Acids,
bases and
pH
, Brønsted–Lowry acids and bases
Arrhenius model of acids & bases:
o Acids dissociate & release H+ ions in aqueous
solution
o Alkalis dissociate & release OH- ions in
aqueous solution. An alkali is a soluble base
o H+ ions are neutralised by OH- ions to form
water: H+(aq) + OH-(aq) H₂O(l)
Brønsted–Lowry model for acids & bases
extends the Arrhenius model to emphasise the
role of proton transfer between species:
o Brønsted–Lowry acid- proton donor
o Brønsted–Lowry base- proton acceptor
, Conjugate acid-base pair
Conjugate acid-base pair- contains 2 species that can be
interconverted by transfer of a proton.
The conjugate base of an acid is the parent acid minus one
proton.
The conjugate acid of a base is the parent base plus one
proton.
Acid-base equilibria involves 2 acid-base pairs
Acid Base Conjugate acid Conjugate base Equation
HNO₂ H₂O H₃O+ NO₂- HNO₂ + H₂O NO₂- + H₃O+
H₂O S2- OH- HS- S2- + H₂O OH- + HS-
Water is amphoteric meaning that it can act as an acid or a
base.
H₃O+ is a hydronium ion formed when H₂O behaves as a base &
accepts a proton to form its conjugate acid. However H+ is
seen written more commonly instead.
, Neutralisation
• Monobasic- 1 hydrogen atom in the acid e.g. HCl
• Dibasic- 2 hydrogen atoms in the acid e.g. H₂CO₃
• Tribasic- 3 hydrogen atoms in the acid e.g. H₃BO₃
Writing equations for complete neutralisation
Decide on whether the acid is mono, di or tribasic.
Write the equation using as many basic units needed to replace the hydrogen
atoms in the acid
Example- complete neutralisation of nitric acid with an excess of NaOH (aq)
HNO₃ is monobasic so has one replaceable H + ion
HNO₃(aq) + NaOH (aq) NaNO₃(aq) + H₂O
Complete neutralisation of sulfuric acid with an excess of NaOH(aq)
H₂SO₄ is dibasic so has 2 replaceable H+ ions
H₂SO₄(aq) + 2NaOH(aq) Na₂SO₄(aq) + 2H₂O(l)
, H+
Acids react with metals, carbonates, metal oxides & alkalis to
form salts. The active species from the acid is H+ (aq) & ionic
equations can be written for each type of reaction to
emphasise the role of H+ ions
Spectator ions are ions that do not change during the
reaction. In the equations, spectator ions can simply be
cancelled out.
Dilute acids undergo redox reactions with some metals to
produce salts & hydrogen gas
Carbonates are bases that neutralise acids to form salt,
water & CO₂
An acid is neutralised by a solid metal oxide or hydroxide
ions to form salt & water
With alkalis, the acid & base are in solution. Salt & water
are produced. The ionic equation for an acid + alkali is :
H+(aq) + OH-(aq) H₂O(l)
bases and
pH
, Brønsted–Lowry acids and bases
Arrhenius model of acids & bases:
o Acids dissociate & release H+ ions in aqueous
solution
o Alkalis dissociate & release OH- ions in
aqueous solution. An alkali is a soluble base
o H+ ions are neutralised by OH- ions to form
water: H+(aq) + OH-(aq) H₂O(l)
Brønsted–Lowry model for acids & bases
extends the Arrhenius model to emphasise the
role of proton transfer between species:
o Brønsted–Lowry acid- proton donor
o Brønsted–Lowry base- proton acceptor
, Conjugate acid-base pair
Conjugate acid-base pair- contains 2 species that can be
interconverted by transfer of a proton.
The conjugate base of an acid is the parent acid minus one
proton.
The conjugate acid of a base is the parent base plus one
proton.
Acid-base equilibria involves 2 acid-base pairs
Acid Base Conjugate acid Conjugate base Equation
HNO₂ H₂O H₃O+ NO₂- HNO₂ + H₂O NO₂- + H₃O+
H₂O S2- OH- HS- S2- + H₂O OH- + HS-
Water is amphoteric meaning that it can act as an acid or a
base.
H₃O+ is a hydronium ion formed when H₂O behaves as a base &
accepts a proton to form its conjugate acid. However H+ is
seen written more commonly instead.
, Neutralisation
• Monobasic- 1 hydrogen atom in the acid e.g. HCl
• Dibasic- 2 hydrogen atoms in the acid e.g. H₂CO₃
• Tribasic- 3 hydrogen atoms in the acid e.g. H₃BO₃
Writing equations for complete neutralisation
Decide on whether the acid is mono, di or tribasic.
Write the equation using as many basic units needed to replace the hydrogen
atoms in the acid
Example- complete neutralisation of nitric acid with an excess of NaOH (aq)
HNO₃ is monobasic so has one replaceable H + ion
HNO₃(aq) + NaOH (aq) NaNO₃(aq) + H₂O
Complete neutralisation of sulfuric acid with an excess of NaOH(aq)
H₂SO₄ is dibasic so has 2 replaceable H+ ions
H₂SO₄(aq) + 2NaOH(aq) Na₂SO₄(aq) + 2H₂O(l)
, H+
Acids react with metals, carbonates, metal oxides & alkalis to
form salts. The active species from the acid is H+ (aq) & ionic
equations can be written for each type of reaction to
emphasise the role of H+ ions
Spectator ions are ions that do not change during the
reaction. In the equations, spectator ions can simply be
cancelled out.
Dilute acids undergo redox reactions with some metals to
produce salts & hydrogen gas
Carbonates are bases that neutralise acids to form salt,
water & CO₂
An acid is neutralised by a solid metal oxide or hydroxide
ions to form salt & water
With alkalis, the acid & base are in solution. Salt & water
are produced. The ionic equation for an acid + alkali is :
H+(aq) + OH-(aq) H₂O(l)
