Le Chatelier’s Principle states that “if a chemical system at equilibrium experiences a change in concentration, temperature, volume, or partial pressure, then the equilibrium shifts to counteract the imposed change and a new equilibrium is established (Atkins,1993). ” In equilibrium the chemical reactions are a two way reactions and if there are changes in various factors, the equilibrium will be pushed to change to form a new equilibrium.
The purpose of this experiment was to portray this very principle and how these pushes in equilibrium of changing the concentration of specific ions or changing the temperature (energy level) will cause the equilibrium to be shifted. It was hypothesized that observable changes in equilibria would be noticeable and observed with these specific pushes if this principle holds true. Materials: The materials used and needed to perform this experiment included an electric hot plate, ice cubes, polyethylene transfer pipets, test tubes, 100 mL beaker, de-ionized water (H2O) and the chemicals : 1 M K2CrO4, 3 M H2SO4, 6 M NaOH, 0. % methyl orange indicator, 0. 1% phenolphthalein indicator, 6 M HCl, 0. 1 M CH3COOH (acetic acid), 1 M NaCH3COO (sodium acetate), 0. 1 M NH3, 1 M NH4Cl, 0. 1 M Fe(NO3)3, 0. 1 M KSCN (potassium thiocyanate), 0. 15 M anhydrous CoCl2 (in methanol), 12 M HCl, 5. 4 M NaCl, 0. 1 M BaCl2, 0. 1 M CaCl2, 0. 5 M H2C2O4 (oxalic acid), 0. 5 M K2C2O4 (potassium oxalate), 6 M NH3. Concentrated chemicals such as 12 M HCl, or even 6 M HCl, are very corrosive. Goggles and gloves must be worn at all times. The mixtures of chemicals must also be accordingly disposed.
Also the heating of methanol causes toxic and flammable vapors, thus requiring this procedure to be performed under a hood. Methods: The first manipulation of equilibrium required 1 M K2CrO4, 3 M H2SO4, and 6 M NaOH. 3 mL of 1 M K2CrO4 was added to a test tube then several drops of 3 M H2SO4 were added. The test tube was swirled to mix the two chemicals and any observed change was recorded. Then several drops of 6 M NaOH were added to the same test tube and the mixture was stirred until a change occurred. This change was then recorded.
Several drops of 3 M H2SO4 was again added to the mixture and stirred. This change was also recorded. In the second equilibrium manipulation, a drop of methyl orange indicator was added to 3 mL of water in a test tube. Two drops of 6 M HCl was then added and any changes were recorded. Following the two drops of HCl, 4 drops of 6 M NaOH was added to the mixture and changes in color were recorded. After this procedure, the experiment of these chemicals was repeated, but the phenolphthalein indicator was used in place of the methyl orange indicator.
In the third equilibrium manipulation, two test tubes were prepared with 3 mL samples of 0. 1 M acetic acid and a drop of methyl orange indicator. To one of the samples 1 M sodium acetate was added by a few drops at a time. With mixing, the changes in color of both tests tubes were compared and recorded. In the fourth equilibrium manipulation, two 3 mL samples of 0. 1 M NH3 was prepared with a drop of phenolphthalein indicator. The initial color of these samples were noted and recorded. Then to one sample, 1 M NH4Cl was added and mixed a few drops at a time.
To the other sample, 6 M HCl was added one drop at a time with mixing. With both samples any change in color and odor was recored. In the fifth equilibrium manipulation, 3 mL of 0. 1 M Fe(NO3)3 was added to 3 mL of 0. 1 M KSCN in a 100 mL beaker. Changes in color were recorded. This mixture was then diluted by adding 50 to 60 mL of water until the deep red color was reduced in intensity. Then 5 mL of this solution was placed in a test tube and 25 drops of 0. 1 M Fe(NO3)3 was added. Any changes in color were recorded. To a second sample of 5 mL of the diluted solution, 1 mL of 0. M KSCN was added and any changes in color were recorded. To a third 5 mL sample of the diluted solution, 5 to 6 drops of 6 M NaOH was added and any changes were recorded. A fourth sample of 5 mL diluted solution was set as a control to compare with the results of the other 3 mixtures. In the sixth equilibrium manipulation, 3 mL of 0. 15 M CoCl2 (in methanol) was placed into a test tube. Then using a pipet, water was added to the chemical to change the blue methanol solution to a pink complex. The pink complex was then equally divided into two portions in two test tubes.
