1. Living By Chemistry SECOND EDITION
Unit 6: SHOWTIME
Reversible Reactions and Chemical Equilibrium

2. Lesson 122: How Pushy
Le Châtelier’s Principle

3. ChemCatalyst FeSCN2+(aq) Fe3+(aq) + SCN–(aq)
Imagine that you have a test tube with a solution of iron (III) thiocyanate ions FeSCN2+. The solution is an equilibrium mixture in which FeSCN2+ dissociates to iron (III) ions, Fe3+, and thiocyanate ions, SCN–.
FeSCN2+(aq) Fe3+(aq) + SCN–(aq)
red yellow K= 0.025
Write the equilibrium constant equation for the equilibrium mixture.
If [Fe3+] M and [SCN-] M, determine the concentration of [FeSCN2+ ].
Explain why the equilibrium mixture is orange.

4. Key Question
What happens to a system at equilibrium when conditions change?

5. You will be able to:
explain how a reversible reaction will shift in response to certain stresses
articulate Le Châtelier’s principle

6. Prepare for the Lab
Work in pairs.

7. Discussion Notes (cont.)
Changing the conditions for a system at equilibrium is often referred to as “stressing” the system.
When a system at equilibrium is stressed, it responds by reducing the effect of the change.
Le Châtelier’s principle: When conditions change for a system at equilibrium, the system will respond by reducing the effect of the change.

8. Discussion Notes (cont.)
If more of one of the products is added to the equilibrium mixture of starting substances and products, this momentarily increases the rate of the reverse process. The concentrations in the mixture adjust until the rates of the forward and reverse processes are equal.
This adjustment results in a new equilibrium mixture that has more starting substance compared with that of the initial solution. The value of the equilibrium constant K is the same for both the adjusted mixture and the initial solution.
You can also change the concentration of gases by altering the volume to change the pressure.

9. Discussion Notes (cont.)
A second change is to raise or lower the solution’s temperature, which also changes the composition of the equilibrium mixture of starting substances and products.
In this case, the changes in composition result in a new value of K depending on if the forward process is exothermic or endothermic.

10. Discussion Notes (cont.)
Because there is a new value of K, the concentrations of starting substances and products change to satisfy the new equilibrium constant equation.
If you transfer heat to an endothermic reaction, more product will form to absorb the added energy. If you transfer heat to an exo-thermic reaction, more starting sub-stance will form to remove the added energy. This is referred to as Le Châtelier’s principle.

11. Wrap Up
What happens to a system at equilibrium when conditions change?
When conditions change for a system at equilibrium, the system will respond by reducing the effect of the change. This is known as Le Châtelier’s principle.
The condition that changes may be temperature, concentration, or pressure. The last two do not change the value of K, but a change in temperature does.
The exact nature of the shift in a reversible reaction is predictable and depends on the type of change encountered by the system.

12. Check-In
Imagine you have an orange solution of M FeSCN2+ (aq). You add Ag+(aq) to the solution. This causes the precipitation of AgSCN(s), thereby removing SCN-(aq) from the solution. Use Le Châtelier’s principle to predict the color change.

13. Discussion Notes (cont.)
Because there is a new value of K, the concentrations of starting substances and products change to satisfy the new equilibrium constant equation.
If the forward process is exothermic (heat is released), the value of K gets smaller as the temperature is raised.
Likewise, if the forward process is endothermic (heat is required), the value of K gets larger as the temperature is raised.