Lec 21 | MIT 5.112 Principles of Chemical Science, Fall 2005

TL;DR

Indicator molecules like phenolphthalein serve as Bronsted acids and change color based on pH changes, with phenolphthalein changing color between 8.2 and 10.0 pH units.

Transcript

The following content is provided by MIT OpenCourseWare under a Creative Commons license. Additional information about our license and MIT OpenCourseWare in general is available at ocw.mit.edu. Continuing our discussion of acid-base theory, I have just drawn here on the board for you the structure of a popular indicator molecule. Indicator molecule... Read More

Key Insights

• 💱 Indicator molecules like phenolphthalein act as Bronsted acids and change their molecular structure and color in response to pH changes during titrations.
• 😥 Indicator molecules can be naturally occurring in plants or synthesized to respond to desired pH ranges.
• 💱 The structure of indicator molecules, such as phenolphthalein, changes when a base interacts with hydrogen ions, resulting in a color change.
• 😥 Phenolphthalein changes color between pH 8.2 and 10.0, from colorless to pink.
• 😒 The use of indicator molecules allows for the detection of pH changes during a titration experiment.
• 😥 pH changes in solutions can affect the functioning of various systems in the environment and the human body.

Questions & Answers

Q: How do indicator molecules work in titrations?

Indicator molecules, like phenolphthalein, act as Bronsted acids and change their molecular structure and color in response to changing pH levels during a titration experiment.

Q: What changes occur in the structure of phenolphthalein during titration?

When phenolphthalein is in a solution with low pH, it is neutral. As the pH increases, a base with a lone pair of electrons will remove a proton from one of the hydrogens in phenolphthalein, resulting in a rearrangement of the molecule's structure and the formation of a negatively charged system.

Q: How can indicator molecules be naturally occurring?

Indicator molecules can be found in some plants and have evolved to respond to pH changes in their environment. These naturally occurring indicators can be used in titrations to detect pH changes.

Q: How can indicator molecules be synthesized?

Indicator molecules can also be synthesized to respond to specific pH ranges. By modifying the structure of the molecule, chemists can create indicators that change color at different pH values, expanding the range of options for titrations.

Summary & Key Takeaways

• Indicator molecules, such as phenolphthalein, are used in titrations to detect pH changes throughout the experiment.

• Phenolphthalein acts as a Bronsted acid and changes its structure and color based on pH levels.

• Indicator molecules can be naturally occurring or synthesized and are selected based on the desired pH response.