I. What Happens When the Conditions of Equilibrium Are Changed?

I. What happens when the conditions of equilibrium are changed?

II. In this experiment, you will investigate what happens when the conditions of equilibrium are changed. Knowing how an equilibrium changes is important for understanding how many natural systems operate. The equilibrium you will be investigating can be described by the following equation.

CuCl4-(aq)<=> Cu2+(aq) + 4Cl-(aq) + heat Green blue colorless

This equilibrium is produced by dissolving copper (II) chloride, CuCl2, in water.

Materials: 10 mL 1.5 M CuCl2 1.2 g solid NaCl test tube holder 5 mL .05 M AgNO3 3 Berol or Pasteur pipettes 6 mL small test tubes 2 well-strips 2 250-mL beakers graduated cylinder heat source ice distilled water balance safety glasses test tube rack III. Procedure Caution: You must wear safety glasses at all times during this experiment. Part A: How do temperature changes affect the equilibrium? 1. Place 5 mL of 1.5 M copper(II) chloride, CuCl2, in a clean, labeled test tube. Place another 5mL of 1.5 M CuCl2 in a second clean, labeled test tube. 2. Place one of the tubes in a test tube rack or empty beaker. This will be used as a reference for comparison. Write down the equation for the equilibrium and underneath it carefully record any observations you might have about the solution. 3. Heat the second tube of copper (II) chloride solution in a boiling water bath for several minutes. Carefully record any observations you make. (Be sure to compare it to the reference solution.) 4. Look at the equation for the equilibrium equation. Try to identify the substances the have increased in concentration and the substances that have decreased in the concentration. 5. Using a test tube holder, place the hot tube of Copper (II) Chloride solution in to an ice-water bath. Carefully record your observations. 6. Look at the equation for the equilibrium. Try to identify the substances that have increased in concentration and the substances that have decreased in concentration 7. Remove the tube from the ice-bath and place it in your test tube rack or in an empty beaker. Part B: How do concentration changes affect the equilibrium? 1. Make sure your well-strips are clean. In each of two wells, place 3 drops of room temperature copper (II) chloride solution. One well will be used as a reference to compare with the other. Write down the equation for equilibrium and underneath it carefully record observations you may have about the solution. Caution: When using pipettes, leave the pipette for each solution in the test tube with the solution so they do not get mixed-up. 2. Add 2 drops of sodium chloride solution, a drop at a time, to one of the wells. (This increases the concentration of chloride ions, Cl-.) Carefully record your observations. 3. Now add one drop of silver nitrate solution to same well that you added the sodium chloride solution to. (This decreases the concentration of chloride ions because the white solid that forms in silver chloride, AgCl .) Carefully record your observations. 4. Look at the equation for the equilibrium. Try to identify the substances that have increased in concentration and the substances that have decreased in concentration. 5. Try to predict what will happen if you now add a few drops of sodium chloride to the same well. Test your predictions; carefully record your observations. 6. Try to predict what will happen if you now add a few drops of silver nitrate to the same well. Test your prediction; carefully record your observations. Part C: What happens if both temperature and concentration are changed? 1. In a clean test tube add a few drops of copper (II) chloride solution. In a second clean test tube, add the same amount of copper (II) chloride. The second test tube will be used for comparison. 2. Write down the equation for the equilibrium and underneath it carefully record any observations you might have about the solution. 3. Alter the equilibrium in different ways; you may use combinations of concentration and temperature change. (See the Note and the Caution below.) Each time, be sure to carefully record what you are doing to the solution and the results.

Note: If the solution is viewed from the side, the dilution of the solution causes a change in the intensity of the color. This effect can be avoided if you look straight down the test tube, using a piece of white paper as a background. Ask your teacher to demonstrate this if you are unsure about it.

Caution: Never look down into a test tube while it is being heated! Wear your safety glasses!

4. With your teacher’s permission, you may repeat any part of this experiment.

Going Further What happens when concentration or temperature is changed gradually? Check with your teacher before continuing.

1. You can investigate the effect of gradually varying the concentration by adding 2 drops of copper (II) chloride solution to each of the wells in a well-strip. Then add 0 drops of sodium chloride to the first well, one drop to the second, 2 drops to the third, and so on. Try to predict what will happen before you do this! 2. See if you can devise your own method for testing the effect of gradually varying the temperature!

IV. Questions

1. What effect does cooling the solution have on the color? From this color change, what can you infer about the change in concentration of CuCl4-(aq) ion and of Cu 2+(aq) ion? 2. What effect does heating the solution have on the color? From this color change, what can you infer about the change in concentration CuCl4-(aq) ion and of Cu 2+(aq) ion? 3. When adding sodium chloride to the equilibrium, which ion do you think was affecting the equilibrium, sodium or chloride? (or both?) How could you test this experimentally? 4. What effect does adding sodium chloride have on the color? From this color change what can you infer about the change in concentration of CuCl4-(aq) ion and of Cu 2+(aq) ion? 5. What did adding silver nitrate do to the chloride concentration? 6. What effect does adding silver nitrate have on the color? From this color change what can you infer about the change in concentration of CuCl4-(aq) ion and of Cu 2+(aq) ion? 7. Try to write a rule for predicting the effect that changing the concentration of one ion has on the other ions in the equilibrium. For example, your rule should predict what would happen to the chloride ion concentration if Cu2+(aq) ions were added to the equilibrium. 8. Try to write a similar rule for the effect of changing temperature for example, your rule should predict what would happen to the concentration of A, (aq), in the following equilibrium if it were heated.

A(aq) + heat  B(aq) Red blue 9. Write a paragraph that briefly describes what you did nad what you found out. Your audience is Another chemistry student who is familiar with lab equipment, but not with this particular Equipment. Be sure to include a topic sentence and a concluding sentence.