Acid/Base Reaction Concepts
Major concepts
 According to Bronsted acid/base theory, an acid is a proton donor (capable of proton
dissociation) and a base is a proton acceptor
 If a Bronsted acid donates a proton, it becomes its conjugate base, which is now capable of
accepting a proton.
 Strong acids completely dissociate in water (and most solvents). This leads to a protonated
solvent, in which the proton is shuttled throughout the solvent.
 Acid/base reactions can be described by an arrow mechanism, which gives information about
order and timing of bond making and bond breaking.
 A strong acid will protonate the most reactive (least stable) lone pair of a molecule
Vocabulary
 Proton dissociation
 alkoxide
 Arrow mechanism
 Conjugate acid/conjugate base
 Strong acids
 hydronium
 Protonated solvent
Students should be able to:
 Draw the conjugate base of any acid
 Recognize HI, HBr, HCl, H2SO4, and H3PO4 as strong acids
 Draw an arrow mechanism for reaction of a strong acid with a solvent
 Explain what an arrow mechanism means for an acid/base reaction
 Predict the product of the reaction of a strong acid with a multi-lone pair base
Daily Problems
1. Draw the conjugate base of these acids, assuming dissociation of the proton in bold.
2. Draw all possible conjugate bases of Vitamin C that are alkoxide ions.
3. In the following reactions, identify the strong acid and the solvent. Then draw an arrow mechanism
and predict the products of these reactions.
4. If a chemist wants to make a dilute aqueous acid, it usually doesn’t matter if he adds HCl or sulfuric
acid to the water. They are basically the same.
A. Draw the products of HCl added to water, and then the products of sulfuric acid added to
water.
B. Why are HCl and sulfuric acid almost interchangeable in making a dilute aqueous acid?
The acid produced in both cases is the same, only the counterion changes.
5. The arrow mechanism for the acid/base reaction below tells about which bonds are being made,
which are being broken, and if they are being broken and made at the same time or different times.
Write out a paragraph that explains this arrow mechanism in words. Try to sound like a chemist: use
functional group names and words like “protonate” and “deprotonate.”
The lone pair on the amine forms a bond with a hydrogen atom while the bond between the hydrogen
and oxygen of hydronium becomes a lone pain on oxygen. The protonation of the amine happens
simultaneously with the Deprotonation of the hydronium.
Cumulative problems
6. In these molecules, which lone pair is the most reactive?
localized
less
electronegative
7. Draw an arrow mechanism and predict the products of the reaction of these strong acids with these
compounds.
Notice the correlation from the answers in #6.
8. Which lone pair in these molecules is protonated with a strong acid? Be sure to consider their “real”
electron distribution with concepts of resonance, induction, hybridization, delocalization, and
aromaticity.
delocalized
delocalized
localized
localized
most reactive
most reactive
Extension problem
9. Acid/base reactions don’t require one of the five strong acids you have learned. In principle, any two
compounds might do an acid/base reaction. (We will learn when they do and don’t.) But this will
require you to figure out which is functioning as the acid and which is functioning as the base. For the
reactions below, use the products to determine which reactant was the acid and which was the base.
Label the reactants. Then label the products as the “conjugate acid” or the “conjugate base.”
B
A
A
CA
CB
B
CB
CA
Based on your answers, why is it better to say that a compound is “functioning as an acid or base” rather
than saying it IS an acid or base?
Notice that the amine can fuction as either an acid or a base depending on the molecule it is paired
with.