Experiment 3_ Exploring the Behavior of Acids

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1 9/25/21 Experiment 3: Exploring the Behavior of Acids Exploring the behavior of an acid/conjugate base pair to understand the “protonated” and “deprotonated” forms of a molecule. Goals : Perform titrations for an unknown organic acid, plot data on a pH curve and take an IR and melting point of your carboxylic acid. Reagent Table : Compound Molecular Weight (g/mol) Density (g/mL) Used Safety Trans-cinnamic acid (C H O ) ₉ ₈ ₂ (most accurate unknown compound) 148 1.25 0.14 g Irritant, causes skin and eye irritation Malonic acid (C H O ) (2nd ₃ ₄ ₄ best unknown compound) 104 1.63 0.14 g Corrosive, Irritant; harmful if swallowed, causes skin and eye irritation

2 Sodium Hydroxide (NaOH) 39.997 2.13 Setup 15 mL in burette ~1.4 mL used in titration Corrosive, causes severe skin burns and eye damage Hydrochloric Acid (HCl) 36.46 1.18 1 mL Corrosive, causes severe skin burns and eye damage, toxic if inhaled Procedure : 1. Obtain a pH meter: a. Remove the electrode from the saturated KCl solution and place the plastic vial in a safe spot to prevent any spilling. b. Then, open the electrode ring on the electrode. c. Lastly, calibrate the pH meter if necessary (follow directions on the back of the pH meter) and rest the electrode in a beaker of water. 2. Weight out approximately 100-200 mg (0.1-0.2g) of your acid and record the exact mass that you will be using in the reagent table. 3. Dissolve the acid in approximately 50 mL of deionized water in a 100-150 mL beaker/flask. Dissolving the acid completely may take time since organic acids may not dissolve well in water which is why you should titrate slowly. 4. Place a stirring bar in the acid solution and place on a magnetic stirrer with the heat off. Most of the solid should disappear. If not, you must titrate very slowly. 5. Put the electrode in the stirring solution, making sure to not hit the end of the electrode with the stir bar. 6. Record the initial pH in your lab notebook. a. If the pH is greater than 2, add about 1 mL of 1M HCl with a poly pipette to ensure that the acid is fully protonated. Record the pH again.

3 7. Set up the titration apparatus by first rinsing a burette with deionized water. Make sure that the stopcock works before continuing. 8. Fill the burette with approximately 20 mL of the 1M NaOH - you will not use all of this 9. Titration: Adding 0.2-0.5 mL increments of 1M NaOH, record the pH every time you make an addition. Continue doing so until you notice that the pH is increasing rapidly. Keep adding NaOH until the pH is above 10. 10. Titration curves: Plot your graph with pH on the y-axis and mL of NaOH on the x-axis. Use a lab computer or sketch accurately in your lab notebook. 11. Return your pH meter to the plastic container with saturated KCl and tighten the cap. Close the electrode ring. 12. Take an IR and a melting point for your carboxylic acid. Data : Melting point: 130-136℃ → 136℃ is the melting point Initial pH: 3.63 pH after adding 1 mL HCl = 1.59 Titration Data Table : Number of additions Amount of NaOH (mL) pH 0 0 1.59 1 0.3 1.70 2 0.3 1.81 3 0.2 2.07 4 0.2 2.47 5 0.2 3.99 6 0.1 8.87 7 0.1 10.04 Amount of NaOH (mL) pH Buffer region (pH 1.70-3.99): 1 mL NaOH Equivalence point (pKa=pH) pKa=2.07 Titration Curve of Unknown Compound

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4 Calculations : Number of moles of NaOH required to move across the buffer zone: 1 mL = 0.001 L 0.001 L× 1 mol 1 L = 0.001 mol = 1.00 × 10 − 3 mol Number of moles of the acid: 1.4 g× 1 mol 135 g = 0.0104 mol = 1.04 × 10 − 3 mol Ratio of moles of the acid to moles of NaOH: 1.04 × 10 − 3 mol 1.00 × 10 − 3 mol = 1.04 mol 1 mol ≈ 1:1 molar ratio Number of moles of trans-cinnamic acid: 1.4 g× 1 mol 148 g = 0.000946 mol = 9.46 × 10 − 4 mol Ratio of moles of trans-cinnamic acid to moles of NaOH: 9.46 × 10 − 4 mol 1.00 × 10 − 3 mol = 0.946 mol 1 mol ≈ 1:1 molar ratio IR spectra from lab :

