1. Chemical Equilibrium

2. The Concept of Equilibrium
No chemical reaction occurs in a single direction
To a certain point, every reaction is reversible, i.e., the products react to reform the reactants
When the speeds of the forward and reverse reactions are equal, and the concentrations of reactants and products no longer change over time, chemical equilibrium is established
Chemical equilibrium is a dynamic process in that the forward and reverse reactions continue, but at identical ratess

3. The Concept of Equilibrium
A physical equilibrium is an equilibrium that involves a single substance
ex.;
A chemical equilibrium is an equilibrium that involves two or more substances

4. The Equilibrium Constant
The equilibrium constant, K, defines the concentrations/pressures of the reactants and products at equilibrium
The equilibrium constant does not have units
For the reaction
Where ax = 1 for a solid or a liquid ax = Px (in atm) for a gas ax = [X] (in mol/L, or M) for a solute
This definition of K is the only one that is always valid with DGo = -RT ln K

5. The Equilibrium Constant
Because ax= 1 for solids and liquids, we can say that ax (the activity) of a solid or liquid does not appear in the expression for K

6. The Equilibrium Constant
When giving an equilibrium constant, it must be accompanied by the corresponding balanced equation, for example
The value of K varies with temperature, so we must also specify the temperature
If K > 1, the equilibrium favours the products (i.e., towards the right)
If K < 1, the equilibrium favours the reactants (i.e., towards the left)

7. The Equilibrium Constant
Example: The equilibrium constant, K, for the reaction is 158 at 1000 K. Calculate PO2 if PNO2 = atm ant PNO = atm.
Solution:

8. Predicting the direction of a reaction
The reaction quotient, Q, is the value obtained when using the actual (non-equilibrium) activities (pressures/concentrations) within the formula of the equilibrium constant
If Q > K, there is too much of the products, so a certain amount of the products must be converted back to reactants; the reaction will shift to the left
If Q < K, there is too much of the reactants, so a certain amount of the reactants must be converted to products; the reaction will shift to the right
If Q = K, the system is at equilibrium; the system does not shift in either direction

9. The Calculation of Concentrations at Equilibrium
If we know the equilibrium constant for a given reaction, one can calculate the concentrations in the equilibrium mixture from the initial concentrations
ex.; Consider the reaction (in an organic solvent, at 200oC, K = 24.0): cis-stilbene trans-stilbene
What are the concentrations of the reactants and products if at the beginning we have mol/L of cis-stilbene (and no trans-stilbene)?

10. The Calculation of Concentrations at Equilibrium
According to the reaction’s stoichiometry, at equilibrium, [trans-stilbene] = x mol/L and [cis-stilbene] = (0.850-x) mol/L
i.e., for each molecule of trans-stilbene produced, we must lose one molecule of cis-stilbene
At equilibrium,

11. The Calculation of Concentrations at Equilibrium
In general:
Express the equilibrium concentrations of all species using the initial concentrations and a single unknown (x) which represents a change in the concentration of one of the species
Express the equilibrium constant in terms of the equilibrium concentrations
The value of the equilibrium constant is known, solve for x
After determining the value of x, compute the equilibrium concentrations of all species

12. The Calculation of Concentrations at Equilibrium
Example: We are mixing H2(g) and I2(g) in a container. The partial pressures are atm and atm, respectively. For the reaction H2(g) + I2(g)  2 HI(g) K = 54.3 at 430oC. At equilibrium, what are the partial pressures of H2(g), I2(g), and HI(g)?

13. The Calculation of Concentrations at Equilibrium
We start with pure A(s) and establish an equilibrium: A(s) ↔ 2 B(g) + 3 C(g). If the pressure of B(g) at equilibrium is atm, calculate the value of ΔGo for this reaction.

14. Le Chatelier’s Principle
Any change in the conditions can shift the equilibrium to the left (reactants) or to the right (products)
Le Chatelier’s principle says that if a system at equilibrium is disturbed, the system reacts to counteract this disturbance
A disturbance may be a change in the concentration of a reactant or product, pressure, volume, temperature, …

15. Le Chatelier’s Principle: Changes in Pressure and Volume
Example: Consider the reaction 2 NOCl(g) NO(g) + Cl2(g). Predict in which direction the net reaction will proceed if there is a decrease in pressure (resulting from an increase in volume) of the system at constant temperature.
Solution: If the pressure is reduced, Le Chatelier’s principle says the system will try to combat this effect by increasing the pressure. It can do this by increasing the total number of moles of gas by shifting the equilibrium towards the products.
N.B. An increase of the total pressure which is accomplished by adding a spectator gas (i.e., one that is not involved in the reaction) has no effect on the partial pressures of reactants and products. It has no effect on the equilibrium position.

16. Le Chatelier’s Principle: Changes in Temperature
Changes in concentrations and pressures do not have an effect on the value of K
However, when temperature changes, the value of K changes
To predict the effect of temperature on the equilibrium position, consider the heat as a reactant or product
For an endothermic reaction, heat is a “reactant”
For an exothermic reaction, heat is a “product”

17. Le Chatelier’s Principle: Changes in Temperature
According to Le Chatelier’s Principle
For an endothermic reaction:
An increase in temperature will move the position of equilibrium towards the products
A decrease in temperature will move the equilibrium position towards the reactants
For an exothermic reaction:
An increase in temperature will move the position of equilibrium towards the reactants
A decrease in temperature will move the equilibrium position towards the products

18. Le Chatelier’s Principle: Catalysts
A catalyst increases the rate of both the forward and inverse reactions by lowering the energetic barrier between the reactants and products (we will learn more about this in the section on chemical kinetics)
A catalyst does not have an influence on the composition of the system at equilibrium
A catalyst merely helps the system reach equilibrium faster

19. Le Chatelier’s Principle
Example: Consider the reaction What would be the effect on the system if: (a) Pressure was increased by a decrease in volume? (b) Pressure was increased by the addition of O2(g)? (c) Temperature was decreased? (d) A catalyst was added?
Solution: (a) Equilibrium would be displaced towards the products (b) Equilibrium would be displaced towards the products (c) Equilibrium would be displaced towards the reactants (d) No effect on the equilibrium position