Brønsted acid
Substances that donate protons (H+) in a chemical reaction.
The Arrhenius theory of acids and bases defines an acid as a substance that releases H+ ions in solution, and a base as a substance that releases OH- ions in solution.
We know that ammonia, NH3, can act as a base and exhibits alkaline properties, but contains no OH- ions. How is this possible?
In this lesson, we will learn about the Bronsted-Lowry theory of acids and bases, which fills in the gaps in the Arrhenius theory.
You have learned that acids are substances with a pH lower than 7.
We can test for acids using universal indicator solution or paper, which turns red or orange, all depending on the strength of the acid.
Blue litmus paper also turns red in the presence of an acid.
An example of an acid that you are likely familiar with is hydrochloric acid.
Hydrogen chloride dissolves in water to give hydrochloric acid.
Hydrogen chloride will dissociate to give a H+ ion and a Cl- ion.
HCl H+ and Cl-
The H+ ion is not just “released”, but it is transferred to another substance.
HCl(g) + H2O(l) H3O+(aq) + Cl-(aq)
For our example, water accepts the H+ ion to form a hydronium ion, H3O+.
It is also known as an oxonium ion.
A Bronsted-Lowry acid is a substance that releases or donates H+ ions in solution, in our case, HCl.
A Bronsted-Lowry base is a substance that accepts H+ ions in solution, in our case, H2O.
Notice that the definition of a Bronsted-Lowry acid or base is not a function of pH.
So an acid-base reaction, according to the Bronsted-Lowry theory, involves the transfer of a H+ ion from one substance to another.
In the introduction, we mentioned that ammonia acts as a base even though it does not release OH- ions, which is stipulated in the Arrhenius theory.
When ammonia dissolves in water, it accepts a H+ ion from water.
NH3(g) + H2O(l) NH4+(aq) + OH-(aq)
Notice that this reaction also involves a H+ ion transfer.
Can you figure out which substance is acting as an acid in this example? Which is acting as a base?
Please pause the lesson to think about this, and resume when you are ready.
In this example, water acts as a Bronsted-Lowry acid because it releases a H+ ion.
Ammonia acts as a Bronsted-Lowry base because it accepts a H+ ion.
A base, according to the Bronsted-Lowry theory, does not need to have a pH greater than 7 or turn red litmus paper blue, or turn universal indicator solution or paper blue or purple.
The only requirement is that it is able to accept H+ ions.
Have you noticed that H2O acted as a base in our first example but as an acid in the second example?
Substances that can act as either an acid or a base are amphoteric.
An easy way to remember this term is to recall that frogs are amphibians – they live on land and water.
In conclusion, according to the Bronsted-Lowry theory, an acid is a substance that dissociates to release or donate H+ ions.
A Bronsted-Lowry base is a substance that accepts H+ ions.
Therefore, an acid-base reaction, according to this theory, involves the transfer of a proton.
We have previously learned about the Bronsted-Lowry theory of acids and bases.
We will now learn about conjugate acids and bases using our knowledge of the Bronsted-Lowry theory.
Let's do a quick warm up.
Vinegar is a 5% aqueous solution of acetic acid, CH3COOH.
Vinegar is used in cooking worldwide and is sometimes used as a cleaning agent.
From the equation which describes the behaviour of acetic acid in water, could you identify the Bronsted-Lowry acid and Bronsted-Lowry base?
Pause and resume when ready.
CH3COOH(l) + H2O(l) CH3COO-(aq) + H3O+(aq)
The Bronsted-Lowry acid is acetic acid because it releases a proton.
Water accepted a proton from acetic acid, so it is a Bronsted-Lowry base.
As we have learned, an acid-base reaction, all according to the Bronsted-Lowry theory, involves the transfer of a proton.
Let's now look at what has been formed from this reaction.
The acetate ion (CH3COO-) is formed when acetic acid released a proton.
The acetate ion is the conjugate base in this reaction.
Can you remember what this (H3O+) is called?
Pause and resume when ready.
This is a hydronium ion, also known as an oxonium ion. The hydronium ion is formed when water accepted a proton from acetic acid. This is the conjugate acid in this reaction.
When a Bronsted-Lowry acid releases a proton, the resulting species is its conjugate base.
When a Bronsted-Lowry base accepts a proton, the resulting species is its conjugate acid.
Acetic acid and the acetate ion is a conjugate pair.
Can you identify the other conjugate pair in this reaction?
Pause and resume when ready.
Water and the hydronium ion is the other conjugate pair.
An acid and its conjugate base is a conjugate pair.
A base and its conjugate acid is also a conjugate pair.
Now, are you ready for a challenge?
For this reaction, try to identify the Bronsted-Lowry acid, Bronsted-Lowry base, the conjugate base, the conjugate acid, and the conjugate pairs.
H2SO4(aq) + NH3(aq) HSO4-(aq) + NH4+(aq)
Grab a piece of paper and pencil, pause the lesson and resume when ready.
The Bronsted-Lowry acid is H2SO4 as it releases a proton to ammonia.
Ammonia accepted the proton, so it is the Bronsted-Lowry base.
HSO4- is the conjugate base of H2SO4
The ammonium ion is the conjugate acid of ammonia.
Therefore, H2SO4 and HSO4- form a conjugate pair, and ammonia and the ammonium ion form another conjugate pair.
Final Summary
In summary, a conjugate base is formed when a Bronsted-Lowry acid releases its proton, and a conjugate acid is formed when a Bronsted-Lowry base accepts a proton.
A Bronsted-Lowry acid and its conjugate base is a conjugate pair, as is a Bronsted-Lowry base and its conjugate acid.