Acid Strength Module – General Chemistry II
GENERAL CHEMISTRY
Module developed by: David L. Cedeño
Acid Strength and Molecular Structure
Introduction
The strength of an acid is related to its molecular structure. For instance, you know that HCl, HBr, and
HI are strong acids, while HF is a weak one. Have you wonder why? Would you be able to tell if HI is
stronger than HBr and HCl?
The strength of an acid in aqueous media is related to its ability to transfer a proton to water. Such
ability is related to the how strong the proton is held by the rest of the molecule (i.e. the bond strength)
and the thermodynamic stability of the resulting conjugated base.
In this project you will calculate the enthalpy change (H) involved in the proton transfer dissociation of
the acid (say HA) according to:
HA(g) + H2O(g) → H3O+(g) + A–(g)
Note that the reaction H is expressed for all reactants and products in the gas phase, but this is for
simplicity because our software cannot model molecules in aqueous solutions. The results as you will
see are quite valid.
You will explore two series of acids: the haloacids (HF, HCl, HBr, and HI) and the chlorinated oxyacids
(HClO, HClO2, HClO3, and HClO4) and will make predictions based on your observations.
You are encouraged to review the following material in your textbook: Section 16. (Acid strength and
structure)
Objectives:
 To understand how the structure and electronic factors in an acid influence the bond strength.
Materials:
You will need Argus Lab 4.0 to carry out the molecular modeling and calculation of DH for proton
transfer. If you do not have Argus, a file is provided which contains all the results you would obtain
using Argus.
This handout provides general instructions to guide you (the warm-up sections) through but assumes you
are already familiar with the basics. The playroom section tells you to build, calculate, and analyze data
for other molecules following similar procedures to the ones learned in the warm-up.
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Acid Strength Module – General Chemistry II
PART A. The haloacid series
Warm-up Section
Using Argus Lab 4.0 built the following acid: HF
Clean up (click
) the molecule and Save (suggested name: hf)
Now calculate the energy for the optimized geometry of the molecule: Go to Calculation then Optimize
Geometry, leave everything as defaulted, except for:
QM: PM3
click OK and run (click
)
Analysis
An output file is generated named (hf.out). Use a text viewer to open it (click
). Scroll down
towards the end of the file to find the Properties section. There you will find the Heat of Formation (in
kcal/mol). This is the calculated enthalpy for making the molecule from its constituent elements
Record the Heat of Formation of HF here: _________ kcal/mol
Now built the water molecule and calculate its Heat of Formation in the same way you did for HF.
Record the Heat of Formation of H2O here: _________ kcal/mol
Now built H3O+ and calculate its Heat of Formation. When setting the Calculation/Optimize Geometry
make sure to adjust the Net Charge (in the Molecule section) to 1, so the number of valence electrons is
8.
Record the Heat of Formation of H3O+ here: _________ kcal/mol
The value of the heat of formation of F– is -31.23 kcal/mol.
Now use the thermodynamic relation below to obtain the H for the acid dissociation (proton transfer)
reaction:
HF(g) + H2O(g) → H3O+(g) + F–(g)
H = H(F–) + H(H3O+) – H(HF) – H(H2O)
where H(X) is the Heat of Formation of a compound X.
Record the H for HF proton transfer here:
_________ kcal/mol
What does the sign of H tells you?
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Acid Strength Module – General Chemistry II
Playroom:
Build, clean and calculate the Heat of Formation for the optimized geometry of the following acids (use
same settings for HF):
HCl, HBr and HI
When calculating H for these acids, you do not need to calculate the Heat of Formation of water and
H3O+ in this step. Use the values obtained above.
Now, complete the following table of results:
Acid
HF
HCl
HBr
HI
H(HA)
H(A–)
Electronegativity
H
(kcal/mol) (kcal/mol) (kcal/mol)
of halogen
-31.23
3.98
-51.22
3.16
-56.24
2.96
-64.62
2.66
Ka
6.8 x 10-4
107
109
1011
Keeping in mind that H measures how much energy is required to transfer the proton. What trends
do you see in the properties tabulated?
Explain the relationships between H and Ka, and how they relate to the electronegativity of the
halogen?
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Acid Strength Module – General Chemistry II
PART B. Oxyacid Series
The oxyacids are those formed by oxygenated atoms (non-metals and metals) in which the proton is
attached to one oxygen atom. For example the acids H2SO4, HClO3, HIO, HNO3 are oxyacids. In this
exercise you will understand the effect of the number of oxygens in the strength of the acid. We will use
the series HClO4, HClO3, HClO2, and HClO.
Warm-up Section:
Use Argus to build HClO4. The picture below indicates how this should be modeled:
Clean up (
) and Save (suggested name: hclo4)
Go to Calculation, then Optimize Geometry. Leave everything as defaulted except the following:
QM: PM3
Calculate Properties: Check to calculate the Mulliken atomic charges
click OK and run (click
)
Analysis
Open the output file (clicking
find the Properties section.
opens the file hclo4.out). Scroll down toward the end of the file to
Record the Heat of Formation here:
_________ kcal/mol
Record the Mulliken Charge for H: ________
Playroom:
Build, clean and calculate the energy and Mulliken charges for the optimized geometries of the
following acids (same as you did for HClO4):
HClO3, HClO2, and HClO
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Acid Strength Module – General Chemistry II
Make sure they have the correct geometry after the Calculation/Optimize Geometry step. HClO3 must
be pyramidal, while HClO2 and HClO are bent.
CalculateH for the generic reaction for the oxyacids, using the heat of formation of water and H3O+
calculated in the previous section:
HA(g) + H2O(g) → H3O+(g) + A–(g)
Complete the following table of results:
Acid
HClO4
HClO3
HClO2
HClO
H(HA)
H(A–)
Charge
H
(kcal/mol) (kcal/mol) (kcal/mol)
of H
-118.78
-142.08
-28.04
-30.03
Ka
1010
10
1.1 x 10-2
3.0 x 10-8
Keeping in mind that H measures how much energy is required to transfer the proton. What trends
do you see in the properties tabulated?
Explain the relationships between H and Ka, and how they relate to the number of oxygens in the
acid. Why is that an increase in the number of oxygens increases the strength of the acid?
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Acid Strength Module – General Chemistry II
Based on your observations in this exercise compare the acids below, which of them is the stronger?
Which one would have the largest H for proton transfer?
Acetic Acid
Trifluoroacetic Acid
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