Investigating Chemical Equilibrium

The reactions presented in this investigation demonstrate observable reversibility. Under changing conditions, the systems will respond to counteract any stress placed upon the equilibrium state. For example, a variation in a precipitate provides the observable evidence that a shift has occurred in the equilibrium concentrations. Part I: Equilibrium involving Thymol Blue 1. Put on safety equipment 2. Obtain 2 – 250mL Erlenmeyer flasks. 3. Label the flasks: flask #1 and flask #2 4. Using a 100mL graduated cylinder, add 100mL distilled water to each flask. 5. Using a 10mL graduated cylinder, add 1 mL thymol blue to each flask. Record the colour. 6. To flask #1, add 0.1M NaOH drop by drop until a definite colour change occurs. Record the colour change and the number of drops required for the colour change. 7. To flask #1, add one drop of 0.1M HCl and swirl the flask. Continue to add HCl drop by drop until a definite colour change is noticed. Record the colour change, and number of drops required to change the colour. (Flask #2 serves as a control; do not stress this flask.) 8. Continue to add 0.1M HCl to flask #1 until a second colour change occurs. Record the colour change, and number of drops required to change the colour. Compare the colour to the control flask. 9. Continue to add 0.1M HCl to flask #1 until a third colour change occurs. Record the colour change, and number of drops required to change the colour. Compare the colour to the control flask. Objectives: 1. To recognize the macroscopic properties of five chemical systems at equilibrium Observations of initial solutions: 100mL distilled water + 1 mL thymol blue Chemistry 12 Name: Investigating Chemical Equilibrium Introduction: Most chemical reactions appear to proceed to completion. Under certain conditions, however, the products may have sufficient energy to reform reactants in a reverse reaction. When reactions continue to occur, the net concentration of each substance remains the same. Le Chatelier’s principle allows us to predict the effects of a stress placed on an equilibrium system. The stress produces a shift in concentrations that may be recognized by changes in the macroscopic properties of the system. 2. To observe shifts in equilibrium concentrations as stresses are applied to the systems. 3. To explain the observations obtained by applying Le Chatelier’s principle. Materials: Apparatus 5 test tubes thymol blue(aq) + H+(aq) ↔ thymol blue+ (aq) (blue) Note: NaOH(aq) reduces the [H+] by reacting with it. (H+ + OH- à H2O Equilbrium: Observations and Analysis: Reagent Added Stress (Ion Added) 1 – 250ml Beaker safety goggles test tube rack 2 – 100 mL beakers lab apron NaOH (step 6) 100mL and 10mL graduated cylinders 2 – 250mL Erlenmeyer flasks plastic gloves EXPLANATION: # of drops required HCl (step 7) Reagents 0.1M HCl Thymol Blue Distilled Water 1.0M HCl 0.1M K2Cr2O7 0.1M NaOH 0.2M FeCl3 or Fe(NO3)3 0.1M K2CrO4 1.0M NaOH 0.2M KSCN 0.1M Ba(NO3)2 6.0M NaOH O.2 M KCl 0.2M Fe(NO3)2 EXPLANATION: HCl (step 8) EXPLANATION: HCl (step 9) EXPLANATION: Predicted Shift Colour Observed Direction of Equilibrium Shift Use collision theory to explain the stress and shift occurring in each case. Part II: Equilibrium involving FeSCN2+(aq) 1. Using a 10mL graduated cylinder, put 1mL 0.20 M Fe(NO3)3 into a 250mL beaker. Record the colour. 2. Record the colour of 0.2M KSCN. 3. Using a 10mL graduated cylinder, put 1mL KSCN into the same 250mL beaker, which already contains 0.20M FeCl3. 4. Swirl the mixture and record the colour. 5. Add 80.0mL distilled water to the 250mL beaker. 