Exp 5. Chemical equilibrium and Le Chatelier’s principle • Exp 3 report is due. • Next class: Exp 6 part ABC only • Chemical equilibrium is achieved when the rates of two opposing chemical reactions become equal. General equilibrium equation aA + bB cC + dD Equilibrium constant: 𝐶 𝐾= 𝐴 𝑐 𝑎 𝐷 𝐵 𝑑 𝑏 K is large, molar concentrations of products > molar concentrations of reactants at equilibrium. K is small, molar concentrations of products < molar concentrations of reactants at equilibrium. Le Chatelier’s principle • A change in factors that determine the equilibrium conditions causes the system to response in a way that counteract the change. • Goal: To study how changes in concentrations (reactants or products), acidity and temperatures affect a chemical equilibrium using observations. Changes in concentration aA + bB cC + dD An increase in reactant concentration (A, B) causes the equilibrium to shift to the right (reducing reactant concentration by forming more products) And vice versa. Changes in acidity are changes in [H+] Changes in temperature • Endothermic reaction: aA + bB + heat cC + dD A temperature increase (heat addition) causes the system to shift to the right (product formation) and vice versa. • Exothermic reaction: aA + bB cC + dD + heat A temperature increase (heat addition) causes the system to shift to the left (reactant formation) and vice versa. Model equilibrium system [Co(H2O)6] 2+ (aq) + 4 Cl- (aq) [Co(H2O)6] 2+ and [CoCl4] 2- [CoCl4] 2- (aq) + 6 H2O (l) have two different colors. By observing the color of the solution, you can deduce how the equilibrium responds to changes. Part A: Determine the color of Co(II) complexes • Which predominant species that reaction 1 and 2 produces? [Co(H2O)6] 2+ or [CoCl4] 2- ? • What is the color? • You will use part A as a reference point for part B! • Check your conclusion of part A with the instructor. Part B: Test tube #1 1. Initial solution: System at equilibrium Prediction: N/A Does the system shift to the left or right? Predict how the color of solution changes Observation: How does the color of the solution change? Do you notice temperature difference? Predominant species: [Co(H2O)6] 2+ or [CoCl4] 2- or a mixture of both? Based on the color of the initial solution, you can deduce whether K is large or small (the system at equilibrium favors products or reactants). Test tube #1 2. Add 12M HCl solution Prediction: Think about what you actually add to the system! HCl  H+ + ClA solution is added ( a solution is a solute dissolved in water) In effect, you are adding H+ (changes acidity), Cl- (reactant) and water (product) to the system. If more than one factors change, which one has the strongest effect on equilibrium? Test tube #2 1. Add 6M HNO3 solution to the initial solution Prediction: HNO3  H+ + NO3 – A solution is added In effect, you are adding H+ (changes acidity), NO3– (nothing) and water (product) to the system. How is this different than test tube #1, step 2: Addition of HCl? Test tube #3 • Add solid LiCl to saturation (keep adding LiCl until it no longer dissolves). • Prediction: LiCl  Li+ + ClNOT a solution (water is NOT added) Dissociation of LiCl in water is an exothermic reaction (produces heat) Test tube #4 • Add solid NH4Cl to saturation • Prediction: NH4Cl  NH4+ + ClNOT a solution Dissociation of NH4Cl in water is an endothermic reaction (consumes heat) Test tube #5 • Manipulate the temperature without changing concentrations or acidity. • Based on observations, you can conclude whether your system is endothermic or exothermic. • NO NEED TO DO TEST TUBES IN ORDER. Post Lab Assignment – Chemical Equilibrium and Le Chatelier’s Principle CHEM 132L General Instructions The entire report must be typed. You and your lab partner may NOT turn in copies of the same tables/ graphs. Each student must make their own tables/ graphs and answer all questions as an individual effort. Include the following information at the top of the first page of your assignment: Name: Lab Partner(s): Lab Section Number: Experiment Title: Due Date: Data and Calculations 1. Summarize your observations for part A. 2. Create typed tables that concisely present the data collected for part B (one table per test tube). 3. Type the equilibrium reaction that is the basis for all of part B. 4. Below each table for part B, type 1-2 sentences that clearly explains what happens to the equilibrium and why the change occurs as related to Le Chatelier’s principle. This explanation should be written for each manipulation, so you will have 16 total explanations. Questions – All answers should be typed and answered in complete sentences. 1. Refer to equation 1 on page 36 of the lab manual. Based on the data collected, summarize the effect of temperature on the position of equilibrium for this reaction. 2. Based on the data collected, is equation 1 on page 36 of the lab manual exothermic or endothermic? Rewrite the equation to include heat on the appropriate side. 3. Does the value of the equilibrium constant (K) for this reaction increase or decrease as the temperature becomes warmer? Explain.
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