3.1.12.1 Brønsted–Lowry acid–base equilibria in aqueous solution

What is a Bronsted-Lowry acid?

Proton donor
Releases H⁺ ions when mixed with H₂O

3 types of bases

Metal …….. eg NaO

Meta ……….. eg) NaOH

Metal ……….. eg) Na_2CO_3

oxides

hydroxides

carbonates

Base vs alkalis solubility

Alkali = A base that is soluble in water. All alkalis are bases
Bases = Insoluble in water

Are most hydroxides soluble or insoluble in water ?

Most hydroxides are SOLUBLE in water so are alkaline

E.g. the alkaline earth metals NaOH (aq)

Common acids, bases and alkalis examples: (they can be in/soluble)

Acids: HCl, H₂SO₄, HNO₃
(Insoluble) Bases: Zn(OH)₂
(Soluble) Alkalis: NaOH, KOH, NH₃

What is a Bronsted-Lowry acid?

and what does it do in water

Proton donor
Releases H⁺ ions when mixed with H₂O

Which arrow sign should be put for weak acids and strong acids?

Weak acids: ⇌ (shows partially dissociation)
Strong acids : → (shows full dissociation)

What is a Bronsted-Lowry base ?

Proton acceptor

Can H₂O act as a base?

Yes

How are strong and weak acids/ bases different?

Strong acids/bases COMPLETELY dissociate/ ionise in aqueous solutions to release H⁺ ions

Weak acids/bases only SLIGHTLY dissociate/ ionise to release small amounts of H⁺ ions

Name a few common weak acids

Ethanoic acid (CH₃COOH)
Phosphoric acid (H₃PO₄)
and other carboxylic acids

Where does the equilibrium lie with weak acids?

Towards the LEFT so the backwards reaction is favoured so not many H⁺ is produced

Name strong acids

HCl hydrochloric acid
H₂SO₄ sulphuric acid
HNO₃ nitric acid

Where does the equilibrium lie with strong acids?

Towards the RIGHT so the forward reaction is favoured so lots of H⁺ is produced

Name Strong Bases

Group 1 metal hydroxides

NaOH, LiOH, KOH, Ca(OH)₂, Ba(OH)₂

Where does the equilibrium lie with strong bases ?

Towards the RIGHT so the forward reaction is favoured so lots of OH⁻ is produced

Name some weak bases

Ammonia (NH₃), Amines

Where does the equilibrium lie with weak bases?

Towards the LEFT so the backwards reaction is favoured so not many OH⁻ ions are produced

Conjugate acid

Each acid is linked to a conjugate base on the other side of the equation

Base + H⁺ ⇌ Conjugate acid

Conjugate base

Each base is linked to a conjugate acid on the other side of the equation

Acid ⇌ Conjugate base + H⁺

What is the conjugate acid of OH⁻ ?

OH⁻ + H⁺ ⇌ H₂O

OH⁻ and H₂O are called a conjugate acid-base pair

What is the conjugate base of HF ?

F⁻

HF ⇌ F⁻ + H⁺

HF and F⁻ are called a conjugate acid-base pair

Whenever a substance acts as a Brønsted-Lowry base, we call the product it forms its...

Conjugate acid

Whenever a substance acts as a Brønsted-Lowry acid, we call the product it forms its...

Conjugate base

Conjugate pairs according to Brønsted-Lowry model

Every acid-base reaction has 2 conjugate pairs

NH₃ (aq) + H₂O (l) ⇌ NH₄⁺ (aq) + OH⁻ (aq)
Conjugate pairs: (NH₃ and NH₄⁺) & (H₂O and OH⁻)

What happens when 2 acids are mixed together?

The stronger acid will act as a Brønsted-Lowry acid, donating a proton
The weaker acid will act as a Brønsted-Lowry base, accepting a proton

Complete the equation for HCl (aq) + CH₃COOH (aq)

HCl (aq) + CH₃COOH (aq) ⇌ Cl⁻ (aq) + CH₃C(OH)₂⁺ (aq)

The HCl is a stronger acid so acts as a proton donor
The ethanoic acid is a weaker acid so acts as a proton acceptor

H₂SO₄ (aq) + CH₃COOH (aq) → HSO₄⁻ (aq) + CH₃C(OH)₂⁺ (aq)
Identify the acid and base according to the Brønsted-Lowry model.

Brønsted-Lowry acid: H₂SO₄
Brønsted-Lowry base: CH₃COOH

Identify the acid and base according to the Brønsted-Lowry model.
NH₃ + NH₃ → NH₄⁺ + NH₂⁻
Give your answers as compound names, rather than chemical formulae.

Brønsted-Lowry acid: ammonia 1
Brønsted-Lowry base: ammonia 2

Ammonium ions can be used as a weak acid in organic reactions. Write an equation for the dissociation of this acid.

NH₄⁺ ⇌ H⁺ + NH₃

Consider the following acid-base reaction:
C₅H₅N + HCl → C₅H₅NH⁺ + Cl⁻
What are the conjugate pairs ?

Conjugate acid = HCl, Conjugate base = Cl⁻

Conjugate acid = C₅H₅NH⁺, Conjugate base = C₅H₅N

Conjugate acid = HCl, Conjugate base = Cl⁻

Conjugate acid = C₅H₅NH⁺, Conjugate base = C₅H₅N

What is a monoprotic acid ? Give 3 examples

● An acid that donates ONE H⁺ ion for every acid molecule.
● So the concentration of the H⁺ ions is equal to the concentration of the acid HCl, HNO₃, HBr

What is a diprotic acid ?

● An acid that donates 2 H⁺ ions for every acid molecule.
● So The concentration of H⁺ ions is 2x the concentration of the acid [H⁺]= 2[Acid]
E.g. H₂SO₄

How do diprotic acids dissociate? Use H₂SO₄ as an example.

First: H₂SO₄ → H⁺ + HSO₄⁻
Second: HSO₄⁻ ⇌ H⁺ + SO₄²⁻

Dissociates 1 proton at a time, and the 1st proton dissociates more fully than the 2nd

Acidity

The higher the concentration of H⁺ the higher the acidity