Introduction
The purpose of this lab was to determine which way equilibrium shifts went by using LeChatelier’s Principle. LeChatelier’s Principle states that change in any of the parameters that determine equilibria will result in a system change, which will cause a change in the equilibria.
The equilibria of three solutions were found by executing and observing a variety of experiments for each.
First the equilibrium shift in a reaction (eq 1) involving the weak acid HSO4- in water was tested.
HSO4-(aq) + H2O (l) ⇌ H3O+(aq) + SO42- (aq) (1)
Next the equilibrium shift in a reaction (eq 2) involving the slightly soluble salt Mg(OH)2 was tested.
Mg2+(aq) + 2OH-(aq) ⇌ Mg(OH)2(s) (2)
Last the equilibrium shift in a reaction (eq 3) involving the hexaaquacobalt(II) ion, Co(H2O)62- was tested.
CoCl42-(alc) + 6H2O(l) ⇌ Co(H2O)62+(aq) + 4Cl-(aq) (3)
Procedure
For the first part of the lab the initial solution was prepared by adding NaHSO4 to five labeled test tubes, and then Thymol Blue as a pH indicator. When the pH is less than two Thymol Blue is red, when it is greater than two Thymol Blue is a red-yellow color. The first test tube was used as a standard. Na2SO4 was added to the second test tube until there was a color change. NaHSO4 was added to the third test tube until there was a color change. The fourth test tube was heated in a hot water bath, then cooled in a ice water bath. The opposite was preformed on the fifth. All observations were recorded.
The second and third parts of the lab were the same as part one but with different solutions and phenolphthalein as the pH indicator. Phenolphthalein is colorless unless the pH is greater than eight, at which point it turns pink. For part two the beginning solution was created from MgCl2 and NaOH, and HCl and Na4EDTA were the solutions added to the appropriate test tubes. For the third part of the lab the beginning solution comprised of water and CoCl42-, and ethyl alcohol, silver nitrate, and HCl were added to the appropriate test tubes.
Detailed procedures may be found in reference 1.
Results
The first part of the lab involved the reaction HSO4-(aq) + H2O (l) ⇌ H3O+(aq) + SO42- (aq). Table 1 contains the data and observations obtained from this reaction.
Table 1. Results of Equilibria for the NaHSO4 solutions.
| Disturbance | Observation | Equilibrium Shift |
| Na2SO4 | Lighter Red/Yellow Tint | Reactants |
| NaHSO4 | No Change | Products |
| Heated | Lighter Pink | Reactants |
| Cooled | Bright Pink | Products |
The Thymol Blue indicator did not work well in the heating/cooling portion of this lab. It should have indicated a basic pH when it was heated and a acidic pH when it was cooled, but it showed the opposite. For this reaction it is known that for this reaction that when the solution is heated it shifts towards the reactants, which is why it was an exothermic reaction.
The second part of the lab involved the reaction Mg2+(aq) + 2OH-(aq) ⇌ Mg(OH)2(s). Table 2 contains the data and observations obtained from this reaction.
Table 2. Results of Equilibria for Mg(OH)2 solutions.
| Disturbance | Observation | Equilibrium Shift |
| HCl | Clear | Reactants |
| Na4EDTA | Darker Pink | Reactants |
| Heated | Lighter Pink | Products |
| Cooled | Increased Precipitate | Reactants |
The pink color from the phenolphthalein caused it to be difficult to accurately discern the quantity of precipitate when it was heated and therefore did not accurately express the shift in equilibrium. For this reaction it is known that heating the solution shifts the equilibrium towards the products, which is why it was an endothermic reaction.
The third part of the lab involved the reaction
CoCl42-(alc) + 6H2O(l) ⇌ Co(H2O)62+(aq) + 4Cl-(aq). Table 3 contains the data and observations obtained from this reaction.
Table 3. Results of Equilibria for CoCl42- solutions.
| Disturbance | Observation | Equilibrium Shift |
| Water | Lighter Pink | Products |
| Silver Nitrate in 100% Ethanol | Precipitate, Purple | Products |
| Ethyl Alcohol | Light Blue | Reactants |
| Hydrochloric Acid | Bright Blue | Reactants |
| Heated | Blue | Reactants |
| Cooled | No Change | Products |
When the solution was heated it shifted towards the reactants, which is why it was concluded that it was an exothermic reaction.
Discussion
In part one the pH indicator did not turn the correct color, which lead to the incorrect conclusion that the reaction was endothermic. It is known that the equilibrium for this reaction lies on the reactant side, which is why it is an exothermic reaction. In part two of this lab the quantity of precipitate in the solution did not noticeably increase, which is why it was thought that it was an exothermic reaction. It is known that in this reaction precipitate should form when heated, therefore indicating that the equilibrium shifted to the products and that it is an endothermic reaction. The probable reason that there was no observation of an increase in precipitate was the presence of phenolphthalein, which made detecting a change in the quantity of precipitate difficult. The reaction of part three of this lab was found to be exothermic, because the equilibrium shifted towards the reactants when the solution was heated.
LeChatelier’s Principle explains that when the temperature, pressure, or concentration of a reaction are changed, an equilibrium will shift as a result, in order to counteract the change. In this lab we changed many of these factors and changed the equilibriums and used LeChatelier’s Principle to aid in determining which side of the reaction the equilibrium lied on.
References
1. General Chemistry Experiments: A Manual of Chemistry 204, 205, and 206, Department of Chemistry, Southern Oregon University: Ashland, OR, 2008. 2009. pp. 101-103
2. Brown, LeMay, Burnsten, Murphy. Chemistry: The Central Science. 11th Edition. Pearson Prentice Hall. Upper Saddle River, NJ. 2009. pp. 649-653.
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