromate, respectively, are:
Equation 1: chromate-dichromate (K2CrO4) equilibrium:
2CrO42- + 2H+ ⇌ H2O + Cr2O72- (eq. 1)
*Note that there is predominantly CrO42- ions present with a small amount of Cr2O72- ions.
Equation 2: dichromate-chromate (K2Cr2O7) equilibrium:
Cr2O72- + 2OH- ⇌ H2O + 2CrO42- (eq. 2)
*Note that there is predominantly Cr2O72- ions present with a small amount of Cr2O72- ions.
Le Chatelier's Principle states that "If a stress is applied to a reaction at equilibrium, the reaction will shift to offset the stress applied." The addition of hydrogen ions or hydroxide ions constitutes a stress, as does the removal of either ion, and so it can be expected that the above equilibria will shift in response to concentration changes in these ions. For example, in eq. 1, if [H+] is increased by adding HCl, there are more ions available for reaction and the reaction will favor the formation of products. Conversely, if [H+] is decreased (by adding OH-) there are fewer ions available for the forward reaction and the reaction shifts reverse to favor reactant formation.
Questions for Part 2:
1. a) Explain how the chromate-dichromate equilibrium (equation 1) shifts in response to an increase or decrease in H+ ions. Express this both in words and as a chemical equation.
b) Explain how the chromate-dichromate equilibrium (equation 1) shifts in response to an increase or decrease in OH- ions. Express this both in words and as a chemical equation.
c) Use Le Chatelier's Principle to explain the above color changes.
2. a) Explain how the dichromate-chromate equilibrium (equation 2) shifts in response to an increase or decrease in H+ ions.
b) Explain how the dichromate-chromate equilibrium (equation 2) shifts in response to an increase or decrease in OH- ions.
c) Use Le Chatelier's Principle to explain the color changes.