Understanding Polyprotic Acid Titrations: Curves & Equivalence Points

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School

Los Angeles Southwest College *

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Course

102

Subject

Chemistry

Date

Jun 11, 2024

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pdf

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Uploaded by GrandChimpanzeePerson885

5/5/24, 3 : 33 PM Beyond Labz 2.0 Assessment Page 1 of 3 https://worksheets.beyondlabz.com/multiembedq.php ? id=7368-7370-7371-7372-7373-7374-7375-7376-7378-7379 Click here to erase all data entry on this page E 6-14: Study of Acid-Base Titrations – Polyprotic Acids In this video we'll show you how to operate the buret: Titrations provide a method of quantitatively measuring the concentration of an unknown solution. In an acid-base titration, 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 point 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. To start this activity, click this link for Study of Acid-Base Titrations – Polyprotic Acids . The lab will load in a new tab. Click back to this tab to read further instructions and complete the questions below. 2. The buret will be filled with 0.3106 M NaOH and 20.00 mL of 0.2556 M H 2 SO 3 will be in the beaker with thymol blue as an indicator. In the Buret section of Live Data click the button that says Save Data . This ensures that all of your titration data is saved to the Lab Book. 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 Saving and the data will appear in a table in the Lab Book. Write complete balanced equations for the two-step neutralization reaction between H 2 SO 3 and NaOH. 00:53 Here are the balanced equations for the neutralization reaction between sulfuric acid (H2SO3) and sodium hydroxide (NaOH) in two steps: Step 1: H2SO3 + NaOH → NaHSO3 + H2O This step involves the neutralization of one hydrogen ion from sulfuric acid with one hydroxide ion from sodium hydroxide, forming sodium bisulfite and water. Step 2: NaHSO3 + NaOH → Na2SO3 + H2O In this step, the remaining hydrogen ion from sodium bisulfite reacts with another hydroxide ion from sodium hydroxide, forming sodium sulfite and water. Edit Insert Formats                        P 3. What was the pH and color of the solution at the beginning of the titration? pH = 1.30

5/5/24, 3 : 33 PM Beyond Labz 2.0 Assessment Page 2 of 3 https://worksheets.beyondlabz.com/multiembedq.php ? id=7368-7370-7371-7372-7373-7374-7375-7376-7378-7379 Color = Red Clear Orange Yellow Blue Green 4. What was the pH and color of the solution at the end of the titration? pH = 12.40 Color = Clear Blue Green Red Yellow 5. Did any additional color changes occur during the titration? During the titration, two other color changes were observed which were yellow and green. 6. Examine the graph of pH vs. volume (blue line) and attach a sketch or screenshot of the titration curve below. Screenshot 2… 3.12.10PM Choose File 7. What happens to the pH around 16 mL and 32 mL? What causes each to occur? In a titration, pH rapidly changes around 16 mL and 32 mL, indicated at two points, equivalence points. At 16 mL, the first hydrogen ion of sulfuric acid is neutralized, and at 32 mL, the second ion. The rapid pH changes because the titrant and analyte react completely in stoichiometric proportions. 8. Examine the graph of conductivity vs. volume (red line). What happens to the conductivity during the titration? As the titration begins, the conductivity initially remains stable at around 23.2 µ S/cm. However, it gradually decreases until approximately 16 mL, when it increases rapidly. It reaches a maximum of around 32 mL, then st decrease again. This behavior can be explained as follows: At the beginning of the titration, the acid (H2SO3) and the base (NaOH) are in low concentrations. Therefore, the conductivity remains stable. As the strong base (NaOH) is added to the weak acid (H2SO3), neutralization reactions lead to the formation of water and salts. These salts initially decrease conductivity. At around 16 mL, the pH increases rapidly, indicating the nearing equivalence point for the titration of the first acidic hydrogen ion of sulfuric acid. At this point, excess hydroxide ions from NaOH contribute to a significa increase in ionic concentration. The conductivity reaches its maximum around 32 mL, corresponding to the equivalence point for titration of sulfuric acid's second acidic hydrogen ion. At this point, all the sulfuric acid is neutralized, resulting in the ma concentration of ions in solution. Beyond the equivalence point, further addition of the base leads to excess hydroxide ions, which can slightly decrease conductivity.

5/5/24, 3 : 33 PM Beyond Labz 2.0 Assessment Page 3 of 3 https://worksheets.beyondlabz.com/multiembedq.php ? id=7368-7370-7371-7372-7373-7374-7375-7376-7378-7379 9. What would cause the change observed in question #8? During the titration process, the conductivity of the solution changes in response to various factors. The addition of a strong base (NaOH) to a weak acid (H2SO3) initiates neutralization reactions that lead to the format water and salts, such as sodium bisulfite and sodium sulfite. These salts contribute to an increase in the concentration of ions in the solution. Additionally, the hydrolysis of these salts can occur, which leads to the form hydrogen sulfite ions (HSO3-) and hydroxide ions (OH-), further increasing the concentration of ions in the solution. Before the equivalence point, the solution contains a combination of weak acid and its conjugate base (H2SO3 and HSO3-), creating a buffer system that helps maintain a stable pH level, which affects the solution's conductivity. However, around 16 mL, the rapid increase in conductivity occurs due to the excess hydroxide ions from the added NaOH, which significantly increases the concentration of ions in the solution. This leads t corresponding increase in conductivity. At the equivalence point (around 32 mL for the second equivalence point), the sulfuric acid has been neutralized, resulting in the maximum concentration of ions in the solution. This leads to the highest conductivity obs during the titration.

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