To one sample 12 M HCl was added one drop at a time until changes were observed. These changes were then recorded. The other sample of the pink complex was then heated to 65 to 70 degrees Celsius using a beaker of hot water on an electric hot plate. After heating the complex, any changes in color were observed and recorded. Then this same complex was cooled in an ice bath, and the restoration of the original pink color was observed and recorded. In the seventh equilibrium manipulation, 2 mL of 12 M HCl was added to 4 mL of 5. 4 M NaCl in a test tube.
Any changes in color from this mixture was noted and recorded. Then in another test tube, 5 to 6 drops of 1 M K2CrO4 was added to 3 mL of 0. 1 M BaCl2. Any changes were then recorded. Then to this mixture 10 to 12 drops of 6 M HCl was added and any changes were then recorded. In the eighth and final equilibrium manipulation, 4 mL of 0. 1 M CaCl2 was mixed with 4 mL of water then placed into three test tubes of equal proportions. To tube 1, 6 to 7 drops of 0. 5 M H2C2O4 was added and 6 to 7 drops of 0. 5 M K2C2O4 was added to test tube 2. The results were then compared and recorded.
Then 10 drops of 6 M HCl was added to test tube 1 and results were recorded. A slight excess of 6 M NH3 was then added to test tube 1 and results were recorded. To find out if the precipitate from the results of test tube 1 was in fact Ca(OH)2, a few drops of 6 M NH3was added to test tube 3. Results: In the first equilibrium manipulation, when H2SO4 was added to K2CrO4, the solution changed in color from light yellow to orange. Then when NaOH was added the solution changed back to its original light yellow color. When H2SO4 was again added, the solution was changed to an orange color again.
The overall reaction of this equilibrium was: 2 CrO42- + 2 H3O+- Cr2O72- + 3 H2O In the second equilibrium manipulation, when 6 M HCl was added to water with indicator, the color changed from yellowish to pink. Then when 4 drops of NaOH was added the color changed back to yellow. The reaction for this equilibrium was: H(indicator) +H2O – H3O+ + indicator – In the third equilibrium manipulation, when a drop of methyl orange was added to acetic acid the color changed from clear to pinkish purple. When sodium acetate was then added to one of the samples, the color changed from reddish pink to orange.
The overall reaction of this equilibrium was: CH3COOH + H2O – H3O+ + CH3OO- In the fourth equilibrium manipulation, when phenolphthalein indicator was added to the NH3 samples the color changed into a purplish pink with pungent odor. When NH4Cl was added to one of the samples, the color changed to a clear solution with no smell. When the 6 M HCl was added to the other sample, the color also changed to clear with no odor. In the fifth equilibrium manipulation, when Fe(NO3)3 was added to KSCN the solution changed from a clear color to a deep brownish red.
When more Fe(NO3)3 was added to the diluted previous solution, there was no change in color and the solution stayed red. When KSCN was added to another diluted sample, the color changed to an even darker blackish red color. When NaOH was added to a third sample of diluted solution, the color changed to an orange color. The fourth sample was a control and there were no changes to the red color. The overall reaction to this equilibrium was: Fe3+ + SCN- – Fe(SCN)2+ In the sixth equilibrium manipulation, when 12 M HCl was added to the pink CoCl2 complex the color changed from pink to purple.
When the second sample of the pink complex was heated the color changed from pink to purple, and the color changed back to purple when the complex was cooled in the ice bath. The reaction to this equilibrium was: CoCl4- + 6 H2O – 4 Cl- + Co(H2O)62+ + energy In the seventh equilibrium manipulation, when 12 M HCl was added to the sodium chloride it resulted in a milky white precipitate on the bottom with a clear fluid on the top. When K2CrO4 was added to the BaCl2, it resulted in a yellow precipitate. When 12 M HCl was then added to this solution the precipitate dissolved into an orange solution.
The reactions to this equilibrium were: 4NaCl + H2O – 4Na+ + 4 Cl- + H2O BaCl2 + H2O + K2CrO4 – BaCrO4 + H2O + 2K- + 2Cl- In the eighth and final equilibrium manipulation, when CaCl2was diluted with water the solution was clear. When H2C2O4 was added to one of the three samples of diluted solution there was a result of a white precipitate. When K2C2O4 was added, there was also a white precipitate that formed. When 6 M HCl was added to tube 1 the precipitate was dissolved and the solution became clear again. When an excess of NH3 was then added to this tube a white precipitate was formed.
