1. 7.4 Predicting the Direction of a Reaction
Equilibrium Constant (Kc) and the Reaction Quotient (Qc)
Le Châtelier’s Principle
SCH4U – Grade 12 Chemistry, University Preparation
Ms. Papaiconomou & Ms. Lorenowicz

2. We learned in previous lessons that…
Law of Chemical Equilibrium
At equilibrium there is a constant ratio between the concentration of reactants and products in any change.
Using ICE charts and the equilibrium equation we can solve for the:
Concentrations at: I = initial, C = change, E = equilibrium
Equilibrium constant
Initial or Equilibrium concentrations of chemical species in a reaction
May need to use the quadratic equation to solve problems.
There may be more than one solution, but only one of the solutions will make sense.
The equilibrium constant (K) describes the extent of a reaction.
K > 1 means equilibrium favours the products (more products made than reactants)
K = 1 means at equilibrium there are equal concentrations of products & reactants.
K < 1 means equilibrium favours the reactants (less products made than reactants)

3. Reaction Quotient, Qc How do you know a reaction is at equilibrium?
How can you predict the direction in which the reaction must proceed to reach equilibrium?
The reaction quotient (Qc) is an expression that is identical to the equilibrium constant expression, but its value is calculated using concentrations that are not necessarily those at equilibrium.

4. Reaction Quotient (Qc)
Q < K  denominator is large
system attains equilibrium by moving to the right, move towards making products
Q = K  system at equilibrium
Q > K  numerator is large
system attains equilibrium by moving to the left, move towards making reactants

5. What happens if we change the concentration of a chemical entity when a reaction is at equilibrium?

6. Le Châtelier’s Principle
A dynamic equilibrium tends to respond so as to relieve the effect of any change in the conditions that affect the equilibrium.
Predicts the way that an equilibrium system responds to change
Involves:
Initial equilibrium system
Shifting “non-equilibrium” state
New equilibrium state
Henry-Louis Le Châtelier, , a French chemist and engineer

7. Change concentration of Reactants
Increase concentration of reactants
Decrease concentration of reactants

8. Change concentration of Products
Increase concentration of products
Decrease concentration of products

9. Effect of Ions in Aqueous Equilibrium Systems
Common ion effect
Involves adding an ion to a solution in which the ion is already present in solution (concentration effect)
Equilibrium shifts away from the added ion

10. Effect of Temperature on Equilibrium
The value of the equilibrium constant changes with temperature, because the forward and reverse reactions are affected
Le Châtelier’s Principle still holds.
The sign of the enthalpy change for the reaction is important.
Endothermic Change (ΔH >0)
↑ temperature, shift equilibrium to right, forming products & Kc increases
↓ temperature, shift equilibrium to left, forming reactants, & Kc decreases
Exothermic Change (ΔH <0)
↑ temperature, shift equilibrium to left, forming reactants & Kc decreases
↓ temperature, shift equilibrium to right, forming products, & Kc increases

12. Effect of Volume & Pressure Changes on Equilibrium
Pressure and Volume are inversely related at a constant temperature.
Pressure is caused by gas molecules striking the walls of a container, so you change pressure by changing the number of gas molecules.
if ↓volume, ↑pressure so, shift to more gas molecules
if ↑volume, ↓pressure so, shift to less gas molecules
What if same number of gas molecules on both sides of equation?
Volume & pressure have no effect on the position of equilibrium.

14. Changes that do not affect the position of Equilibrium Systems
Catalyst
A catalyst speeds up the rate of a reaction.
Overall effect is to lower the activation energy, which increases the rate of reaction, of both the forward and reverse reactions.
Catalysts do not affect the position of equilibrium, only the time taken.

15. Changes that do not affect the position of Equilibrium Systems
Inert Gases (N2 & Noble Gases)
Inert gases do not react with other gases. This will not be part of the equilibrium system.
The equilibrium position of the system will not change.
The presence of the inert gas changes the probability of successful collisions for both the reactants and products equally, resulting in no shift in the equilibrium system.

17. Chickens and Equilibrium