1. Kinetics Class #4 OB: reactions that are in dynamic equilibrium and how to “push” them forward, or reverse using LeChatelier's Principle.

2. Most chemical reactions are “one way”, meaning that once they happen, they are done. Spontaneous reversals in chemistry are not common, because most reactions are without enough energy to go the other way. The Potential Energy diagrams show us the huge energy demand it would take to go backwards. Usually the energy required is “lost” to the environment. Some reactions are reversible because the energy requirements are much less, for a variety of reasons. One of the most important reactions that is reversible, is the simultaneous synthesis of ammonia from nitrogen and hydrogen, and the decomposition of ammonia into nitrogen and hydrogen, shown below… N 2 + 3H 2 2NH 3

3. When a reaction is reversible (easily) and you let it happen, you will end up with both synthesis and decomposition. Both a forward and a reverse reaction will both happen, and the “system” will reach a dynamic equilibrium. If enough synthesis occurs, and enough ammonia forms, then it will start to decompose faster. That would create excess nitrogen and hydrogen, setting off a push to synthesize. Over time, the rate of the forward reaction is equal to the rate of the reverse. This is dynamic equilibrium. NOTE: In a dynamic equilibrium situation, you do NOT necessarily have equal masses, or equal moles on both sides of the reaction, but you do have equal rates of forward and reverse reactions.

4. If you combine nitrogen gas and hydrogen gas (in a closed system) together, they will form into ammonia (and release some energy. As soon as some ammonia forms, it will decompose to nitrogen and hydrogen gases. It will synthesize into ammonia again, then it decompose again. Depending upon the conditions that you start with, of pressure and temperature, and of initial concentrations of gases, a balance of nitrogen, hydrogen and ammonia will form. This is a dynamic equilibrium.

5. nitrogen and hydrogen ammonia and energy

6. nitrogen and hydrogen ammonia and energy The rate of the forward reaction (synthesis) equals the rate of the reverse reaction (decomposition).

7. nitrogen and hydrogen ammonia and energy What happens if we pump in a bunch of ammonia gas to this closed system?

8. ammonia, ammonia, ammonia, ammonia, + energy nitrogen and hydrogen If this situation happens, a lot more decomposition will happen because there is so much ammonia, the reverse reaction will push backwards until a new balance is reached.

9. Professor LeChatelier gave us what is now known as LeChatelier's Principle: A chemical system in dynamic equilibrium will stay at equilibrium, and if a chemical stress is applied, the system will shift to counter act this stress, until a new equilibrium is reached.

10. The chemical stresses that could be applied are limited to these: Change in pressure Change in temperature Add reactants Remove reactants Since the reactions are reversible the word “reactants” refers to reactants or products.

11. N 2 + 3H 2 2NH 3 + energy This closed system is in dynamic equilibrium. Let’s apply some stresses, and see which way the system will “push” to create a new dynamic equilibrium. Add nitrogen Add hydrogen Add ammonia Add energy (heat) Add pressure

12. N 2 + 3H 2 2NH 3 + energy This closed system is in dynamic equilibrium. Let’s apply some stresses, and see which way the system will “push” to create a new dynamic equilibrium. Add nitrogen Add hydrogen Add ammonia Add energy (heat) Add pressure

14. N 2 + 3H 2 2NH 3 + energy This closed system is in dynamic equilibrium. Let’s apply some stresses, and see which way the system will “push” to create a new dynamic equilibrium. Remove nitrogen Remove hydrogen Remove ammonia Remove energy (cool system) Lower pressure

15. N 2 + 3H 2 2NH 3 + energy This closed system is in dynamic equilibrium. Let’s apply some stresses, and see which way the system will “push” to create a new dynamic equilibrium. Remove nitrogen Remove hydrogen Remove ammonia Remove energy (cool system) Lower pressure

16. 4Al + 3O 2 2Al 2 O 3 + Energy Here, the forward reaction is exothermic, the reverse is endothermic. The forward reaction is synthesis, the reverse is decomposition. This reaction is at dynamic equilibrium. We cannot know how much aluminum, oxygen, or aluminum oxide is on hand, but we do see that the rate of synthesis (forward) is equal to the rate of the reverse (decomposition). The reactions continue, but the amounts remain constant. Let’s stress this out now… Add aluminum oxide Remove oxygen Remove heat (cool system) Add aluminum Heat system Increase pressure

17. 4Al + 3O 2 2Al 2 O 3 + Energy Let’s stress this out now… Add aluminum oxide Remove oxygen Remove heat (cool system) Add aluminum Heat system Increase pressure

18. A couple of things to pay attention to… Pressure only has an effect on gases, it has NO effect on solids, liquids, or aqueous solutions. If a stress “stops” a forward reaction, the reverse keeps happening. If a stress “stops” a reverse reaction, the forward keeps happening. At least for a while, until a new balance is reached. Sometimes we “play” in chemistry, and use a reaction that is not so easily reversed with LeChatelier. If the double arrows are there, the reaction does go both ways.

19. CH 4(G) + 2O 2(G) CO 2(G) + 2H 2 O (G) + energy Add methane Add water Add heat Remove carbon dioxide Add heat Remove methane Remove carbon dioxide Increase pressure Decrease pressure

20. CH 4(G) + 2O 2(G) CO 2(G) + 2H 2 O (G) + energy Add methane Add water Add heat Remove carbon dioxide Add heat Remove methane Remove carbon dioxide X Increase pressure X X Decrease pressure X Both sides have equal moles of gases, pressure is steady