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Le CHATELIER’S PRINCIPLE LABORATORYDIRECTIONS: Each group will rotate from one station to the next on the teacher’s command. Each station will take approximately 10 minutes. STATION 1 ACID – BASE INDICATOR EQUILIBRIUMAn indicator is a dye that can gain or lose hydrogen ions to form substances that have different colors. For simplicity, the uncharged indicator molecule may be represented as HIn, and the anionic indicator molecule after the loss of a hydrogen ion may be written as In-. Bromothymol blue will be used as the indicator in this activity. To start, measure approximately 2 mL of distilled water in a small test tube. Add 5 drops of 0.04% bromothymol blue. Swirl gently and record observations.Add a couple of drops of 0.1 M HCl. Record changes below.Add a couple of drops of 0.1 M NaOH. Record changes below.Play around with adding HCl and NaOH to the test tube. Record what you see.Rinse out the test tube and place back in the test tube rack.ReactionsHIn(aq) ? H+(aq) + In-(aq)H+(aq) + OH-(aq) H2O(l)ProcedureObservationsInitial color of water and bromothymol blue solutionAdd 0.1 M HCl dropwiseAdd 0.1 M NaOH dropwiseAdditional drops of HCl and NaOHSTATION 2 FORMATION OF COBALT COMPLEX IONSWhen cobalt (II) chloride hexahydrate (CoCl2?6H2O) is dissolved in ethyl alcohol, three different solute species are present: Co+2 cations, Cl- anions, and water molecules. These can react to form two different complex ions: Co(H2O)6+2, where the cobalt ion is surrounded six water molecules, and CoCl42-, in which the metal ion is surrounded by 4 chloride ions. To start, label three test tubes A-C and place them in a test tube rack (may already be done for you). Fill a 250 mL graduated cylinder half way with tap water and slowly heat on a hot plateUsing a 10-mL graduated cylinder, add 2 mL of the cobalt chloride solution to each test tube A-C. NOTE: the exact volume is not important, but try to keep the volume of the solution approximately equal in each test tube.Add 6.0 M HCl, using a dedicated pipet, dropwise to test tube B and record observations.Add 0.1 M AgNO3, using a dedicated pipet, dropwise to test tube B and record observations.Add distilled water, using a dedicated pipet, dropwise to test tube C and record observations.Add 5-6 grains of CaCl2 to test tube C and record observationsPlace test tube C in an ice water bath for 2-3 minutes and record observationsTransfer test tube C from the ice water bath to the hot water bath for 2-3 minutes and record observationsEmpty all beakers and rinse all test tubes. REACTIONS[Co(H2O)6]+2(aq) + 4Cl-(aq) ? [CoCl4]2-(aq) + 6H2O(l)Ag+(aq) + Cl-(aq) AgCl(s)PROCEDUREOBSERVATIONSInitial color of cobalt chloride solution (control)Add 6.0 M HCl dropwise to test tube BAdd 0.1 M AgNO3 dropwise to test tube BAdd distilled water dropwise to test tube CAdd 5-6 grains of CaCl2 to test tube CTest tube C placed in ice-water bath for 2-3 minutesTest tube C placed in hot-water bath for 2-3 minutes STATION 3 SOLUBILITY OF CARBON DIOXIDEWhen carbon dioxide dissolves in water, it forms a weakly acidic solution due to the following reversible reaction:2CO2(g) + H2O(l) ? CO2(aq) + H+(aq) + HCO3-(aq)The hydrogen ion concentration in solution depends on the amount of dissolved carbon dioxide. According to Henry’s Law, the amount of gas dissolved in solution is proportional to the pressure of the gas above the solution. This explains why pop stays carbonated best when the cap of the 2 L is screwed on tight. The tightness on the cap keeps the pressure high above the pop in the 2 L allowing more carbonation to stay in the liquid. Obtain approximately 10 mL of seltzer water in a 50-mL beaker.Add about 20 drops of 0.04% bromocresol green indicator. Swirl to mix the solution and record observations. Draw up about 10 mL of the seltzer/indicator solution into a 30-mL syringe. Seal the syringe by pushing a tip cap firmly onto its open endPull back on the syringe to create a decrease in pressure. Hold the plunger steady and shake until bubbles stop forming. Record observations.Push syringe in to increase pressure. Hold the plunger steady and record observations.Reactions2CO2(g) + H2O(l) ? CO2(aq) + H+(aq) + HCO3-(aq)ProcedureObservationsInitial color of seltzer water and bromcresol green solutionPull back on syringe to create a decrease in pressurePull back on syringe to create an increase in pressure ANALYSIS QUESTIONSSTATION 1 Bromthymol blue is an indicator for acids and bases. This means the indicator will change color based upon whether a solution is acidic (greater concentration of H+ ions) or basic (lower concentration of H+ ions). Bromthymol blue is green at pHs between 6.0-7.6. Bromthymol blue yellow at pHs below 6.0 and blue at pHs above 7.6.Estimate the pH of the solution after HCl was added. When HCl was added did the rate of the forward or reverse reaction increase? What substance increased in concentration? Estimate the pH of the solution after NaOH was added. When NaOH was added, did the rate of the forward or the reverse reaction increase? When NaOH is added, which substance did it react with and remove from the reaction? Use your observations to explain how you know this reaction is reversible.STATION 2Use the following information to help answer the next questions: [Co(H2O)6]+2 is a pink color and [CoCl4]2- is a blue color. When HCl was added to test tube B, explain what happened. You must reference the following in your answer: How HCl changed the concentration of either a reactant or productHow the rate of either the forward or reverse reaction changedHow your observations reiterate how you answered parts a and b. When AgNO3 was added, how did this affect equilibrium? Also include how the reaction was able to get back to equilibrium.Use evidence from your data table to explain how adding CaCl2 affected equilibrium. Is this reaction exothermic or endothermic? Use evidence from your data table to answer this question.STATION 3Use the poster in the front of the room for bromcresol green which informs you of colors and the resulting pH. Estimate the initial pH of the solution. Is this solution, initially, acidic or basic? Explain what causes this. When the pressure increases, how does the pH change? Use Le Chatelier’s Principle to explain this.INTEGRATING IT ALL TOGETHERWhen a chemical is manufactured, chemists and chemical engineers choose conditions that will favor the production of the desired product as much as possible. They want the forward reaction to occur more quickly than the reverse reaction. In the early 20th century, Fritz Haber developed a process for the large-scale production of ammonia from its constituent elements. Some of his results are summarized in the chart below.*Each experiment began with a stoichiometric mixture of H2 and N2. Write the balanced chemical equation, including the heat term, for the synthesis of ammonia from its constituent elements. Based on the results above, explain the effect of temperature on the equilibrium position of the reaction. Explain the effect of pressure on the equilibrium position of the reaction. The optimal conditions to synthesize ammonia are high pressures and low temperatures. However, each factor comes with a drawback: high pressures require strong pipework and hardware, and at low temperatures the reaction is slow. In order to get high yields of ammonia at lower pressures and higher temperatures, ammonia is removed from the system as it is formed. Use LeChatelier’s Principle to explain why this is so effective. ................
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