LAB: EQUILIBRIUM AND LeCHATELIER’S PRINCIPLE



Lab: Equilibrium and LeChatelier’s Principle

LeChatelier’s Principle states that: If an equilibrium system is subjected to a stress, the system will react to remove the stress. To remove a stress, a system can only do one of two things: form more products using up reactants, or reverse the reaction and form more reactants, using up products. In this experiment you will form several equilibrium systems. Then, by putting different stresses on the systems, you will observe how equilibrium systems react to a stress.

The purpose of this demo experiment is to investigate the effect of reaction conditions on the reversible formation of cobalt complex ions. When cobalt (II) chloride hexahydrate (CoCl2 • 6H2O) is dissolved in ethyl alcohol, three different solute species are present: Co+2 cations, Cl-1 anions, and water molecules. These can react to form two different complex ions, Co(H2O)6+2, where the cobalt ion is surrounded by six water molecules, and CoCl4-2, in which the metal ion is surrounded by four chloride ions. The net ionic equation for the reversible reaction in this experiment is shown below. The colors of the complex ions are shown underneath their formulas.

Co(H2O)6+2 + 4Cl- ↔ CoCl4-2 +6H2O

Pink Blue

Observations/Data Table

|Part A. Effect of Concentration |

|Test tube |Reagents |Observations |

|A |CoCl2 in alcohol (control) |PURPLE |

|B |CoCl2 in alcohol + water |Purple to Pink |

|C |CoCl2 in alcohol + HCl |Pink to blue |

|D |CoCl2 in alcohol + water + CaCl2 |Pink to Blue (slowly) |

|E |CoCl2 in alcohol + acetone |Pink to BRIGHT Blue |

|F |CoCl2 in alcohol + silver nitrate |Purple to Pink and Cloudy White |

|Part B. Effect of Temperature |

| |CoCl2 in alcohol |

|B | |

| |Initial color |Purple |

| |Final color after heating to 75-80 °C |Blue |

| |CoCl2 in alcohol |

|C | |

| |Initial color |Purple |

| |Final color after cooling to 0-5 °C |Pink |

Post-Lab Questions

1. Based on the initial color of the cobalt chloride solution (test tube A), what complex ions are present in this solution? Explain.

2. Which complex ion was favored by the addition of water to the original solution (test tube B)? Use LeChatelier’s Principle to explain the observed color change.

3. Which complex ion was favored by the addition of hydrochloric acid and calcium chloride in test tubes C and D, respectively? Why?

4. Select the correct choices in the following statements to summarize the answers to Questions #2 and 3.

a. Adding a species which appears on the right side of an equation will shift the equilibrium to the (left/right) side of the equation.

b. Adding a species which appears on the left side of an equation will shift the equilibrium to the (left/right) side of the equation.

5. Acetone is a polar solvent that attracts water molecules. Use this fact and LeChatelier’s Principle to explain the color change that was observed when acetone was added to the solution in test tube E.

6. a. Silver chloride is a white solid that is insoluble in water. Write a net ionic equation for the reaction of silver ions and chloride ions to form silver chloride.

b. Based on this reaction, did the concentration of chloride ions in test tube F increase or decrease when silver ion (in the form of silver nitrate) were added?

c. Use these facts and LeChatelier’s Principle to explain the changes observed in test tube F when silver nitrate was added. Be specific.

7. Select the correct choices in the following statements to summarize the answers to Questions #5 and 6.

a. Removing or decreasing the concentration of a species which appears on the right side of an equation will shift the equilibrium to the (left/right) side of the equation.

b. Removing or decreasing the concentration of a species which appears on the left side of an equation will shift the equilibrium to the (left/right) side of the equation.

8. Which complex ion was favored when the solution was heated? Which complex ion was favored when the solution was cooled? Use these results to determine whether heat should be included on the reactant or product side of the equation. Write the energy term (heat) directly in the equation.

Co(H2O)6+2 + 4Cl- ↔ CoCl4-2 +6H2O

9. Use LeChatelier’s Principle to explain the color changes that resulted from heating and cooling the solutions.

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