CAChe Lab: Acids and Bases
Created by: Rebecca A. Kruse
Pre-lab Assignment:
Read “Calculating Electron Density and Electrostatic Potential with CAChe” and complete the section of the
laboratory procedure denoted (Prior to lab).
Pre-lab Discussion:
Acids and bases make up a broad range of compounds but their precise definitions have evolved over time.
In 1884 Arrhenius defined an acid as any substance that releases hydrogen ions (protons, H +) when mixed with
water; he defined a base as any substance that releases hydroxide ions (OH-). Arrhenius’s definitions were often
used to define the properties of acids and bases. Acids are molecular substances that (1) ionize when added to
water, (2) corrode active metals to produce hydrogen gas, (3) affect the colors of acid-base indicators, (4)
neutralize bases, and (5) have a sour taste. Bases are molecular substances that (1) affect the colors of acidbase indicators, (2) neutralize acids, (3) taste bitter and feel slippery, and 4) are electrolytes.
In 1923, Brønsted and Lowry broadened the definitions stating that an acid is any substance that can donate a
proton to another substance (proton donor) and a base is any substance that can accept a proton from another
substance (proton acceptor). According to Brønsted-Lowry theory, an acid-base reaction occurs when one
substance donates its most loosely held proton to another substance. This transfer of a proton illustrates the
characteristics of the conjugate acid-base pair (conjugate pair for short):
conjugate pair


HA + B  A- + BH+


conjugate pair
Also in 1923, Lewis further broadened the definitions of acids and bases stating that acids are electron pair
acceptors and bases are electron pair donors. While Lewis’s definitions have shaped the study of modern
organic chemistry (chemistry of carbon-containing compounds), they are usually covered in advanced chemistry
courses and will not be a focus here.
Generally the Brønsted-Lowry definitions of acids and bases are used to describe the molecular basis of acidbase chemistry. Again acids are defined as proton donors:
HA + H2O  A- + H3O+
The HA bonds in stronger acids are polarized H +A- so that H is already “proton-like.” Consequently, an acid’s
proton donating ability, or acid strength, which can be described by the acid dissociation constant, Ka, is
usually correlated with the partial charge on hydrogen, which can be modeled with an electrostatic potential map.
In this lesson you will investigate the nature of simple first-row compounds by comparing certain molecular
properties with their electrostatic potential maps.
Purpose:
In this exercise you will investigate the relationships between electronegativity, experimental Ka, partial atomic
charges, and electrostatic potential maps generated with CAChe for first row compounds.
Lab Procedure:
(Prior to lab) Use online resources, your textbook, or other reference texts such as the Merck Index or the CRC
Handbook to find the following:
1. Electronegativities of H, C, N, O, and F.
2. Experimental Ka values for CH4, NH3, H2O, and HF.
Follow the directions provided in “Calculating Electron Density and Electrostatic Potential with CAChe” to do
the following:
3. Open CAChe.
4. Calculate electrostatic potential maps for CH4, NH3, H2O, and HF.
5. Compare partial atomic charges and electrostatic potential maps for each molecule.
Questions:
Use your data to answer the following:
1. Which molecule contains the most electron-poor hydrogen (largest +)?
2. Which molecule contains the least electron-poor hydrogen (smallest +)?
3. What relationship, if any, exists between the partial atomic charge on hydrogen and the
electronegativity of the atom bonded to hydrogen? Make a general statement if possible and provide
evidence.
4. What relationship, if any, exists between the partial atomic charge on hydrogen and the experimental
Ka value? Make a general statement if possible and provide evidence.
5. List the compounds in the order of increasing acidity based on your general statements in 3 and 4.
Suggestions for Further Work/Inquiry:
Choose any household acid (there are many) then complete the following:
a. Obtain its 2-D stick structure and experimental K a using online resources, your textbook, or other
reference texts such as the Merck Index or the CRC Handbook.
b. Generate an electrostatic potential map for the acid.
c. Predict the most acidic proton in the molecule using the information collected and the general
statements you made above.
d. Create a PowerPoint slide that includes the name and common use(s) of your selected acid, the
information and prediction from a, b, and c, and any interesting discoveries you made about your acid
during this investigation.