Unit Plan : Chemical Equilibrium and Solubility Shannon B. Carpenter TE 40 04/03/01 This unit plan is intended to cover two weeks and would be appropriate for an introductory and/or advanced placement chemistry high school class. This unit introduces them to equilibrium and solubility of a solution, a topic that they have not yet been introduced to. The prerequisites for this class are that students must be able to balance basic reactions, be able to calculate the molarity (concentration) of a solution and understand the properties of solutions. Central Questions: What processes in our body or nature undergo some sort of equilibrium? How could you relate the concept of equilibrium to a game of tug-of-war? What is hard water and how can it be softened? Content: One data/example of this unit plan is reversible reactions, because only reversible reactions can undergo a state of equilibrium. Natural equilibrium systems such as the beating of your heart and the relationship between predator and prey in an environment are observations and examples that can be seen in the real world. Some other examples for the solubility section of the unit plan are the concepts of precipitation of ions, and its applications such as iron concentrations in drinking water and soap scum in hard water. Some patterns of this unit are the concentration of solutions and how alterations of the system such as temperature or pressure affect reaction rates. Another pattern of the solubility section would be the concept of like dissolves like. Main theories of the unit include Law of Chemical Equilibrium and Le Chatelier s Principle. The Law of Chemical Equilibrium states that every reversible reaction proceeds to an equilibrium state that can be described by a specific equilibrium constant. Le Chatelier s Principle states that a reversible reaction at equilibrium will shift to offset a stress, or change in conditions, imposed on a system. Science Statements: 1. All processes in nature want to undergo a state of equilibrium.. Equilibrium can only occur in reversible reactions. 3. If the conditions (i.e. pressure, temperature, amount of a substance) of the system are altered at equilibrium, the equilibrium position will shift in the direction that tends to reduce that change in conditions. 4. Like dissolves like. 5. The process in which ions leave a solution and regenerate an ionic solid is called precipitation. Objectives: * All objectives with an asterisk are ones that are either constructing or reflecting. Source for all objectives: Prentice Hall-Chemistry. 1. Describe a reversible reaction and define chemical equilibrium. Knowledge. Determine the equilibrium constant for a given reaction. Knowledge
3. Analyze the extent of a reaction from its equilibrium constant. Applying Knowledge 4. Explain Le Chatelier s Principle and be able to describe how changes in concentration, pressure, and temperature affect a reaction at equilibrium. Knowledge 5. Relate the Haber process to Le Chatelier s Principle. Applying Knowledge 6. Compare and define dissolution and precipitation of a solution. Knowledge 7. Explain how the solubility product can be used to predict if a precipitate will form when two aqueous solutions are mixed. Applying Knowledge 8. Be able to write net ionic equations to describe the formation of an aqueous solution. Knowledge 9. *Construct an investigation of how cobalt (II) chloride is used to predict the weather. Diagnose possible explanations for what you discover. Developing Understanding, Constructing Objective 10. *Discuss the hazards of lead including the modern sources of lead, the role it may have played in ancient history, and the role it plays today. Developing Understanding, Reflecting Objective Activities: 1. Demo-Equilibrium Demonstration Using Aquariums-Demonstrates when a reaction reaches equilibrium, as well as reinforcing the characteristics of a system at equilibrium. The demonstration attempts to eliminate the misconception that, when a system is in equilibrium, the two sides of the reaction are the same. It emphasizes that the equilibrium is reached when the rate of the forward reaction equals the rate of the reverse reaction. Two Volunteer students will each use a test tube to remove water, which has been colored by food coloring, from his/her tank and place it in the other person's tank at a 1:1 ratio. The volunteers can keep doing this until asked to stop (reached equilibrium). Questions: What is equilibrium? What is happening to the amount of water in the two tanks? What would happen to the equilibrium if we gave one person a larger test tube? Source: http://educ.queensu.ca/~science/main/concept/chem/c05/c05desu1.html (Obj. 1 and 4) Knowledge, Modeling. Laboratory Investigation-The Color