Brønsted-Lowry Theory of Acids and Bases (definitions) Chemistry Tutorial

Key Concepts

• A Brønsted-Lowry acid is a proton donor.⁽¹⁾
• A Brønsted-Lowry base is a proton acceptor.
• A conjugate base is the product after a Brønsted-Lowry acid has donated a proton.
• A conjugate acid is the product after a Brønsted-Lowry base has accepted a proton.
• Monoprotic acid: Brønsted-Lowry acid is capable of donating only 1 proton.
• Polyprotic acid: Brønsted-Lowry acid is capable of donating more than 1 proton.

  Polyprotic acids can also be referred to by a name that designates how many protons can be donated:

  ⚛ Diprotic acid (di=2): a Brønsted-Lowry acid that can donate 2 protons

  ⚛ Triprotic acid (tri=3): a Brønsted-Lowry acid that can donate 3 protons

• Amphiprotic substance can either donate or accept a proton.

For reactions of Brønsted-Lowry acids and bases, go to the Proton-Transfer Reactions Tutorial

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Brønsted-Lowry Definition of Acids and Bases

A Brønsted-Lowry acid is a species that donates a proton.

Consider hydrogen chloride, HCl.
As a gas, HCl_(g), is a covalent compound in which the H atom provides 1 electron to the bonding pair of electrons and Cl provides 1 electron to the bonding pair of electrons as shown in the Lewis Structure (electron dot diagram) below:

When HCl is acting as a Brønsted-Lowry acid, it donates a proton, H⁺, that is a hydrogen atom that has lost its electron. This electron is retained by the Cl atom to form the chloride ion, Cl^-.

This chloride ion, Cl^-, is referred to as the conjugate base of the acid HCl.
Why is Cl^- any kind of base?
Because a Brønsted-Lowry base is defined as a species that is capable of accepting a proton, H⁺.

The chloride ion, Cl^-, with a complete octet of electrons (4 pairs of electrons) can accept a proton (a hydrogen atom with no electrons, H⁺) to form HCl as shown below:

Note that the chloride ion, Cl^- has provided both electrons to be shared between the Cl and H atoms in order to form the covalent bond between them (see Coordinate Covalent Bond (Dative Bond) Tutorial).

In this case, HCl is referred to as the conjugate acid of the base Cl^-

You will need to know the following definitions:

• A Brønsted-Lowry acid is a species that donates a proton (H⁺).
• A Brønsted-Lowry base is a species that accepts a proton (H⁺).
• A Brønsted-Lowry conjugate base is the species that results after an acid has dontated a proton.
• A Brønsted-Lowry conjugate acid is the species that results after a base has accepted a proton.

The table below lists some acid/conjugate base pairs, and, some base/conjugate acid pairs for acids that can donate only proton (referred to as monoprotic acids):

In the table above, we only listed acids that are generally capable of donating only 1 proton, these are called monoprotic acids (mono=1).
When a monoprotic acid loses a proton it produces a conjugate base which is an anion that contains no hydrogen atoms capable of being donated as protons (H⁺).
But there are lots of other species out there that are capable of donating more than one proton and these are referred to as polyprotic acids ...

Brønsted-Lowry Polyprotic Acids

A polyprotic acid is a Brønsted-Lowry acid that can donate more than one proton (poly = lots).

Consider H₂SO₄ which contains 2 hydrogen atoms, both of which can be donated as protons.
When a molecule of H₂SO₄ donates 1 proton it produces the conjugate base HSO₄^-.
But HSO₄^- could also act as a Brønsted-Lowry acid by donating 1 proton to produce the conjugate base SO₄^2-.
Overall, H₂SO₄ is a polyprotic acid, a Brønsted-Lowry acid capable of donating more than 1 proton.
Because H₂SO₄ is a Brønsted-Lowry acid capable of donating 2 protons, we can also refer to it as a diprotic acid (di=2).

Now consider H₃PO₄ which contains 3 hydrogen atoms, all of which can be donated as protons.
When H₃PO₄ donates 1 proton it produces the conjugate base H₂PO₄^-.
But H₂PO₄^- can also act as a Brønsted-Lowry acid by donating 1 proton to produce the conjugate base HPO₄^2-.
And HPO₄^2- can act as a Brønsted-Lowry acid as well by donating 1 proton to produce the conjugate base PO₄^3-.
Note that PO₄^3- has no hydrogen atoms that could be donated as protons so it is only able to act as a Brønsted-Lowry base, that is, it could accept a proton.
H₃PO₄ is a polyprotic acid because it is capable of donating more than 1 proton.
Because H₃PO₄ can donate 3 protons it is also referred to as a triprotic acid (tri=3).

You will need to remember the following definitions:

• Monoprotic acid: Brønsted-Lowry acid able to donate only 1 proton, H⁺
• Polyprotic acid: Brønsted-Lowry acid able to donate more than 1 proton, H⁺

  ⚛ Diprotic acid: Brønsted-Lowry acid able to donate 2 protons, H⁺

  ⚛ Triprotic acid: Brønsted-Lowry acid able to donate 3 protons, H⁺

The table below lists some polyprotic acids:

Now, if you think about sulfuric acid (H₂SO₄) for example, when it donates a proton it produces the conjugate base HSO₄^-.
And we said that HSO₄^- can itself act as an acid by donating a proton to produce its conjugate base SO₄^2-.
So why can't HSO₄^- act as a Brønsted-Lowry base and accept a proton to produce its conjugate acid H₂SO₄?
Well it can! HSO₄^- can either donate a proton (act as an acid) or accept a proton (act as a base) so it is referred to as an amphiprotic species or amphiprotic substance ...

