Activity 1: Concentration Effects

Introduction

In this laboratory activity you will investigate chemical reactions of the type A + B Æ C. You will then add several reagents to the reaction mixture to see the effect of each on the reaction.

Purpose

This laboratory activity will help you gain an understanding of a new type of chemical reaction.

Safety

1. Wear protective goggles throughout the laboratory activity.

2. Dispose of the chemicals as your teacher directs.

Procedure

1. Place 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(NO₃ )₃, iron(III) nitrate, solution to the KSCN solution and stir. Make notes about the appearance of the Fe(NO₃)₃solution and of the combined solutions.

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

4. In a second test-tube, put one drop of the Fe(NO₃)₃ solution. Mix and observe any changes that occur. Write your observations.

5. In the third test-tube, put a few crystals of solid KSCN (about the size of a small pea). Mix gently while observing and record your observations.

6. In the fourth test-tube, put one drop 0.1 M AgNO₃, silver nitrate solution. Mix and observe any changes that occur. Write your observations.

7. In the fifth test-tube, put a few crystals of solid NaF, sodium fluoride (about the size of a rice grain). Mix gently while observing and record your observations.

8. Thoroughly wash your hands before leaving the laboratory.

Data Analysis and Concept Development

1. Write comparisons of the solutions in the five test-tubes.

2. Potassium ions (K⁺ ) and nitrate ions (NO₃ ^-) are not active in any of the chemical reactions of this activity. Such "inactive" ions are sometimes termed "spectator ions." They are present because they are carried along with the "active" part of the chemical involved in the reaction. What are the active substances in the reaction between Fe(NO₃ )₃ and KSCN?

3. The product of this reaction is FeSCN²⁺ . Write a balanced equation for the reaction studied in this laboratory activity.

4. What evidence is there for a chemical reaction between Fe(NO₃)₃ and KSCN?

5. Identify the cause of the color of the solution formed in Step 2.

6. Why does the color change when one drop of Fe(NO 3 ) 3 solution is added to the solution formed in Step 2?

7. Why does the color change when solid KSCN is added to the solution formed in Step 2?

8. What is the cause of the changes observed when the AgNO₃ solution is added? Write an equation to represent these changes.

9. Why does the color change when solid NaF is added to the solution?

Implications and Applications

Ammonia gas, NH₃, can be formed by the reaction of hydrogen gas, H₂ , and nitrogen gas, N₂. The formation of ammonia is exothermic. An equilibrium is established according to the equation:

Predict the effects of each of the following on the equilibrium system. (Does the equilibrium shift to the right, to the left, or remain unchanged?)

1. Adding more nitrogen gas or hydrogen gas at constant volume and temperature.

2. Removing some ammonia gas at constant volume and temperature.

3. Heating the system at constant volume.

4. Decreasing the pressure of the system by changing the volume of the reaction vessel at constant temperature.

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