Colourful Solutions > Proton transfer reactions > Conjugate acid-base pairs

The Mad Science Lab

Standard level

Conjugate means "partner", hence an acid may have a "partner" base, formed by the loss of a hydrogen ion.

Syllabus ref: R3.1.2

Reactivity 3.1.2 - A pair of species differing by a single proton is called a conjugate acid–base pair.

  • Deduce the formula of the conjugate acid or base of any Brønsted–Lowry base or acid.

Guidance

Tools and links

  • Structure 3.1 - What is the periodic trend in the acid–base properties of metal and non-metal oxides?
  • Structure 3.1 - Why does the release of oxides of nitrogen and sulfur into the atmosphere cause acid rain?

Conjugate acid - base pairs

The idea of conjugate acid - base pairs comes from the idea that all reactions are fundamentally reversible (if not in practice, at least in theory). When a proton is transferred to another species, that product is then capable, in turn, of transferring the proton back - in other words it is itself capable of behaving as an acid.

As this acid was created by accepting the proton in the first place it is called the conjugate (paired) acid of the original base. We say that the two species form an acid - conjugate base pair.

Example: Finding the conjugate partner in a reaction involves considering the reverse reaction or considering where the proton (hydrogen ion) goes to. In the following example consider the hydrogen ion and remember that the hydrogen ion donator is the acid.

CH3COOH + H2SO4 → CH3COOH2+ + HSO4-

On the left hand side it is the sulfuric acid that donates the proton. It is behaving as an acid. Its conjugate base is the species on the other side of the equation that would accept the proton to go in the reverse direction.

On the right hand side it would be the hydrogen sulfate ion (HSO4- ) that accepts the proton and therefore it is the conjugate base of the sulfuric acid.

On the left hand side of the equation the ethanoic acid accepts a proton and is therefore the Brønsted-Lowry base. It has a conjugate acid on the right hand side that can release a proton - this is the CH3COOH2+ion.

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Conjugate acid - base pair examples

Look at the following equations as examples of conjugate acid - base pairs. Remember that the conjugate partners are found by considering the reverse reaction.

acid
base
conjugate base
conjugate acid
H2SO4
+
CH3COOH
→
HSO4-
+
CH3COOH2+
             
CH3COOH
+
NH3
→
CH3COO-
+
NH4+
             
H2O
+
H2O
→
H3O+
+
OH-
             
HCl
+
H2O
→
Cl-
+
H3O+
             
proton donor
proton acceptor
⇋
proton acceptor
proton donor

Logically, if the reactions involve the transfer of a proton (hydrogen ion) from the acid to a base the structures of the acid and its conjugate base must differ in structure by that hydrogen ion.

Look at the acid and conjugate base in the above equations, and ou can see that the acid-conjugate base pairs differ by one hydrogen ion, as do the base-conjugate acid pairs.

The Syllabus states that the position of the hydrogen ion should be indicated. The hydrogen ion being transferrred to the base is accepted by the base. It must be accepted in a location where there is a lone pair of electrons to hold it.

In the first equation above, it is the lone pair on the oxygen atom of the ethanoic acid that accepts the proton. It is therefore correct to write the formula of the conjugate acid of the ethanoic acid (base) as CH3COOH2+, as this indicates that the proton is accepted by the oxygen (or one of them) of the carboxyl group. It would be incorrect to write the formula as CH4COOH+ as this would suggest that the hydrogen has been accepted by the wrong part of the structure.

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Brønsted-Lowry Summary

Worked examples

Q821-01 In the equilibrium shown below, which are the two conjugate bases?

CH3COOH(aq) + H2O ⇋ H3O+ + CH3COO-(aq)

  1. CH3COOH and H2O
  2. CH3COO- and H3O+
  3. CH3COOH and H3O+
  4. CH3COO- and H2O
Answer

A conjugate base of an acid accepts a proton (hydrogen ion), therefore on the left hand side water is the base and on the right hand side the ethanoate ion is the base.

Response D is correct, CH3COO- and H2O
Q821-02 Which statement describes the Brønsted-Lowry behaviour of H2O molecules in aqueous solutions?
  1. They cannot act as either acids or bases
  2. They can act as acids but not bases
  3. They can act as acids or bases when reacting with each other
  4. They can act as acids when reacting with HCl molecules
Answer

In aqueous solution the following reaction can be defined:

H2O + H2O → H3O+ + OH-

It may be seen that water is acting as both a base and an acid - response C

Q821-03 The ionisation of sulfuric acid is represented by the equations below:

H2SO4(aq) + H2O(l) ⇋ H3O+(aq) + HSO4-(aq)

HSO4- (aq) + H2O(l) ⇋ H3O+(aq) + SO42-(aq)

Which of the following is the conjugate base of HSO4-?

  1. H2O(l)
  2. H3O+
  3. H2SO4
  4. SO42-
Answer

The conjugate base is the species formed by removal of a proton from the HSO4- ion.

Therefore the answer is SO42-, response D
Q821-04 Which one of the following species can act as both a Brønsted-Lowry acid and base in aqueous solution?
  1. CH3COOH
  2. NO3-
  3. H3PO4
  4. OH-
Answer

The species must be able to release or accept a proton.

The only possible alternative is CH3COOH - response A
Q821-05 In the equilibrium below:

CH3COOH(aq) + H2O(l) ⇋ CH3COO-(aq) + H3O+(aq)

Which species represent a conjugate acid-base pair?

  1. CH3COOH / H2O
  2. CH3COO- / H3O+
  3. H2O / CH3COO-
  4. H3O+ / H2O
Answer

Conjugate acid-base pairs must only differ by one hydrogen ion.

Correct answer = H3O+ / H2O - response D

Q821-06 A Brønsted-Lowry base is defined as a substance which:
  1. accepts H+ ions.
  2. produces OH- ions.
  3. conducts electricity.
  4. donates protons
Answer

Brønsted-Lowry bases accept protons (hydrogen ions).

Correct response = A

Q821-07 Which of the pairs below is a conjugate acid- base pair in the following reaction?

HNO3 + H2SO4 ⇋ H2NO3+ + HSO4-

  1. HNO3 and H2SO4
  2. HNO3 and H2NO3+
  3. HNO3 and HSO4-
  4. H2NO3+ and HSO4-
Answer

Conjugate acid-base pairs only differ by one proton (hydrogen ion)

Therefore the correct answer = HNO3 and H2NO3+ ; response B
Q821-08 In the reaction:

2HCO3- ⇋ H2CO3 + CO32-

the hydrogen carbonate ion, HCO3- is functioning as

  1. a Brønsted-Lowry acid only.
  2. a Brønsted-Lowry base only.
  3. both a Brønsted-Lowry acid and a Brønsted-Lowry base.
  4. neither a Brønsted-Lowry acid nor a Brønsted-Lowry base.

Answer

The hydrogen carbonate ion, HCO3- is both losing and gaining a hydrogen ion. It is therefore functioning as both an acid and a base. Response C
Q821-09 In the reaction between the hydrogen sulfate ion and water, HSO4- + H2O ⇋ H3O+ + SO42-, the water acts as:
  1. an acid
  2. a base
  3. a salt
  4. an inert medium
Answer

The water is accepting a proton (hydrogen ion) and is therefore a base.

Response B
Q821-10 Which one of the following species acts as an acid according to the Brønsted model of acids and bases?
  1. O2-
  2. NH4+
  3. CH4
  4. NH2-
Answer

Brønsted theory deals with acids as proton (hydrogen ion) donors and bases as proton acceptors. The only one that can lose a proton, i.e. donate it, is NH4+

Response B

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