6. Pour 5.0mL of solution into 5 separate test tubes, labeled A to E. 7. For the next few steps, record your observations in the appropriate table. Record the “stress” involved, state which ion concentration was changed and if it had increased or decreased in concentration. 8. To test tube B, add 10 drops of 0.20M KCl. 9. To test tube C, add 10 drops of 0.20M Fe(NO3)3 10. To test tube D, add 10 drops of 0.20M KSCN. 11. To test tube E, add 10 drops of 6.0M NaOH. Observations of initial solutions: 0.20 M FeCl3 0.2M KSCN KSCN and FeCl3 mixture Equilibrium: Fe3+(aq) pale rust + Precipitation Reaction: Fe3+(aq) + SCN-(aq) clear FeSCN2+(aq) dark red Added Ions - B Cl C Fe3+ D SCN E OH- - Predicted Shift 1. 2. 3. 4. Label two test tubes, #1 and #2. To test tube#1, place 10 drops of 0.1M K2CrO4. To test tube #2, place 10 drops of 0.1M K2Cr2O7 To each test tube, add, in alteration, 1.0M NaOH drop by drop until a colour change occurs in one of the test tubes. Record your results. 5. To each test tube, add in alteration, 1.0M HCl drop by drop until a colour change occurs in one of the test tubes. Record your results. 6. Repeat steps 1-3, but add 1.0M HCl first until a colour change occurs, and then add 1.0M NaOH. 7. To a new test tube labeled tube #3, add 10 drops of 0.1M K2CrO4 and 2 drops of 1.0M NaOH. 8. To test tube #3, add 0.1M Ba(NO3)2 solution drop by drop until a change is noted. Record the number of drops added. 9. To test tube #3, add 1.0M HCl drop by drop until a colour change is noted. Record the number of drops added. 10. Label a test tube: test tube #4. 11. To test tube #4, add 10 drops of K2Cr2O7. 12. To test tube #4, add 2 drops of 1.0M HCl, then add 10 drops of Ba(NO3)2 drop by drop. 13. To test tube #4, add 1.0M NaOH drop by drop until a change is noted. 14. Label two test tubes: test tube #5 and #6. 15. To test tube #5, add 10 drops of 0.1M K2CrO4. 16. To test tube #6, add 10 drops of 0.1M K2Cr2O7. 17. To test tubes #5 and #6, add 10 drops of 0.1M Ba(NO3)2 to each. Record your results. Equilibrium: 2CrO42-(aq) + OH-(aq) Fe(OH)3 (aq) 2H+(aq) Acid Cr2O72-(aq) + Orange H2O(l) Precipitation Reaction: Observations and Analysis: Test tube Be sure that you are able to use collision theory to explain the stress and shift occurring in each case. Part III: Equilibrium involving Chromate and Dichromate Ions Predicted Observation Actual Observation Reason for the Shift (increase or decrease of [Fe3+] or [SCN-]) Ba2+ (aq) + CrO42- (aq) BaCrO4 (s) Additional Notes: 1. HCl(aq) contributes H+ to this equilibrium 2. NaOH(aq) reduces the [H+] by reacting with it. (H+ + OH- à H2O) 3. Ba(NO3)2 is used as a test for chromate (CrO42-). The Ba2+ ion forms a precipitate with CrO42- but not with Cr2O72-. So when barium nitrate is added to one of these solutions, a cloudiness would indicate that some CrO42- is present in the solution. The more the cloudiness, the more precipitate is formed, hence the more CrO42- is present. Part III: Chromate and Dichromate Ions Part III: Chromate and Dichromate Ions Predictions: Observations and Analysis Stress (Ions Added) Predicted Shift (left/ right) ¬/® Predicted Observations ~colour changes? ~formation/amount of precipitate? clear or cloudy? Tube # 1 and 2 0.1M K2CrO4 0.1M K2Cr2O7 Initial colour OH1.0M NaOH added H+(HCl) 1.0M HCl added OH-& Ba(NO3)2 1 and 2 Initial colour as a test for chromate 1.0M HCl added H+ & Ba(NO3)2 1.0M NaOH added as a test for chromate CrO42-& Ba(NO3)2 3 +1.0M +0.1M Ba(NO3)2 NaOH as a test for chromate Cr2O72-& Ba(NO3)2 Initial colour +1.0M HCl 0.1M K2CrO4 4 Initial colour +1.0M HCl +0.1M Ba(NO3)2 +1.0M NaOH as a test for chromate 0.1M K2Cr2O7 5 and 6 0.1M K2CrO4 (tube #5) 0.1M K2Cr2O7 (tube #5) Add 0.1M Ba(NO3)3 Be sure that you are able to use collision theory to explain the stress and shift occurring in each case.