of Heat- Explore a reversible reaction in liquid crystal observing the color changes and temperature of the air surrounding the crystal. Questions: What happens to the equilibrium position as more heat is added? What happens when heat is removed? Relate the concepts of this laboratory exercise to mood rings and explain if they do indicate how a person is feeling. Source: Prentice Hall-Chemistry. (Obj. 4) Knowledge, Modeling 3. *Worksheet-Equilibrium in a Bottle-Examine various pictures (A-E) showing the molecules per unit volume over a 0-minuite period in a mock experiment. Sample Questions: In which of the pictures A-E is the system at equilibrium? How can you tell? How would the reaction have proceeded if Alejandro had started with only A B in the bottle? Source: Prentice Hall-Chemistry. (Obj.4) Knowledge, Coaching 4. *Journal-Enrich-Economic Equilibria-Relate Le Chatelier s Principle to a simple economic system. For example, how would the economic system react if a new manufacturing
technology cut the cost of producing a compact disc in half? Prentice Hall-Chemistry. (Obj. 4) Application, Coaching 5. *Laboratory Investigation-Le Chatelier s Principle Using Cobalt Chloride and Water- Students will experimentally vary the concentration of water, Cl - ions and temperature to see its affects and then try to reverse these affects to get the solution back to its original properties. Source: Tom Dubyne, Haslett High School Retired Teacher. (Obj. 4,9) Knowledge, Coaching/Fading 6. *Laboratory Exercise-Solubility Rules-Students will survey a large number of combinations of ions simply by mixing solutions of various ions and observing what happens (i.e. does a precipitate form). From these results, students will derive the main solubility rules by making connections between the observed results and the ion s placement in the periodic table. Source: http://www.onlink.net/~bernas/solubility.htm. (Obj. 6 and 7) Applied Knowledge, Coaching. 7. Lecture-There will be a formal one-day lecture focusing on teaching the students how to write net ionic equations and how to use the solubility rules to predict whether or not when two solutions are mixed if a precipitate will form or if they will be soluble in each other. (Obj. 7) Knowledge, Modeling 8. *Laboratory Exercise-Soap vs. Detergents-Students will investigate the chemical action of soap vs. detergents in hard water and the use of a precipitation reaction to soften hard water. Source: http://www.cci.unl.edu/chemistry/labs/labs1.html#introduction. (Obj. 7) Applied Knowledge, Modeling/Coaching 9. Journal-Students will write a magazine article describing the hazards of lead including the modern sources of lead, the role it may have played in ancient history (Fall of the Roman Empire), and the role it plays today. Make sure they also include the chemistry behind their arguments. (Obj. 6 and 7) Developing Understanding, Fade 10. *Homework Set-Students will be given a homework set covering these two objectives (Obj.4 and 7). See attached sheet. Knowledge, Fade Evaluation Plan: Some of my assessment will come from the activities that are asterisked in the above activity list. My formal assessment will be based on about 3 reading pop-quizzes covering main concepts and terminology (multiple choice) and 1 exam covering the two chapters which will be all short answer with some extra credit possibilities. Sample Test Questions: Teeth and bones are composed mainly of calcium phosphate, Ca 3 (PO 4 ) (s). Predict the direction of the reaction and possible outcomes if the following changes are made to an aqueous solution of calcium phosphate: increase calcium ions, decrease calcium ions, phosphate ions decrease, phosphate ions increase, more calcium phosphate is added. A local group believes that a lake is polluted with lead ions. The company accused of polluting the lake is denying any such pollution. How could you test for the ions?
Sample Homework Set 1. Explain the process by which an ionic compound attains equilibrium in an aqueous solution.. Predict whether each change would favor the forward or reverse reaction in the following reaction: a) Adding HI b) Adding H c) Removing I d) Removing HI HI ( g) H ( g) + I ( g) 3. At equilibrium, what effect would decreasing the pressure have on this reaction: Explain your answer. g NO( g) + O ( g) NO ( )? 4. What type of reaction is a precipitation reaction? Describe what happens to the ions during a precipitation reaction. 5. Use the solubility rules to determine whether or not the following are soluble or insoluble: a) Na SO 4, b) HgCl, c) Al(NO 3 ) 3, d) PbS and e) MgCO 3
6. Determine whether a precipitate will form when 70.0 ml of a 0.0040 M Ca(NO 3 ) solution is mixed with 30.0 ml of a 0.010M Na SO 4 solution at 5 C.