Application of LeChatelier’s Principle to Chemical Equilibria

Introduction

The purpose of this lab was to use LeChatelier’s Principle to determine the direction of equilibrium shifts in reactions. 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. 

Observations

For the first reaction an initial solution was prepared by adding crystal NaHSO₄ and Thymol Blue as a pH indicator to five test tubes. 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 and NaHSO₄ was added to the third until there was a color change. The fourth test tube was heated in a hot water bath, and the fifth was cooled in a ice water bath. Observations and the determined direction of equilibrium shift can be seen in Table 1. It was determined that this was an exothermic reaction, because the equilibrium shifted to the reactants when the solution was heated and shifted to the products when cooled.

Table 1. Observation and interpretation of chemical and physical perturbations to the aqueous bisulfate equilibrium.

HSO4-(aq) + H2O (l) ⇌  H3O+(aq) + SO42- (aq)
Perturbation	Change in Appearance	Equilibrium Shift
Na2SO4 Added	Lightened to Pink/Yellow	Reactants
Crystal NaHSO4 Added	Darkened to Purple	Products
Heated Solution	Lightened	Reactants
Cooled Solution	Darkened	Products

For the second reaction the initial solution was created in five test tubes with MgCl₂ and NaOH. The first test tube was used as a standard, HCl was added to the second, and Na₄EDTA was added to the third. The fifth test tube was cooled and the sixth was heated. Observations and the determined direction of equilibrium shift for this reaction can be seen in Table 2. It was determined that this reaction was exothermic.

Table 2. Observation and interpretation of chemical and physical perturbations to the __ equilibrium.

Mg2+(aq) + 2OH-(aq) ⇌  Mg(OH)2(s)
Perturbation	Change in Appearance	Equilibrium Shift
NaOH Added	Precipitate	Products
HCL Added	Precipitate disappeared	Reactants
Na4EDTA Added	More Precipitate Formed	Products
Heated Solution	Less Precipitate	Reactants
Cooled Solution	More Precipitate	Products

The initial solution for the third reaction comprised of water and CoCl₄^2-. The first test tube was a standard, ethyl alcohol was added to the second, silver nitrate to the third, and HCl to the fourth. The fifth test tube was cooled and the sixth was heated.

The observations and the direction of the equilibrium shift can be found in Table 3.

Table 3. Observation and interpretation of chemical and physical perturbations to the __ equilibrium.

CoCl42-(alc) + 6H2O(l) ⇌  Co(H2O)62+(aq) + 4Cl-(aq)
Perturbation	Change in Appearance	Equilibrium Shift
H2O Added	Pink	Products
Ethyl Alcohol Added	Blue	Reactants
Silver Nitrate Added	Pink and Precipitate Formed	Products
HCl Added	Blue	Reactants
Heated Solution	Blue	Reactants
Cooled Solution	Pink	Products

Discussion

LeChatelier’s Principle states that when the parameters of a reaction are changed, the equilibrium will shift in order to counteract the change. In this lab we changed many parameters in the reactions and used LeChatelier’s Principle to aid in determining which side of the reaction the equilibrium lied on.

All three reactions were found to be exothermic.