Chem 2 - Chemical Equilibrium VI: Heterogeneous Equilibria

Chem 2 - Chemical Equilibrium VI: Heterogeneous Equilibria

1. Chemical Equilibrium (Pt. 6) Heterogeneous Equilibria 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. Heterogeneous Equilibria A heterogeneous equilibrium system is one in which more than one phase is present. For instance, and reaction involving both gases and solids would be a heterogeneous system.

3. Heterogeneous Equilibria Calculation of equilibrium concentrations or partial pressures for heterogeneous equilibria is very similar to the process for homogeneous equilibria (already discussed). The essential difference is that the activity for solids and pure liquids in the equilibrium constant expression is always equal to “1”.

4. General Example for Heterogeneous Equilibria For reactions involving more than one phase (solids, liquids, and gases), The relationship between the value of the equilibrium constant K and the concentrations of reactants and products is   “1” to any power is still just “1” 

5. Example: Decomposition of CaCO 3 (s) CaCO 3 (s) in a sealed container at 25 C Let time pass… CaCO 3 (s) CaCO 3 (s) CO 2 ( g) + CaO (s)

6. Example: Decomposition of CaCO 3 (s) Let time pass… CaCO 3 (s) CaCO 3 (s) in a sealed container at 25 C CaCO 3 (s), CaO (s), and CO 2 (g) in a sealed container at 25 C Now, CaCO 3 (s) and CaO (s) CaCO 3 (s) CO 2 ( g) + CaO (s)

7. Example: Decomposition of CaCO 3 (s ) and the Equilibrium Constant Expression The activity for solids and pure liquids is “1”   CaCO 3 (s) CO 2 (g) + CaO (s)

8. What Does This Mean? It doesn’t matter how much CaCO 3 (s) we start with, the partial pressure of CO 2 (g) in the closed container is always the same (at a given T).   CaCO 3 (s) CO 2 (g) + CaO (s)

9. Use the ICE Tables to organize given information and variables for heterogeneous equilibrium calculations. ICE Tables and Heterogeneous Equilibria I C E CaCO 3 (s) CO 2 (g) + CaO (s)

10. Allow CaCO 3 (s) to decompose in a closed container. At a certain temperature, the equilibrium constant is 1.04. Determine the partial pressure of CO 2 once the system reaches equilibrium. Example Calculation CaCO 3 (s) CO 2 (g) + CaO (s)

11. Allow CaCO 3 (s) to decompose in a closed container. At a certain temperature, the equilibrium constant is 1.04. Determine the partial pressure of CO 2 once the system reaches equilibrium. ICE Tables and Heterogeneous Equilibria I C E CaCO 3 (s) CO 2 (g) + CaO (s)

12. Notice: We don’t specify “how much” of each solid is decomposed or formed. ICE Tables and Heterogeneous Equilibria I C E CaCO 3 (s) CO 2 (g) + CaO (s) +x used some made some

13. ICE Tables and Heterogeneous Equilibria CaCO 3 (s) CO 2 (g) + CaO (s) I C E +x used some       x made some  

14. A system has to have at least “some” of everything at equilibrium. As long as we have a little of each solid, the system has the same equilibrium partial pressure for CO 2 (g). Central Concept CaCO 3 (s) CO 2 (g) + CaO (s)

15. If a reaction has to have at least “some” of everything at equilibrium, would it matter if we added more CaCO 3 ? (Assume no effect on volume.) Would the partial pressure for CO 2 (g) change? NO! Central Concept- Adding Solids (or Pure Liquids) CaCO 3 (s) CO 2 (g) + CaO (s)  

16. Next up, The Reaction Quotient Q for Nonequilibrium Systems (Pt 7)