Chemical Equilibrium

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Chemical Equilibrium: Le Chatelier Principle

By

Sarah Ramos and Kristina Todorovic

Chemistry 203 DEN

Dr. Mohamed El-Maazawi

Part A. Acid-Base Indicators
Purpose

In this part of the experiment, we will find a reagent that will shift the acid-base equilibrium reaction described by Equation (2) in one direction and then a second reagent that will cause the equilibrium position to shift back in the opposite direction.

Introduction

An acid–base indicator is a substance that changes color as the pH of a solution changes. Consistent with LeChâtlier’s principle, if a solution containing the indicator becomes acidic (e.g., with the addition of H+), the equilibrium will shift to the left, and the solution will become yellow. On the other hand, if the solution becomes basic (e.g., if OH– ions are added, which would reduce the [H+]), the equilibrium will shift to the right and the solution will become purple. At intermediate pH values there will be a mixture of both HB and B–, and the solution will take on a green color. Some indicators exhibit only two colors and some exhibit a wide range. Each indicator must be individually studied to determine its behavior as a function of pH.

Le Chatelier's Principle states that if an equilibrium system is subjected to a stress, the system will react to remove the stress by either: forming more products using up reactants, or reverse the reaction and forming more reactants, using up products. In this experiment several equilibrium systems will be formed. By putting different stresses on the systems, we will observe how equilibrium systems react to a stress. Acid-base indicators are large organic molecules that can gain and lose hydrogen ions to form substances that have different colors. It is the color of the ion that will indicate how the equilibrium system is being affected.

Procedure

Approximately 5 mL of deionized water was added to a test tube and a few drops of the methyl violent indicator solution. The color of our solution was reported. Our solution from Step 1 currently contains one form of methyl violet indicator. Now predict which of the two 6 M reagents you obtained, the strong acid or the strong base, will cause a color change in your solution by making the methyl violent indicator shift to its other form. The 6 M reagent of our choice was added drop-by-drop and if our solution changes color, the color of the solution and formula of the reagent was reported. If the addition of our reagent did not result in a color change we tried other reagents until you were successful. Equilibrium systems are reversible which means that it is possible to shift a reaction left or right repeatedly by changing the appropriate conditions. Another 6 M reagent was tested that will cause our solution from Step 2 to revert to its original color. The 6 M reagent of our choice was added drop-by-drop and if our solution changed color, the formula of the reagent was reposted. If the addition of our second reagent did not result in a color change, we tried other reagents until we were successful.

Data, Calculations, and Results

Original color of methyl violet in water is Purple.
Reagent|Color Change|Return to Original Color|
6 M NaOH (aq)|Clear|
6 M NH4OH (aq)|Clear|
6 M HCl (aq)|Yellow|addition of 6 M NaOH(aq) ® Purple|
0.3 M HCl (aq)|Blue|addition of 6 M NH4OH(aq) ® Purple|

Addition of 6 M NaOH (aq) to the Yellow solution (6 M HCl (aq) + H3O+ (aq) + Mv - (aq)) causes the equilibrium to shift back and returns the solution to its original Purple color.

Addition of 6 M NH4OH (aq) to the Blue solution ( 6 M HCl (aq) + H3O+ (aq) + Mv - (aq) ) causes the equilibrium to shift back and returns the solution to its original Purple color.

HMv (aq) + H2O (l) ® H3O+ (aq) + Mv - (aq)

Discussion

Methyl violet in water is a Purple color. The addition of NaOH or NH4OH to methyl violet in water caused the solution to become clear,...
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