23. Acid-Base Titrations Part I

Catherine Drennan introduces the topic of strong acids and bases, focusing on their behavior during acid-base titrations. She explains the concept of the equivalence point, where the moles of acid equal the moles of base, and emphasizes the importance of recognizing this point in a titration curve. The lecture also touches on the practical aspects of titrations in a lab setting, including the use of pH indicators and the challenges of identifying the endpoint accurately.

A teaching assistant presents a practical demonstration using a natural dye from red cabbage, which changes color depending on the pH of the solution it's added to. The TA explains how the dye's molecular structure changes in acidic versus basic conditions, resulting in different colors. The demonstration includes testing various household substances like lemon juice, water, vinegar, and ammonia to illustrate the concept of acidity and basicity. An unexpected experiment with dry ice and the resulting color change due to carbonic acid formation is also discussed, tying the demonstration to global warming and the impact of CO2 on the environment.

The script delves into the calculations involved in determining the pH of solutions before and after adding a strong acid to a strong base. It explains the process of calculating moles of hydroxide ions and hydronium ions, adjusting for the volume of the solution, and using these values to find the pH and pOH. The importance of understanding the stoichiometry of the reaction and the significance of the equivalence point, where the pH is neutral (7), is highlighted. The calculations are used to predict the pH at various stages of the titration, including before and beyond the equivalence point.

The lecture continues with an analysis of titration curves for weak acids with strong bases and vice versa. It describes the buffering region where the pH remains relatively constant due to the presence of a weak acid and its conjugate base. The equivalence point is identified as the stage where all the weak acid has been neutralized by the strong base, resulting in a pH greater than 7. The concept of the half-equivalence point is introduced, where half the moles of acid have been neutralized. The lecture concludes with a practical reminder to start problem sets early to understand the application of these concepts.

The script outlines a step-by-step approach to solving titration problems, emphasizing the importance of recognizing the type of problem being addressed at different stages of the titration curve. It provides examples of how to calculate the pH at various points during the titration of a weak acid with a strong base, including the initial state, the buffering region, the half-equivalence point, the equivalence point, and beyond. The rapid change in pH near the equivalence point is highlighted, illustrating the precision required in titration experiments.

The lecture notes review the process of titrations, specifically focusing on the curves for weak acids with strong bases and weak bases with strong acids. The importance of accurately drawing these curves and understanding the buffering region is stressed. The lecture also introduces the next unit on oxidation-reduction, providing a smooth transition from acid-base chemistry to a new topic. The script provides a detailed example of a titration problem involving formic acid, guiding students through the calculations needed to determine the pH at various stages of the titration.

The script discusses the creation of buffer solutions through the reaction of a weak acid with a strong base, leading to the formation of the conjugate base. It explains how to calculate the pH of these buffer solutions using both the Ka expression and the Henderson-Hasselbalch equation. The importance of checking the assumptions made during these calculations is emphasized, particularly ensuring that the change in ionization is small relative to the initial concentration. The lecture also covers the calculation of pH at the half-equivalence point and the equivalence point in a titration scenario.

The lecture concludes with a summary of the key points covered in the titration lecture, including the calculation of pH at various stages and the importance of understanding the buffering region. The script then transitions to the next topic, oxidation-reduction, indicating a continuation of the lecture in the next class. The emphasis is on the importance of starting problem sets early to apply the concepts learned in class.