Chem 2 - Chemical Equilibrium IX: Le Chatelier's Principle and Pressure - Volume Changes

Chem 2 - Chemical Equilibrium IX: Le Chatelier's Principle and Pressure - Volume Changes

1. Chemical Equilibrium (Pt. 9) Le Chatelier’s Principle and Pressure-Volume Changes By Shawn P. Shields, Ph.D. This work is licensed by Dr. Shawn P. Shields-Maxwell under a Creative Commons Attribution- NonCommercial - ShareAlike 4.0 International License .

2. Recall: “Stressing” a Chemical System Chemical reactions at equilibrium like to stay at equilibrium. Any factor that disrupts the equilibrium state is considered a “stress.”

3. Stresses and Le Chatelier’s Principle A stress is some change to the system, which can include Adding/removing product or reactant Changing the pressure on a system Changing the volume of a system Changing the temperature

4. Le Chatelier’s Principle When a chemical reaction at equilibrium is stressed, it will shift the equilibrium point in the direction that counteracts the stress applied.

5. Increase the pressure on the system… What happens to the volume (at const T) … Will the equilibrium shift? 2 NO 2 (g) N 2 O 4 (g) Example Problem: Increasing the Pressure on the System PV = nRT  P  V = constant

6. When the volume is reduced (or the external pressure is increased) , the equilibrium shifts in the direction of the smaller number of moles of gas. The system tries to “reduce the pressure” inside the container. Moles of Gas and Pressure/Volume Changes 2 NO 2 (g) N 2 O 4 (g)

7. Now let’s add an inert gas (N 2 ) to the reaction flask at constant volume (and T) … What happens? Will the equilibrium shift? 2 NO 2 (g) N 2 O 4 (g) Example Problem: Adding N 2 (g)

8. Add inert gas (N 2 ) at constant volume (and T) … The equilibrium does NOT shift! Why? Constant V (and T) Example Problem 2 NO 2 (g) N 2 O 4 (g)

9. The total pressure in the flask has increased, but the partial pressure of NO 2 and N 2 O 4 remains the same. Recall: Dalton’s Law of Partial Pressures   2 NO 2 (g) N 2 O 4 (g)

10. Now let’s add an inert gas (N 2 ) at constant pressure (and T) and compare… What happens? Will the equilibrium shift? Compare: Add N 2 (g) at Constant (External) Pressure 2 NO 2 (g) N 2 O 4 (g)

11. Add an inert gas (N 2 ) at constant pressure and temperature … Hint: If the total pressure is constant , what must be happening to the volume? O_o Add N 2 (g) at Constant (External) Pressure 2 NO 2 (g) N 2 O 4 (g)

12. Add an inert gas (N 2 ) at P and T… The total pressure is constant , so the volume must be expanding . Add N 2 (g) at Constant (External) Pressure 2 NO 2 (g) N 2 O 4 (g)

13. As the volume expands ( const T), the partial pressure of each gas goes down . PV = nRT  P  V = constant Add N 2 (g) at Constant (External) Pressure 2 NO 2 (g) N 2 O 4 (g)

14. When the partial pressure each reacting gases goes down (upon expanding the container), the equilibrium shifts in the direction of the larger number of moles of gas. The system tries to “replace” the lost gas pressure. We Still Don’t Know (yet!) 2 NO 2 (g) N 2 O 4 (g)

15. The equilibrium shifts in the direction of the larger number of moles of gas. So, t he reaction shifts toward the reactants. (2 moles gas versus 1 mole gas) Moles of Gas and Pressure/Volume Changes 2 NO 2 (g) N 2 O 4 (g)

16. Reduce the volume (at const T)… In which direction does the equilibrium shift? Mini Quiz: Moles of Gas and Volume Changes 4 A (g) B (g ) + C (s)

17. the equilibrium shifts in the direction of the smaller number of moles of gas. The equilibrium shifts in the forward direction as it tries to “reduce the pressure” inside the container. Mini Quiz Solution 4 A (g) B (g ) + C (s)

18. Next up, Le Chatelier’s Principle and Temperature Changes ( Pt 10)