Chemistry 30
Acids and Bases: The Bronsted-Lowry Definition
Can you remember the physical characteristics of acids and bases in the space below? What
characteristics do they have in common, and which are unique?
Physical Characteristics
Acids
Bases
Arrhenius Definition
Recall the Arrhenius definition of an acid and the Arrhenius definition of a base:
Acids are compounds that dissolve in water to form hydrogen (H+) ions.
Bases are compounds that dissolve in water to form hydroxide (OH-) ions.
This definition of acids turns out to be inadequate, because there are substances that do not
dissolve in water to form ions such as these but are still acids and bases. Because of this, a
model that accounted for these additional substances was derived.
Chemistry 30
Bronsted-Lowry Definition of Acids and Bases
Working independently, two different scientists developed a model for acids and bases that
better describes their characteristics and reactivity. Their names were Johannes Bronsted and
Thomas Lowry, and each (on their own) developed what is now known as the BronstedLowry Theory of acids and bases.
Acids are substances that act as hydrogen (or proton) donors.
Bases are substances that act as hydrogen (or proton) acceptors.
Let’s examine both acids and bases using this new definition, and see how our understanding
of them can change.
Chemistry 30
For acids, there is very little change in what we think of as an acid. We've already examined how acids
can produce a hydrogen ion. What does it mean, however, when we say it donates a proton?
First of all, how does a hydrogen ion relate to a proton? That's really what a hydrogen ion is - a single
proton. The most common form of a hydrogen atom consists of one proton and one electron (and no
neutrons). Hydrogen typically forms an ion by losing it's electron - all that remains is a single proton.
HCl (g)→ H+(aq) + Cl-(aq)
HCl(g) + H2O(l) → H3O+(aq) + Cl-(aq)
The first equation shows HCl acting as an Arrhenius acid by dissociating to form hydrogen
ions. In the second equation, we see HCl acting as a Bronsted-Lowry acid by donating its
proton to a water molecule. The resulting solution contains chloride ions, as well as
hydronium ions. It seems obvious that if something is donating protons, something else needs
to be accepting them. We see that water is acting as the material that accepts a hydrogen ion.
Hydronium ions can be thought of as another way of writing hydrogen ions. Often, the
dissociation of water is actually written in the following way:
H2O(l) + H2O(l) → H3O+(aq) + OH-(aq)
Chemistry 30
Let’s take a closer look at how a base can accept a hydrogen ion. By examining ammonia we
can do this quite easily.
NH3 (g) + H2O(l)
NH4+(aq) + OH–(aq)
In this reaction, ammonia is clearly acting as a Bronsted-Lowry base. We can see from looking
at both the reactant and product side of the equation that ammonia has accepted a hydrogen
ion from water. The resulting products are an ammonium ion and a hydroxide ion. Therefore,
even though NH3 does not dissolve in water to form hydroxide ions, it still produces them in
solution.
So, a base according to the Bronsted-Lowry definition is any molecule that is able to accept
hydrogen ions from another molecule. An electron dot model may help see how ammonia
accepts hydrogen ions from a water molecule:
Although technically it is not the definition for a base, it is often easy to associate bases with
hydroxide ions when we think about them in solution.
Chemistry 30
Conjugate Acid-Base Pairs
It is important to remember the concept of chemical equilibrium when we are discussing the
reactions of acids and bases. Consider the reaction of ammonia with water that we just
discussed above:
NH3 (g) + H2O(l)
NH4+(aq) + OH–(aq)
Remember that the chemical reaction is happening in both directions. On the left side of the
equation (the reactant side), ammonia is acting as a base and accepting a hydrogen ion from
water. On the right side of the equation, the ammonium ion (which was formed when
ammonia accepted a hydrogen ion) is acting as an acid and donating its proton to the
hydroxide ion. Therefore, in this situation, ammonium is acting as an acid.
What we have in this situation are CONJUGATE ACID-BASE PAIRS.
Conjugate acid-base pairs differ from each other by the presence or absence of a single
hydrogen ion (proton). Every acid has a conjugate base, and every base has a
conjugate acid.
Chemistry 30
In the ammonia reaction, water acted as an acid because it donated a proton (hydrogen ion) to
ammonia:
NH3 (g) + H2O(l) ↔ NH4+(aq) + OH–(aq)
Compare this to another reaction we looked at earlier when we saw how hydrochloric acid
acted as an acid by donating a proton to water:
HCl(g) + H2O(l) → H3O+(aq) + Cl-(aq)
In this reaction, water is acting as a base because it accepts a proton from HCl.
Because of its ability to act as both an acid and a base, we call water an amphiprotic
substance. There are many other types of amphiprotic substances, and often-times they are
also referred to as amphoteric substances. It is important to remember that the two terms
mean the same thing, but in this class I will only use the term amphiprotic.
Example: HSO4- is an amphiprotic substance. Write a reaction with water in which HSO4- acts
as an acid, as well as a reaction in which HSO4- acts as a base.
HSO4– + H3O+ ↔ H2SO4 + H2O
HSO4– accepts a proton from H3O+
HSO4– + OH– ↔ H2O + SO42-
HSO4– gives (donates) a proton to OH–