Chemical EquilibriumChemical Equilibrium Nancy Zipprich D.D. Eisenhower H.S. 201 Indiana St. 12700 Sacramento Park Forest, IL 60466 Blue Island, IL 60406 708-747-4136 708-597-6300 Objectives: The student will: 1. distinguish between reactions that go to completion and those that are reversible. 2. explain the concept of chemical equilibrium. 3. understand how Le Chatelier's Principle works on a chemical reaction at equilibrium. Materials needed: (for teacher demonstration with student participation) matches, "Match Box" car that changes colors water scooping demo: 2 equal sized battery jars (or 2 2000 mL beakers), 2 medium plastic cups, one smaller plastic cup cup demo: 4 or 5 sets of: 2 plastic cups (may or may not be same size), water, 2 eyedroppers (or 2 straws of different sizes), plain water and colored water NO2-N2O4 demo: 3 sealed tubes containing these gases at equilibrium, beaker full of ice, beaker full of boiling water, empty beaker, hotplate cobalt (II) chloride hexahydrate equilibrium: this chemical solution is made by putting .6 g into 20 mL ethanol, safety goggles, apron, test tube, rack and holder, dropper bottles full of: (1) distilled water, (2) .1 M silver nitrate, (3) conc. HCl, beaker full of ice water, beaker full of boiling water Strategy: 1. Demonstrate how a toy (such as hot wheels that change color) will change color in response to different temperatures. Hopefully this will instill curiosity about reversible reactions. 2. Burn a match and get the class to realize this is a reaction that has gone to completion. Explain other reactions that go to completion (ex. complete combustion, rusting, decomposition) and put a sample equation of this type on the board, discussing its one way arrow and arrangement. match + oxygen gives ash + carbon dioxide + water + HEAT 3. Review the placement of energy on the right side as meaning an exothermic or energy releasing reaction and energy on the left as endothermic or an energy absorbing reaction. 4. Perform the water scooping demonstration using two battery jars and either the same size or different size cups to scoop with. The water level starts out filled on one side and empty on the other. Ask the students to make predictions about what will happen. Then after their predictions have been verified or disproven, draw a comparison between this demo and a reversible reaction that has reached chemical equilibrium. No water spilled = a closed system. Equal scooping technique in opposite directions = forward and reverse reactions proceed at the same rate. Eventually the water levels do not change. The levels are not necessarily equal. Define chemical equilibrium (not only are forward and reverse reaction rates equal, but the concentrations of reactants and products eventually become constant at equilibrium). Explain that under VERY specific conditions, most all chemical reaction are reversible (when in a closed system). Chemical equilibria usually occurs in all gaseous or all aqueous systems. (Irwin Talesnick, Idea Bank Collection, Vol I, Idea #1). 5. Have five students come to the front of class and do a similar experiment, but this time using two beakers or transparent cups (do not have to be the same size). One cup will have plain water in it and the other will have colored water. Mark original water levels. Water transfer will be accomplished by using eyedroppers (identical technique of transfer, as in last demo) and the students will tell when they have reached equilibrium and why. (Irwin Talesnick, Idea #284) 6. This will serve as a review of equilibrium learned thus far and perhaps it will come out in discussion that it does not matter how much reactant or product there is to start with, a certain equilibrium will be reached at that particular temperature. 7. Demonstrate a real chemical equilibrium using the NO2-N2O4 gas tubes in both cold and hot water. At this point introduce the concept that a chemical equilibrium's position can be shifted by certain factors. Temperature is one of them. Define Le Chatelier's Principle in relation to this demo. According to Le Chatelier, equilibrium systems can also be stressed so that they shift to relieve this stress. Changes in pressure and changes in concentration of reactant or product are two ways to stress this closed system. (This gas demo is in most H. S. Chemistry texts.) 8. EMPHASIZE that whatever is done to stress a system at equilibrium, the system tries to relieve the stress by doing the exact opposite. 9. Perform the demonstration using the cobalt II chloride dehydrated-hydrated complex. A closed system's equilibrium system can be shifted by changing temperature, changing concentration of reactants and products and changing pressure (Le Chatelier's Principle). In this demo, pressure was not demonstrated as a means to shift equilibrium. The color can be shifted from blue to pink by either adding more pure water or by putting the tube in cold water. The color can be shifted from pink to blue by either adding conc. HCl, putting in some .1 M AgNO3 or by heating the tube. Tie in how the color can be shifted in this demo and ask the class to predict how to get the color to shift after thoroughly reviewing Le Chatelier's Principle. Lee Summerlin, Chemical Demonstrations, American Chemical Society, Washington D.C., 1987. 10. Review the objectives. 11. Ask the class for examples of phenomena or toys that could be examples of equilibria. For homework, ask them to design a "controlled" paper wad fight that would simulate the idea of establishing chemical equilibrium. The best idea will be carried out the next day. This is a great learning experience, but pick a class that you really trust! Return to Chemistry Index