Week 6 homework 3parts
CHE 105 – Week 6 Formative Assessment. Instructions: • • • • • Each student will work on this assignment alone. Assignments with identical answers will earn grades of zero. If you have questions, you are to ask me, your instructor, not your peers. The assessment will be printed out, completed, scanned, or photographed and uploaded to the upload area. Only assignments uploaded to the class will be graded. You must show ALL work. Submissions with answers only will not be evaluated. Formats: The following formats will be evaluated: • Microsoft Word Note: if you cannot scan your work before submitting, you are welcomed and encouraged to do the following: • • • • Take an image of each page of your work with your phone. Copy/paste those images to a Word document. Make sure the image size is the same as the page. Submit one image per page. Save the file as a Word document. Submitted files in the following formats will NOT be evaluated: • Pages • Lone Jpeg images • Any other file formats *********************************************** The Week 6 Formative Assessment Assignment will test your understanding of colligative properties. Le Chatelier’s Principle Formative Assessment Part 1: Application: This formative assessment involves study of equilibrium and Le Chatelier’s Principle. This lesson is designed to provide examples that will enable you to understand concept of equilibrium and what causes it to shift to the right or to the left. Specifically, the assessment is designed to dispel the notion that equilibrium in a chemical reaction implies equal amounts of reactants and products. Nothing could be farther from the truth! Equilibrium, in reality, exists in all situations in chemistry and chemical reactions, because all processes are, to some degree, reversible. Whenever the temperature, concentration of reactants or products, or partial pressure of various components of the reaction are changed, Le Chatelier’s Principle applies, and equilibrium will shift toward the reactants or products in a way that will resolve the stress on the system. Once a reaction reaches equilibrium, the rates (speed of change) of the forward and backward reactions are equal, but the amounts of reactants and products are usually unequal. Equilibrium is reached when the ratio of reactants and products is the most stable. When a stress is placed on a system which is at equilibrium, the system acts in a way to counteract that stress and restore equilibrium. The stress causes a new equilibrium. This week we have studied Le Chatelier’s Principle. Items needed to complete this assignment: • • • This document Two clear glasses of the same size Water Before we begin, please print out this Formative Assessment, and address the following questions: 1. Based on your understanding at this moment, provide an example of equilibrium: a system which is not changing. Propose a stress which might change the system. 2. Take the two glasses which are the same size. Fill them each with different amounts of water. One container should have significantly more liquid. Let’s call this one A. The one with less liquid will be B. 3. Consider what will happen if we pour water from A to B. Describe the overall change which will happen to B when this happens. Think about the situation in terms of equilibrium. Pour the water as described in #3. Think of container B as the ‘system.’ From state 1 (before the pour) to state 2 (after), will there be a large change or a small change? Why? Complete this pour and describe what happens. 4. Now reverse the situation: consider only system A. If we pour B into A, how does A change, from state 1 (before the pour) to state 2 (after)? Will there be a small change or a large change? Why? Complete this pour and describe what happens. 5. Now, compare your answers from Questions 3 and 4. Which one had the greater change from equilibrium? Apply Le Chateliers Principle to this and describe which system – after the pour, required more shifting to get back to an equilibrium. 6. Was the new equilibrium the same as the old one? If different, how were they different? 7. Which new equilibrium was faster to achieve? Why? 8. Describe the differences between the stress on both systems as they moved towards the new equilibrium (one glass full/one glass empty). 9. When doing a pour, the amount of water in container A could be considered to be reactants and the amount of water in B to be products. When transferring the water from A to and the amount of water in B to be products. a. When transferring the water from A to B this is a forward reaction. b. When transferring the water from B to A this is a reverse reaction. Describe the states of reaching equilibrium for both forward and reverse reactions. Part 2: Application: 1. Open the following website: http://phet.colorado.edu/simulations/sims.php?sim=Reversible_Reactions This website has a link that will enable you to download a computer simulation of reversible reactions. In the simulation, you can manipulate temperature, activation energy between reactants and products, and potential energy of the reactants and products. For now, the activation energy will be left at the zero. 2. Play with the simulation. You can easily see the “weighting” of the reaction between reactants and products by looking at the letters A and B at the top of the computer program. Begin by moving the potential energy of A and B to be at equal levels and ensuring that there is no barrier between them in terms of activation energy. 3. Before adding molecules of A for the first demonstration, what do you predict will be the relative number of molecules present at equilibrium? Why do you think so? Give your answer here: 4. In the simulation, add approximately 75 atoms of molecule A to the container and start the simulation. What do you see happening? 5. Which way did the equilibrium go? 6. Without doing the experiment, what do you think would happen if we only added molecules of B instead of A? 7. Now heat the system and start with only molecules of A. Describe what happens in terms of the equilibrium being achieved. 8. Add an equal number of molecules of B to the system while it’s heated. What happens to the equilibrium? You may need to do this a few times to see what’s happening. 9. Using the simulator, generate a system at equilibrium. Describe your system: Number of molecules of A:_________ Number of molecules of B:_________ Heat:________ Now change the system Direction of shift for equilibrium Why does it change? Increase molecules of A Increase molecules of B Add heat Remove heat Increase the activation energy Part 3: Application: Here is how Le Chatelier’s Principle: For a reversible process at equilibrium, when conditions of concentration, temperature, or pressure are changed, the reaction shifts in a direction that will counteract the stress and restore equilibrium. 1. Now that you have seen this principle in action, both in the glasses and on the simulation, please restate this principle in your own words again – and provide a real example from the world you know. 2. Describe in a paragraph what it means for an actual reaction to be at equilibrium, and provide a specific example of a reaction. Make sure to describe relative rates of forward and backward reactions. 3. For a theoretical process A + B ↔ C + heat, describe what you expect at equilibrium if: a. Additional A were added b. The amount of C was decreased as it was formed c. The temperature was increased d. The pressure was increased Assessment adapted from Le Chatelier’s Principle Demo by Laura Conrad, last updated 8/13/2102 at PhET, University of Colorado Boulder, retrieved https://phet.colorado.edu/en/contributions/view/3377
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