notes Unit 12 - Acid-base Equilibria

ACID - BASE EQUILIBRIA

The Bronsted – Lowry definition of acid and bases.

According to the Bronsted Lowry theory, an acid is a proton donor. Base is a proton accepter.

Eg: 1. HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (l)
During the reaction the proton (H+ ion) donated by the HCl acid is accepted by the OH- ion in NaOH
to form water. Therefore, HCl is an acid, NaOH is a base.

HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (l)
(H+ + Cl- + Na+ + OH- → Na+ + Cl- + H2O)

Ionic equation :- H+(aq) + OH-(aq) → H2O(l)


2. NH3 (g) + HCl (g) → NH4Cl (s)
Base acid salt

According to the Bronsted Lowry theory, some compounds in the gaseous phase are even
considered to be acids and bases.

During the reaction the H+ ions donated by HCl, have been accepted by NH3 to form NH4+

(NH3 + H+ → NH4+)

Conjugate acids and bases
▪ Consider the ionization in the week acid CH3COOH, in water.


CH3COOH (aq) + H2O (l) CH3COO - (aq) + H3O+ (aq)
Acid Base Conjugate Conjugate
Base Acid

▪ In the forward reaction, the CH3COOH acts as an acid, donating a proton (H+) to a water molecule. Water
accepts the proton and acts as a base.
H2O + H+ → H3O+
The acceptance of H+ is carried out via the lone electron pairs on the oxygen of H2O.


Bases always have a lone pair/lone pairs to accept H+ ions. Acid always lose H+ ions to bases.

▪ In the reverse reaction, the H3O+ ion acts as an acid, donating a proton (H+) to the CH3COO - ion, which of
course acts as a base.

H3O+ → H2O + H+



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,▪ In the before equilibrium reaction, it is always possible to find the two acids (CH3COOH and H3O+) and two
bases (H2O and CH3COO -).
▪ In each case, the acid on one side is formed from the base on the other side. They are called conjugate
acid base pairs.
▪ CH3COOH is the conjugate acid of CH3COO -, which means that CH3COO - is the conjugate base of CH3COOH.

Similarly, H3O+ is the conjugate acid of H2O, so H2O is the conjugate base of H3O+.



Acid Base + H+


Water has both the properties of an acid and a base.

▪ In the reaction between H2O and NH3, water acts an acid as it donates a proton to NH3, hence NH3 acts as
a base.

NH3 (aq) + H2O (l) NH4+ (aq) + OH- (aq)
Base Acid Conjugate Conjugate
acid base
▪ H2O behaves as a base when it reacts with substances such as CH3COOH, accepting a proton in the
reaction.

CH3COOH (aq) + H2O (l) CH3COO - (aq) + H3O+ (aq)
Acid Base Conjugate Conjugate
Base Acid


Water is an amphoteric substance as it has both acidic and basic properties.



Strong and week acids
▪ Strong acids
It’s an acid that is completely ionized/dissociated, in aqueous solutions.

Hence a higher concentration of H+ ions is produced.

Eg: HCl, HNO3, H2SO4

▪ HCl (aq)→ H+ (aq) + Cl- (aq)

[ HCl (aq)+ H2O (l) →H3O+(aq) + Cl-(aq) ]

▪ HNO3 (aq)→H+ (aq) + NO3- (aq)
[HNO3 (aq) + H2O (l) → H3O+ (aq) + NO3- (aq)]


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, ▪ Weak acids
It’s an acid that is partially ionized/dissociated, in aqueous solutions.
Therefore, a low concentration of H+ ions is resulted.

Eg: CH3COOH – Ethanoic acid
CH3CH2COOH - Propanoic acid
H2CO3 – Carbonic acid

CH3COOH (aq) CH3COO- (aq) + H+ (aq)
[CH3COOH (aq) + H2O (l) CH3COO- (aq) + H3O+ (aq) ]

▪ The substances in a weak acid are in an equilibrium system, therefore the equilibrium expression
for the above equilibrium is as follows:




Strong and week bases
Strong Bases
It’s a base that is completely ionized/dissociated, in aqueous solutions.
As a result of the complete ionization, a higher concentration of OH- ions is formed, thus the ability to
accept protons is greater.

Eg: NaOH, KOH, Ba(OH)2
NaOH (aq) →Na+ (aq) + OH- (aq)
Ba(OH)2 (aq) → Ba2+ (aq) + 2 OH- (aq)


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