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Strength of Conjugate Acids and Strength of Conjugate Bases Tutorial

Key Concepts

acidconjugate basedominant reaction
stronger weaker
strong
acids
HClO4ClO4-weak
bases
acid H+ + base
H2SO4HSO4-
HClCl-
HNO3NO3-
H3O+H2O
moderately
weak
acids
H2SO3HSO3-moderately
weak
bases
acid H+ + base
(equilibrium)
HSO4-SO42-
HFF-
CH3COOHCH3COO-
HNO2NO2-
H2SHS-
HSO3-SO32-
NH4+NH3
HCO3-CO32-
weak
acids
H2OOH-strong
bases
acidbase + H+
HS-S2-
OH-O2-
weaker stronger
conjugate acidbase

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Strong Acid - Weak Conjugate Base Pair

A strong Brønsted-Lowry acid is one which has strong tendency to donate a proton3:
Strong acids include H3O+, HCl and HNO3.
For example, in water, a strong acid like hydrochloric acid readily donates a proton to a water molecule:

acid + base
(water)
conjugate base
of HCl acid
+ conjugate acid
of base H2O
HCl + H2OCl- + H3O+

The equilibrium position lies so far to the right that we usually assume that the HCl molecule completely dissociates in water:
acid + base
(water)
conjugate base
of HCl acid
+ conjugate acid
of base H2O
HCl + H2OCl- + H3O+

This means that the reverse reaction does not occur to any appreciable extent, that is Cl- does not accept a proton from H3O+.
So Cl- must be a weak base, it has very little tendency to accept a proton.

The conjugate base of a strong acid is a weak base.

Moderately Weak Acid - Moderately Weak Conjugate Base Pair

A moderately weak Brønsted-Lowry acid has only a slight tendency to donate a proton.
Moderately weak acids include CH3COOH, HNO2, H2S, NH4+, HCO3-

For example, acetic acid (ethanoic acid) is a moderately weak acid in aqueous solution.
Some of the acetic acid (ethanoic acid) molecules dissociate in water, producing acetate ions (ethanoate ions) and protons.
Some of the acetate ions (ethanoate ions) and protons reform acetic acid (ethanoic acid), so that the system is in equilibrium:

acid + base
(water)
conjugate base
of HCl acid
+ conjugate acid
of base H2O
CH3COOH + H2OCH3COO- + H3O+

Since both the forward and reverse reactions are occurring to some extent, acetic acid (ethanoic acid) which is donating a proton is considered a moderately weak acid, and acetate ions (ethanoate ions) which are accepting protons are considered moderately weak bases.

The conjugate base of a moderately weak acid is a moderately weak base.

Weak Acid - Strong Conjugate Base Pair

A weak Brønsted-Lowry acid has very little tendency to donate a proton.
Weak Brønsted-Lowry acids include H2O2, CH3OH and H2O.

For example, methanol is a weak Brønsted-Lowry acid:

acid + baseconjugate base
of CH3OH acid
+ conjugate acid
of H2O base
CH3OH + H2OCH3O- + H3O+

Since very little of the methanol dissociates to produce protons, the equilibrium position lies very far to the left:
acid + baseconjugate base
of CH3OH acid
+ conjugate acid
of H2O base
CH3OH + H2OCH3O- + H3O+

The tendency for CH3O- to gain a proton is much greater than the tendency for CH3OH to lose a proton, so CH3OH is a weak acid, but CH3O- is a strong base.

The conjugate base of a weak acid is strong base.

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Strong Base - Weak Conjugate Acid Pair

A strong Brønsted-Lowry base has a strong tendency to accept a proton.
Strong Brønsted-Lowry bases include OH-, CH3CO- and HO2-.
For example, the hydroxide ion is a strong base:

base + acidconjugate acid
of OH- base
+ conjugate base
of acid H3O+
OH- + H3O+H2O + H2O

The equilibrium position for this reaction lies so very far to the right that we assume the reaction goes to completion:
base + acidconjugate acid
of OH- base
+ conjugate base
of acid H3O+
OH- + H3O+H2O + H2O

Since the water molecules formed as the conjugate acid of OH- have very little tendency to donate protons, they are only very weak acids.

