LeChatelier's Principle Mini-Lab

By: Shaun Lamar, Jenna Williams, and Emily Davidson

Activity One: Solubility of Carbon Dioxide

Chemical Reaction

2CO2 (g) + H2O (l) ↔ CO2 (aq) + H+ (aq) + HCO3-1 (aq)

Background


According to the inert gas law, PV=nRT, there is an inverse relationship between pressure and volume (as shown in the graph).


That means that when volume increases, pressure decreases. A product of the given reaction for this activity is H+. Its concentration depends on the concentration of dissolved carbon dioxide. The concentration of carbon dioxide is proportional to the pressure of the gas above the dissolved solution.


According to Henry’s Law, the amount of gas dissolved in solution is proportional to

the pressure of the gas above the solution. The hydrogen ion concentration in solution therefore depends on the amount of dissolved carbon dioxide.

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Procedural Steps

  1. Begin by creating the initial conditions.

  2. Obtain approximately 10 mL of fresh seltzer water in a beaker.

  3. Add about 20 drops of 0.04% bromocresol green indicator. Swirl to mix the solution.

  4. Draw up about 10 mL of the seltzer/indicator solution into a 30 mL syringe.

  5. Seal the syringe by pushing a tip cap firmly onto its open end.

  6. Take a picture that shows the initial color of the seltzer/indicator solution.

  7. Decrease the pressure on the seltzer/indicator solution in the syringe by expanding the volume.

  8. Take another picture that shows the new color of the seltzer/indicator solution.

  9. Record the results.

Observations

The first picture shows the initial color of the seltzer/indicator solution. It has a pH of 7.0 and is a light green color.


The second picture shows the final color of the seltzer/indicator solution after the pressure was decreased. It has a pH of 8.0 and is a teal color.

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Results

We changed the system by decreasing the pressure or by increasing the volume. This caused a shift to the side with more moles of gas, which in this case was a shift towards the reactants. This was supported by the change in color which displayed an increase in pH from 7.0 to 8.0. At first, the reaction was at equilibrium which is why it had a pH of 7.0 and was a light green color. When the reaction began to favor the reactants, the pH increased to 8.0 and was a teal color signaling the presence of a weak base. Bases have higher pH values. A shift to the products was not present because that would’ve signaled a decrease in pH. Acids have a lower pH level and the HCO3-1 acid in the products would’ve created a lower pH level if it was the favored side.

Activity Two: Solubility of Magnesium Hydroxide

Chemical Reactions


Mg(OH)2 ➝ Mg+2 + 2OH-1


HCl + 2OH- ↔ H2O + Cl-1

Background

The active ingredient in milk of magnesia is magnesium hydroxide crystals, used as an over-the-counter antacid remedy. Magnesium hydroxide forms a suspension in water due to its low solubility—0.0009 g/100 mL in cold water and 0.004 g/100 mL in hot water.

Procedural Steps

  1. Begin by creating the initial conditions.

  2. Obtain 10 mL of the milk of magnesia solution and add this to a 250 mL beaker.

  3. Add approximately 50 mL of distilled water.

  4. Add 5-10 drops of universal indicator solution.

  5. Swirl with magnetic stir plate and bar to mix the solution.

  6. Take a picture to record the initial color of the magnesium hydroxide dissolved in water.

  7. Next add 20 mL of hydrochloric acid solution (3 M) to the beaker.

  8. Take a picture to record the new color of the solution.

  9. Then add 10 mL of magnesia solution to the beaker.

  10. Take a picture to record another new color of the solution.

  11. Record the final results.

Observations

Results

We changed the concentration of the reaction by adding more reactants. By adding more reactants, the equation shifted to the right to favor the products. This is supported by the color changes. Initially, the first reaction was a purple color and had a pH of around 10.0. This means that it produced a base since bases have high pH levels. In the second reaction, when 20 mL of HCl was added, a light pink color was produced and had a pH range around 4.0. This means that an acid was produced since the pH level is low. When 10 mL of magnesia solution was added next, the solution turned a green color. This meant that the pH was at 7.0 and the reaction was at equilibrium due to the product of water. The reaction then began to turn a teal/light blue color that had a pH range around 8.0. The reaction once again produced a base since the pH level was high.