Activity 2: Temperature Effects

Introduction

In this laboratory activity you will study two chemical systems and observe the effect of temperature changes on them: (1) the aqueous ammonia system and (2) the Fe³⁺ /SCN ­ system first studied in Laboratory Activity 1 .

Purpose

To determine the effect of temperature on chemical systems.

Safety

1. Wear protective goggles throughout the laboratory activity.

2. CAUTION: The test-tube will be hot after heating. Be sure to use a test-tube holder.

3. Concentrated ammonia solution has a strong odor. Avoid breathing the ammonia fumes. Be careful not to spill any concentrated ammonia solution on yourself. If you do spill any, wash the area with large amounts of water and notify your teacher.

4. Dispose of the chemicals as your teacher directs.

Procedure

Part A

1. Place about 400 mL distilled water in a large beaker.

2. Add one drop concentrated ammonia solution. Stir well.

3. Fill two large test-tubes half full of this solution.

4. Add one drop phenolphthalein indicator to each test-tube. The solution should be pink, but not dark pink (see teacher-made solution for exact color).

5. Gently heat one of the test-tubes over a burner until the solution begins to boil. Reserve the other test-tube as a ÒstandardÓ for comparison. Record your observation of any changes that occur. Compare the heated solution with that of the unheated ÒstandardÓ solution.

6. Let the tube cool for a few seconds, then place it in a stream of cold water. Record any changes that occur.

Part B

1. Put about 50 mL 0.005 M KSCN, potassium thiocyanate, solution in a 100-mL beaker. Note the appearance of the solution.

2. Add about five drops 0.2 M Fe(NO 3 ) 3 , iron(III) nitrate, solution to the KSCN solution and stir. Make notes about the appearance of the Fe(NO 3 ) 3 solution and the combined solution.

3. Fill each of three test-tubes about one-third full of the solution made in Step 2. Label one test-tube as the ÒstandardÓ and place it in your test-tube rack.

4. Place one of the test-tubes in a boiling water bath for 5 min. Place a second test-tube in a salt-ice water bath for 5 min. Compare the solution in each bath with your ÒstandardÓ solution at room temperature.

5. Thoroughly wash your hands before leaving the laboratory.

Data Analysis And Concept Development

1. What ion accounts for the pink color of the aqueous ammonia solution?

2. Write a chemical equation for the reaction between ammonia and water resulting in the ion identified in Question 1.

3. What evidence is there that a chemical change occurred when the test-tube was heated? Write a chemical equation for this chemical change.

4. What effect did cooling the solution have on the aqueous ammonia solution? Write a chemical equation for this chemical change.

5. Is the reaction involved in Question 4 exothermic or endothermic? How do you know?

6. Write a net ionic equation for the reaction between iron(III) nitrate and potassium thiocyanate.

7. Analyze the equation you wrote in Question 6 in a manner similar to that of the aqueous ammonia system and determine whether this reaction is exothermic or endothermic.

8. Generalize about the effect of heat on reversible chemical reactions.

Implications and Applications

1. Why would the aqueous ammonia reaction not work well if too much ammonia were added?

2. Discuss how a change in temperature of a liquid solvent affects the solubility of a solid solute.

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