5 IR spectra of trans-cinnamic acid from database : C=O O-H

6 IR spectra of malonic acid from database: C=O O-H

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7 Analysis : Melting point : The melting point of the unknown compound was observed to be in the range of 130-136 degrees celsius. With this range, two of the unknown compounds can be considered: malonic acid (melting point of 136 ) and trans-cinnamic acid (138 ). ℃ ℃ pKa : The equivalence point of the titration graph shows when pH is equal to the pKa value. This is the point where the moles of acid and the moles of base would neutralize. On our titration curve, the equivalence point is equal to 2.07. When comparing this pKa value to malonic acid and trans-cinnamic acid, the pKa value is closer to the one for malonic acid. Malonic acid possesses a pKa value of 2.85 while trans-cinnamic acid has a pKa of 4.51. The lower pKa value for the unknown compound indicates that the acid can give up its proton easier, demonstrating a strong acid. While the term “strong” seems contradictory to this statement, O-H C=O

8 strong acids have weaker bonds as they are more reactive and unstable compared to weak acids. Reactions tend to favor the side with the weaker acid as they want to reach stability. When examining the buffer zone, one question that arises is why is the left edge of the buffer zone difficult to see compared to the right edge? This could be because as mentioned earlier, strong acids possess a small pKa. Once the equivalence point is reached, there is a very sharp jump in pH because strong acids are very unstable and reactive. Once the amount of NaOH exceeds the 1 mL buffer zone, the pH skyrockets, making the right edge of the buffer zone easier to see. Determining the unknown compound by pKa alone will not be accurate because we had to perform the titrations by hand. With our titration data, we had a large jump in pH from the 4th addition of NaOH to the 6th. From the 4th to 5th addition of NaOH, the difference in pH was 1.52, and from the 5th to 6th the difference in pH was 4.88. Because we performed the titrations by hand, controlling the amount of NaOH per addition was very hard to control. With the lack of points on our titration curve, the equivalence point may not be accurate because we were not able to measure the pHs with additions of NaOH less than 0.1 mL. Protons : The ratio of moles of the acid to moles of NaOH demonstrates a 1:1 ratio. This indicates that only one proton will be present in the acid. Also, because there was only one buffer region on the titration curve, this also demonstrates that there is only one proton present on the unknown compound. As relating to the potential compounds, this pattern aligns perfectly with the ratio of moles of trans-cinnamic acid to moles of NaOH as it also demonstrates a 1:1 ratio. This is further proven by the structure of trans-cinnamic acid as this compound possesses one proton. On the other hand, malonic acid possesses two protons in its structure, which does not match with the 1:1 molar ratio and the one buffer region with the unknown compound. IR Spectra : At first glance, all the IR spectra displayed above demonstrate similar wave patterns. More specifically, both demonstrate the structure of a carboxylic acid composed of a hydroxyl group (O-H) and a carboxyl group (C=O). When comparing the database IR spectrum for trans-cinnamic acid and malonic acid, one could argue that they both align with the spectra from the lab as they display the same prominent peaks (both being carboxylic acids). But when looking at the wave patterns of the spectra from the lab and the one of trans-cinnamic acid, the size of the waves are more similar at first glance. For example, the O-H peak for malonic acid is

9 more broad and spread out compared to the O-H peaks in the spectra of trans-cinnamic acid and the IR spectra collected in the lab. Even without identifying the major peaks in the spectra, the “filler” peaks from the lab IR also display similar wavenumber positions as the database IR for trans-cinnamic acid. Conclusion : Based on the analysis, the categories listed above support the idea that the unknown compound given to us is similar to trans-cinnamic acid. As with the unexpected equivalence point value, perhaps this resulted from the fact that our compound was not capable of dissolving fully even during the titration, making it harder for us to obtain precise pH points for the titration curve. If we added more trials to this experiment we could fully understand if the lower pKa value was because of human error and/or procedure error. Even with this discrepancy, I believe that based on the number of protons and the IR spectrum specifically that the unknown compound is trans- cinnamic acid.

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