VCL 17-2 Weak Acid-Strong Base Titrations

Transcription

1 VCL 17-2 Weak Acid-Strong Base Titrations Titrations provide a method of quantitatively measuring the concentration of an unknown solution. In an acidbase titiration, this is done by delivering a titrant of known concentration into an analyte of known volume. Titration curves (graphs of volume vs. ph) have characteristic shapes. The equivalance point of the titration, or the points where the analyte has been completely consumed by the titrant, is identified by the point where the ph changes rapidly over a small volume of titrant delivered. In this assignment, you will observe this titration curve by titrating the weak acid CH 3 COOH (acetic acid) with the strong base NaOH. You will also predict the ph at the equivalence point, validate your prediction experimentally, and then calculate the equilibrium constant for the neutralization reaction. 1. Start Virtual ChemLab and select Weak Acid-Strong Base Titrations from the list of assignments. The lab will open in the titration laboratory. 2. Click the Lab Book ad open it. The buret will be filled with NaOH and ml of CH 3 COOH will be in the beaker with phenolphthalein as the indicator. Click the Save button in the Buret Zoom View window. The horizontal position of the orange handle is off for the stopcock. Open the stopcock by pulling down on the orange handle. The vertical position delivers solution the fastest with three intermediate rates in between. Turn the stopcock to one of the fastest positions. Observe the titration curve. When the volume reaches 40 ml, double-click the stopcock to stop the titration. Click Stop in the Buret Zoom View. A blue data link will be created in the lab book. Click on it to view the data. If you need to repeat the titration, click in the Stockroom to enter, click on the clipboard and select preset Experiment #3 Weak Acid-Strong Base. 3. The beaker contains M CH 3 COOH and the buret contains M NaOH. Write a complete balanced equation for the two-step neutralization reactions between CH 3 COOH and NaOH. The following questions can be answered by examining the Plot window and the Data Viewer window, and the balanced net ionic equation. 4. Indicate the species present at the equivalence point, and predict whether the ph at the equivalence point will be ph >7, ph<7, or ph approximately 7. Explain why you made this prediction. 5. Explain the graph ph vs. volume (blue line) and sketch the titration curve on the following page. Mark the equivalence point as halfway between the top and bottom shoulders of the curve. Based on your graph, what is the ph at the equivalence point? How does it compare with your predicted ph?

2 6. Calculate the value of the equilibrium constant for the balanced net ionic equation for the weak acid-strong base titration.

3 VCL 17-5 Study of Acid-Base Titrations Polyprotic Acids Titrations provide a method of quantitatively measuring the concentration of an unknown solution. In an acidbase titiration, this is done by delivering a titrant of known concentration into an analyte of known volume. (The concentration of an unknown titrant can also be determined by titration with an analyte of known concentration and volume.) Titration curves (graphs of volume vs. ph) have characteristic shapes. The graph can be used to determine the strength or weakness of an acid or base. The equivalence point of the titration, or the points where the analyte has been completely consumed by the titrant, is identified by the point where the ph changes rapidly over a small volume of titrant delivered. For polyprotic acids, there will be multiple equivalence points. In this assignment, you will observe this titration curve by titrating the weak acid H 2 SO 3 with the strong base NaOH. 1. Start Virtual ChemLab and select Study of Acid-Base Titrations-Polyprotic Acids from the list of assignments. The lab will open in the titration laboratory. 2. Click the Lab Book ad open it. The buret will be filled with NaOH and ml of H 2 SO 3 will be in the beaker with thymol blue as an indicator. Click the Save button in the Buret Zoom View window. The horizontal position of the orange handle is off for the stopcock. Open the stopcock by pulling down on the orange handle. The vertical position delivers solution the fastest with three intermediate rates in between. Turn the stopcock to one of the fastest positions. Observe the titration curve. When the volume reaches 40 ml, double-click the stopcock to stop the titration. Click Stop in the Buret Zoom View. A blue data link will be created in the lab book. Click on it to view the titration data. If you need to repeat the titration, click in the Stockroom to enter, click on the clipboard and select preset Experiment #7 Polyprotic Acid-Strong Base. 3. The beaker contains M H 2 SO 3 and the buret contains M NaOH. Write a complete balanced equation for the two-step neutralization reactions between H2SO3 and NaOH. The following questions can be answered by examining the Plot window and the Data Viewer window. 4. What was the ph and color of the solution at the beginning of the titration? 5. What was the ph and color of the solution at the end of the tiriation? Did any additional color changes occur during the titration? 6. Examine the graph of ph vs. volume (blue line) and sketch the titration curve on the following page.

4 7. What happens to the ph around 16 ml and 32 ml? What causes each to occur? 8. Examine the graph of conductivity vs. volume (red line) and sketch the titration curve on the graph above. 9. What happens to the conductivity during titration? 10. What would cause the change observed in question 9?

5

6

7