CAChe Lab: Modeling a Titration
Created by: Rebecca A. Kruse
Pre-lab Assignment:
Read “Calculating Electron Density and Electrostatic Potential with CAChe.”
Pre-lab Discussion:
Reviewing Ahhrenius’s definitions of acids and bases from the last lab, you’ll recall that among the properties of
acids are the abilities to affect the colors of acid-base indicators and neutralize bases. Bases, the counterparts of
acids, likewise affect the colors of acid-base indicators and neutralize the acids.
An acids-base titration is used to determine the unknown concentration of an acid or base by monitoring its
neutralization reaction with a standard of known concentration in the presence of an indicator. For example, a
base of unknown concentration may be titrated with an acid standard of known concentration or the reverse. The
endpoint is the point of the acid-base titration in which just enough of the standard solution has been added to
completely neutralize the solution of unknown concentration. Indicators are used to signal this point of the
titration, and are generally selected according to the type of titration (strength of acid and based used in the
titration). When the endpoint of the titration has been reached, one drop of standard solution past the endpoint will
result in a color change of the indicator.
Many common household substances are acid and bases; tomorrow you will be performing a titration of a weak
household acid (unknown solution) with a weak household base (standard solution). To better understand the
titration you will perform in the wet lab, we would like for you to think about the titration on the molecular level
today. In this short lab exercise, you will use electrostatic potential maps to model each species involved in the
acid-base titration. You will then be asked to answer a few questions pertaining to the acid-base titration and
rationalize your answers.
Purpose:
In this exercise you will use CAChe to generate and compare electrostatic potential surfaces of each of the
components of an acid-base titration in order to simple molecular principles underlying acid-base titrations,
neutralization, and indicators.
Lab Procedure:
Follow the directions provided in “Calculating Electron Density and Electrostatic Potential with CAChe” to do
the following:
1. Open CAChe.
2. Generate and compare electrostatic potential surfaces for the weak household acid and base, and
their conjugate acid and base, respectively.
3. Generate and compare electrostatic potential surfaces for the two structural forms of the indicator that
are provided.
Questions:
Use your data to answer the following:
1. Based on the electrostatic potential surfaces, identify the most electron-poor hydrogen (largest +, red
region) of the acid.
2. Based on the electrostatic potential surfaces, identify the most electron-rich region (smallest +, blue
region) of the base.
3. What other substances are present in this aqueous titration and why are they important?
4. What substance is in excess at the beginning of the titration (i.e. before the endpoint)?
5. What substance is in excess at the end of the titration (i.e. after the endpoint)?
6. What substance is in excess at the endpoint?
7. Describe what happens to the indicator’s structure and electrostatic potential map just one drop
beyond the endpoint?
8. Predict the part of the indicator’s structure that is responsible for the visible color change.
Suggestions for Further Work/Inquiry:
From the list provided by your teacher select an indicator with a pH range that is significantly different from the
one modeled today then complete the following:
a. Use online resources, your textbook, or other reference texts such as the Merck Index or the CRC
Handbook to find the 2-D structures for both forms of the indicator.
b. Generate and compare electrostatic potential surfaces for its two forms.
c. Predict the part of the indicator’s structure that is responsible for the visible color change then
compare that portion of the structure to the part predicted in Question 8.
d. How are they similar? Different?