The conjugate acid of a strong base is only a weak acid.

Moderately Weak Base - Moderately Weak Conjugate Acid Pair

A moderately weak Brønsted-Lowry base has only a slight tendency to accept a proton.
Moderately weak Brønsted-Lowry bases include, NH3, CO32-, HS-, NO2- and CH3COO-.

For example, ammonia, NH3, is a moderately weak base.
There is some tendency for NH3 to accept a proton to form NH4+, but the NH4+ also has some tendency to donate a proton to form NH3.
The reaction forming NH4+ is in equilibrium with the reaction forming NH3:

base + acidconjugate acid
of NH3 base
+ conjugate base
of acid H2O
NH3 + H2ONH4+ + OH-

Since both the forward and reverse reactions are occuring to some extent, it can be seen that the tendency for NH3 to accept a proton is similar to the tendency for NH4+ to donate a proton, so that the NH3 is considered to be a moderately weak base while the NH4+ is considered to be a moderately weak acid.

The conjugate acid of a moderately weak base is a moderately weak acid.

Weak Base - Strong Conjugate Acid Pair

A weak Brønsted-Lowry base shows very little tendency to gain a proton.
Weak Brønsted-Lowry bases include H2O, Cl- and NO3-.

For example, the chloride ion is a weak Brønsted-Lowry base:

base + acidconjugate acid
of Cl- base
+ conjugate base
of acid H2O
Cl- + H2OHCl + OH-

Since Cl- displays almost no tendency to accept a proton, the equilibrium position lies very far to the left:
base + acidconjugate acid
of Cl- base
+ conjugate base
of acid H2O
Cl- + H2OHCl + OH-

This means that any HCl that might form, readily donates a proton to reform the Cl-.
Cl- has little tendency to gain a proton so it is a weak base, but its conjugate acid, HCl, has an enormous tendency to donate a proton it is a strong acid.

The conjugate acid of a weak base is a strong acid.

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Footnotes:

1. The terms "acid" and "base" refer to a Brønsted-Lowry acid and a Brønsted-Lowry base.
(See the tutorial on Acid and Base Definitions)

2. The term "strong" is quite well defined for a dilute solution of monoprotic acid, such an acid is strong if its percentage dissociation (ionisation) is approximately 100%.
The term "weak" is not well defined, that is, a weak acid is one that only partially dissociates (ionises).
For this reason there is a continuum of weakness of acid, that is, some weak acids are weaker than others.
We have chosen to use the terms "weak" for those that undergo very little dissociation, and the term "moderately weak" for those that are not so "weak".
You will also see comparative terms such as weak, very weak, and, very, very weak (even feeble) used to describe this continuum of weakness.
Unless you have been given a list of acids to memorise as weak, very weak etc, these terms are quite arbitrary, and should only be used to compare the strength of one acid with another.
The strength of a weak acid is best described using the value of its acid dissociation constant (acid ionisation constant), Ka (or pKa).
It is far more important that you understand that the strength of an acid (or base) and its conjugate base (or acid) is based on the equilibrium position for the dissociation (ionisation) reaction.

3. IUPAC prefers the term "hydron" rather than "proton" for the positively charged hydrogen ion.
We use the term proton because most of the naturally occurring hydrogen is hydrogen-1, 1H.
When this isotope of hydrogen loses its electron, what is left is just a proton.
However, there is a very small percentage of naturally occurring hydrogen-2, 2H.
When this hydrogen atom loses its electron, the result is a nucleus containing both a proton and a neutron.
"Hydrogen" refers to the naturally occurring mix of hydrogen-1 and hydrogen-2, "hydron" refers to the naturally occurring mix of positively charged ions of naturally occurring hydrogen and is a much better description than "proton".
Most of the literature uses "proton" rather than "hydron".