Finally when NH3was added to the third test tube, a very light precipitate also formed. The reactions for this equilibrium manipulation were: Ca2+ + C2O42- – CaC2O4 (s) K2C2O4 – 2 K+ + C2O42- H2C2O4 + H2O – H3O+ + HC2O4- + H2O – H3O+ + C2O42- NH3 + H3O+ – NH4+ + H2O Discussion: The first manipulation was the manipulation of chromate ion-dichromate ion equilibrium. The higher concentration of H2SO4 pushed the color to change orange. When NaOH was added the concentration was pushed back to the initial equilibrium. Then when H2SO4 was added again the concentration was pushed back to the orange color.
These changes were due to the addition of hydroxide ions exerting an effect on the equilibrium. The second equilibrium manipulated was weak acid – base indicator equilibrium. The color changes of the indicators in these reactions were due to the acid HCl being added pushing the equilibrium to be more acidic, and when the base NaOH is added the color changes indicating that the indicator is changed back to its basic form. The third equilibrium manipulated was a weak acid weak base equilibrium. In this manipulation, the addition of acetate ions produces a change in color of the indicator.
This showed that the indicator had been changed from its acidic form to its base form. This also means that the hydrogen ion concentration became smaller when sodium acetate was added. The fourth equilibrium manipulated was also a weak acid weak base equilibrium. The indicator was added to a weak base of NH3 thus changing its color and odor. When the NH4Cl was added, the ion concentrations changed and pushed the indicator back to its acidic form which is colorless and odorless. The addition of HCl also pushed the equilibrium to a more acidic solution resulting in the same consequences.
The manipulation of the fifth equilibrium was the manipulation of a complex ion equilibrium. In this equilibrium cations attract negatively charged ions to form complexes. The Fe3+ cation attracted the negatively charged SCN- ion thus forming Fe(SCN)2+forming the dark red color. When more Fe(NO3)3 was added to this solution the addition of Fe3+ ions caused no change because these positive cations do not attract each other. When more KSCN was added, the higher concentration of SCN- ions pushed the equilibrium into a darker red solution.
When NaOH was added in the third sample it pushed the formation of the complex Fe(OH)3 which is very insoluble. This resulted in the change to an orange color. The manipulation of the sixth equilibrium was the manipulation of a temperature dependent equilibrium of complex ions. The cobalt complex in methanol is blue and tetrahedral as the complex in an aquo complex is octahedral and has a pink color. The equilibrium between the two forms involves a significant energy change and is temperature dependent.
When HCl was added the pink aquo complex was pushed to change the complex pack to its tetrahedral form this was also true when the solution was heated due to its requirement of substantial energy. When the solution was cooled however the equilibrium was pushed back to its original octahedral pink form. The seventh manipulation was that an equilibrium of saturated solutions. When HCl was added to the NaCl solution, the Cl- ion concentration increased. This increase in concentration pushed the Na+ ions within the solution to react more and form a NaCl precipitate.
The same explanation could be accounted for saturated barium chromate. When K2CrO4was added to barium chloride, the Ba2+ ions binded with the CrO42- ions to create a barium chromate complex. When HCl was added, the Cl- ion concentration increased, thus pushing the equilibrium back to the formation of barium chloride. The eighth manipulation applied the law of chemical equilibrium to solubility equilibrium. When H2C2O4 and K2C2O4 were added to the CaCl2, the increase of C2O42- concentration caused a precipitate of Ca2C2O4.
When HCl was added the concentration of Cl- ions increased thus pushing the equilibrium back to CaCl2. When NH3 was added it was possible that some OH- ions were generated, thus pushing the equilibrium to a precipitate of Ca(OH)2. Conclusion: Ultimately Le Chatelier’s Principle was apparent in the experiments performed. The principle states that “if a chemical system at equilibrium experiences a change in concentration, temperature, volume, or partial pressure, then the equilibrium shifts to counteract the imposed change and a new equilibrium is established (Atkins,1993). The hypothesis was supported in the sense that when the equilibria were pushed in one direction by a change in concentration or temperature, there was an obvious change in equilibrium. This change in equilibrium was noted and observed by a change of color or the formation of a precipitate.
Atkins, P. W. (1993). The Elements of Physical Chemistry (3rd ed. ). Oxford University Press.