Amphiprotic Substances

An amphiprotic substance is one which can either:

• donate a proton (act as a Brønsted-Lowry acid)

  OR

• accept a proton (act as a Brønsted-Lowry base)

The conjugate base of a polyprotic acid will be an amphiprotic substance.
Consider the diprotic acid H₂SO₄ which donates a proton to produce its conjugate base HSO₄^-.
HSO₄^- can act as a Brønsted-Lowry acid by donating a proton to produce its conjugate base SO₄^2-
HSO₄^- can act as a Brønsted-Lowry base by accepting a proton to produce its conjugate acid H₂SO₄
HSO₄^- is an amphiprotic species, it is able to either donate a proton or accept a proton.

Consider the diprotic acid H₂CO₃ which donates a proton to produce its conjugate base HCO₃^-.
HCO₃^- can act as a Brønsted-Lowry acid by donating a proton to produce its conjugate base CO₃^2-
HCO₃^- can act as a Brønsted-Lowry base by accepting a proton to produce its conjugate acid H₂CO₃
HCO₃^- is an amphiprotic species, it is able to either donate a proton or accept a proton.

A compound doesn't have to be an ion in order to be amphiprotic, but it does need to contain a hydrogen atom that can be donated as a proton, and, it does need to be capable of accepting a proton.

Water, H₂O, is an example of an uncharged molecule⁽⁴⁾ that is amphiprotic.

H₂O can act as a Brønsted-Lowry acid by donating a proton to produce its conjugate base OH^- (known as the hydroxide ion).

H₂O can act as a Brønsted-Lowry base by accepting a proton to produce its conjugate acid H₃O⁺ (known as the oxidanium ion, or oxonium ion, or, in older textbooks as the hydronium ion⁽⁵⁾).

Ammonia, NH₃, is another example of an uncharged molecule that is amphiprotic.

NH₃ can act as a Brønsted-Lowry acid by donating a proton to produce its conjugate base NH₂^-

NH₃ can act as a Brønsted-Lowry base by accepting a proton to produce its conjugate acid NH₄⁺

You will need to remember the following definition:

• An amphiprotic species is a species that is able to either donate a proton or to accept a proton.

The table below lists some amphiprotic substances and their conjugate acids and conjugate bases:⁽⁶⁾

A word of warning...

Compounds can contain hydrogen atoms yet NOT act as Brønsted-Lowry acids because they do not donate a proton to form a conjugate base.
Methane⁽⁷⁾, CH₄, contains 4 hydrogen atoms yet none of these hydrogen atoms are generally lost as protons in a chemical reaction so CH₄ is NOT a Brønsted-Lowry acid.
Ethane⁽⁷⁾, CH₃-CH₃, contains 6 hydrogen atoms yet none of these hydrogen atoms are generally lost as protons in a chemical reaction so CH₃-CH₃ is NOT a Brønsted-Lowry acid.

Compounds can contain lots of hydrogen atoms but have only 1 hydrogen atom that can be donated as a proton, these are Brønsted-Lowry monoprotic acids.
Formic acid (methanoic acid)⁽⁸⁾, HCOOH, has only 1 hydrogen atom that can be donated as a proton which is the hydrogen atom covalently bonded to the oxygen atom.

HCOOH is a Brønsted-Lowry monoprotic acid, it donates a proton to form the conjugate base HCOO^- (formate ion, or, methanoate ion).

Acetic acid (ethanoic acid)⁽⁸⁾, CH₃-COOH, is also a Brønsted-Lowry monoprotic acid, it can donate a proton to produce its conjugate base CH₃-COO^- (acetate or ethanoate ion).

In order to classify a substance as a Brønsted-Lowry acid or base we need to understand the structure and behaviour of the substance.

Sample Questions with Worked Solutions

Question 1. What is the molecular formula of the conjugate base of H₂S ?

Solution:

1. Define "conjugate base":

   A Brønsted-Lowry acid donates (loses) a proton (H⁺) to form a conjugate base.

2. Identify the Brønsted-Lowry acid in the question:

   H₂S must be the Brønsted-Lowry acid because we are asked to find its conjugate base

3. Write an equation to represent the Brønsted-Lowry acid donating (losing) a proton (H⁺) to produce its conjugate base:

   acid	→	proton	+	conjugate base
   H2S	→	H+	+	HS-
   H S H	→	H+	+	H S −

4. State your answer:

   HS^- is the conjugate base of the acid H₂S

Question 2: Explain why H₂PO₄^- is said to be amphiprotic.

Solution:

1. Define "amphiprotic":

   An amphiprotic substance is one that can either:
   (i) donate (lose) a proton
   or
   (ii) accept (gain) a proton

2. Write an equation to show H₂PO₄^- donating (losing) a proton:

   H₂PO₄^- → H⁺ + HPO₄^2-

3. Write an equation to show H₂PO₄^- accepting (gaining) a proton:

   H₂PO₄^- + H⁺ → H₃PO₄

4. H₂PO₄^- is amphiprotic because :

   H₂PO₄^- can either:
   (i) lose a proton to form HPO₄^2-
   or
   (ii) gain a proton to form H₃PO₄