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

Key Concepts

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:

H


Cl

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-.

H+ +


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:

H+ +


Cl

H


Cl

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:

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):

Monoprotic Acid/Conjugate Base Pairs, and, Base/Conjugate Acid Pairs
acid (2) −H+
conjugate base   base +H+
conjugate acid
HF
hydrofluoric acid
  F-
fluoride
F-
fluoride
  HF
hydrofluoric acid
HCl
hydrochloric acid
Cl-
chloride
Cl-
chloride
HCl
hydrochloric acid
HBr
hydrobromic acid
Br-
bromide
Br-
bromide
HBr
hydrobromic acid
HI
hydroiodic acid
I-
iodide
I-
iodide
HI
hydroiodic acid
HNO3
nitric acid
NO3-
nitrate
NO3-
nitrate
HNO3
nitric acid
HNO2
nitrous acid
NO2-
nitrite
NO2-
nitrite
HNO2
nitrous acid
HClO4
perchloric acid
ClO4-
perchlorate
ClO4-
perchlorate
HClO4
perchloric acid
HClO3
chloric acid
ClO3-
chlorate
ClO3-
chlorate
HClO3
chloric acid
HClO2
chlorous acid
ClO2-
chlorite
ClO2-
chlorite
HClO2
chlorous acid
HClO
hypochlorous acid
ClO-
hypochlorite
ClO-
hypochlorite
HClO
hypochlorous acid
HBrO4
perbromic acid
BrO4-
perbromate
BrO4-
perbromate
HBrO4
perbromic acid
HBrO3
bromic acid
BrO3-
bromate
BrO3-
bromate
HBrO3
bromic acid
HBrO2
bromous acid
BrO2-
bromite
BrO2-
bromite
HBrO2
bromous acid
HBrO
hypobromous acid
BrO-
hypobromite
BrO-
hypobromite
HBrO
hypobromous acid
HCN
hydrogen cyanide
CN-
cyanide
CN-
cyanide
HCN
hydrogen cyanide

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 ...

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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 SO42-.
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 HPO42-.
And HPO42- can act as a Brønsted-Lowry acid as well by donating 1 proton to produce the conjugate base PO43-.
Note that PO43- 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:

The table below lists some polyprotic acids:

  Polyprotic acid Reason
Diprotic acids H2CO3
carbonic acid
able to donate 2 protons
H2SO4
sulfuric acid
able to donate 2 protons
H2SO3
sulfurous acid
able to donate 2 protons
Triprotic acids H3PO4
phosphoric acid
able to donate 3 protons
H3PO3
phosphorous acid(3)
able to donate 3 protons

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 SO42-.
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 ...

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Amphiprotic Substances

An amphiprotic substance is one which can either:

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 SO42-
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 CO32-
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:

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

conjugate acid +H+
amphiprotic substance −H+
conjugate base
H3O+
oxidanium, oxonium or hydronium
  H2O
water
  OH-
hydroxide
NH4+
azanium or ammonium
NH3
ammonia
NH2-
azanide or amide
H2CO3
carbonic acid
HCO3-
hydrogencarbonate
CO32-
carbonate
H2SO4
sulfuric acid
HSO4-
hydrogensulfate
SO42-
sulfate
H2SO3
sulfurous acid
HSO3-
hydrogensulfite
SO32-
sulfite
H3PO4
phosphoric acid
H2PO4-
dihydrogenphosphate
HPO42-
hydrogenphosphate
H2PO4-
dihydrogenphosphate
HPO42-
hydrogenphosphate
PO43-
phosphate
H3PO3
phosphorous acid
H2PO3-
dihydrogenphosphite
HPO32-
hydrogenphosphite
H2PO3-
dihydrogenphosphite
HPO32-
hydrogenphosphite
PO33-
phosphite

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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.

formic acid
(methanoic acid)
−H+
formate ion
(methanoate ion)
 

O

     
   
       
   
 
   
H

C

O

H
       
   
−H+
 

O

     
   
       
   
 
   
H

C

O

       
   
HCOOH −H+
HCOO-

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).

acetic acid
(ethanoic acid)
−H+
acetate ion
(ethanoate ion)
CH3−COOH −H+
CH3−COO-

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+ + HPO42-

  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 HPO42-
    or
    (ii) gain a proton to form H3PO4


Footnotes:

(1) Hydrogen exists naturally as a mixture of isotopes.
For a hydrogen atom that has lost an electron, IUPAC prefers the term "hydron" derived from the name hydrogen, or the name hydrogen(1+).
This means acids donate a hydron while bases accept a hydron.
But because most of the naturally occurring hydrogen atoms on Earth have only 1 proton and 0 neutrons in the nucleus, when hydrogen atoms lose an electron the vast majority of the ions produced will simply be protons, 1H+ (protium is the IUPAC systematic name).
The term "proton" has been in use for so long that it is likely to continue for a long time to come.

(2) How do you name inorganic acids? Tricky question. There are the "acid names" we have used here, the "hydrogen names" and you could also name them using IUPAC systematic additive or substitutive nomenclature, or based on their composition.
For example, HCN can be named as:
hydrogen cyanide (compositional name)
hydridonitridocarbon (additive nomenclature)
methylidyneazane (substitutive nomenclature)
hydrogen(nitridocarbonate) (hydrogen name)
formonitrile (functional organic nomenclature).

(3) Note the spelling here! The element with the symbol P is spelt phosphorus, while the oxoacid (oxyacid) H3PO3 is named as phosphorous acid.

(4) You can also use the term "electrically neutral" to refer to a compound that has no net overall charge (no formal charge), that is, a compound that is not an ion, an uncharged molecule.
But beware! Neutral also refers to a solution in which neither acid nor base is in excess.
Water is "neutral" in these acid-base terms as well as being "electrically neutral", whereas NH3(aq) for example is not neutral in an acid-base sense but is "electrically neutral" in that the molecule has no net charge (no formal charge).

(5) The IUPAC document seems quite emphatic when it comes to this question of how to name H3O+.
IUPAC recommends the name oxidanium (systematic substitutive nomenclature), but accepts the alternative oxonium (non-systematic) but does NOT accept hydronium as an acceptable name. Hydronium is referred to as an obsolete name.
Refer to IUPAC Red Book Nomenclature of Inorganic Chemistry IUPAC Recommendations 2005.

(6) How do you name the conjugate base of an inorganic acid? Another tricky question for the same reason as above. We have chosen to use the "hydrogen names" because we think you are more likely to know these and recognize them.
NOTE: in the names of the anions derived from inorganic acids, the "hydrogen" word is attached as a prefix to the rest of the name,
For example, HCO3- is known as the hydrogencarbonate ion NOT as the hydrogen carbonate ion.
However, organic chemists are more likely to refer to HCO3- as the hydrogen carbonate ion rather than as the hydrogencarbonate ion.

(7) For IUPAC nomenclature of alkanes, go to Naming Straight-chain Alkanes Tutorial

(8) For IUPAC nomenclature of alkanoic acids go to Naming Straight-chain Alkanoic Acids Tutorial