IB CHEMISTRY:
TOPIC 8 ACIDS AND BASES NOTES
8.1 Theories of acids and bases
Brønsted-Lowry theory
• According to the Brønsted-Lowry theory, a substance behaves as an acid when it
donates a proton to a base. A substance behaves as a base when it accepts a proton
from an acid. Thus:
o A Brønsted-Lowry acid is a proton (H+) donor
o A Brønsted-Lowry base is a proton (H+) acceptor
• Acids can be a combination of hydrogen ions (𝑯! ) and an anion. Examples include:
𝐻𝐶𝑙, 𝐻𝑁𝑂" , 𝐻𝐶# 𝐻" 𝑂#
• Bases can be a combination of hydroxide ions (𝑶𝑯$ ) and metal cations. Examples
include: 𝑁𝑎𝑂𝐻, 𝐾𝑂𝐻
o However, sometimes a hydrogen next to a metal signifies a base such as 𝑁𝑎𝐻
• Acid-Base Conjugate Pairs: A conjugate pair is two species which differ by a single
proton:
o The acid will become the conjugate base
o The base will become the conjugate acid
Conjugate-Base Pair Question:
Label the conjugate acid-base pairs in the following reactions:
𝐂𝐇𝟑 𝐂𝐎𝐎𝐇 + 𝐇𝟐 𝐎(𝐠) ⇌ 𝐂𝐇𝟑 𝑪𝑶𝑶$ + 𝐇𝟑 𝐎!
Conjugate pair 1: CH" COOH/CH" 𝐶𝑂𝑂$
𝐶𝐻" 𝐶𝑂𝑂𝐻 is the acid because it donates a H! , so CH" 𝐶𝑂𝑂$ is conjugate base
Conjugate pair 2: H# O/H" O!
H# O is the base because it accepts a H! , so H" O! is the conjugate acid
• To find the conjugate acid of a species, add one H!
• To find the conjugate base of a species, takeaway one H!
, • Before the Brønsted-Lory theory acids and bases were distinguished by their taste, acids
tasted sour
Amphiprotic
• Some substances can behave as either acids and bases, depending on what they react
with, and can therefore donate or receive protons. Such substances are said to be
amphiprotic
o Amphiprotic: A chemical species capable of accepting and donating protons,
thus able to act as a Brønsted-Lowry acid and a
base
• This means the species must be able to accept or donate
a proton to another species
• H2O is amphiprotic as it is able to donate a proton to form a OH$ or accept a proton to
form H" O! therefore acting as a Brønsted-Lowry acid or base
• HCO3-, HSO4-, H2PO4-, CH3COOH, NH3
Amphiprotic Species Question:
Write an equation to show hydrogen phosphate acting as and an acid and a base in
water.
Acting like an Acid: HPO' #$ + 𝐇𝟐 𝐎(𝐠) ⇌ 𝐏𝐎𝟒 𝟑$ + 𝐇𝟑 𝐎!
Acting like a Base: HPO' #$ + 𝐇𝟐 𝐎(𝐠) ⇌ 𝐇𝟐𝐏𝐎𝟒 $ + 𝐎𝐇$
Amphoteric
• Amphoteric: A species that can act as an acid or base, including reactions that do not
involve a proton
• Amphiprotic specifically relates to the Brønsted-Lowry acid-base theory, where the
emphasis is on the transfer of a proton. Amphoteric is a more general term that isn’t
just confined to proton transfer
• All amphiprotic species are also amphoteric but not all amphoteric species are
amphiprotic
TOPIC 8 ACIDS AND BASES NOTES
8.1 Theories of acids and bases
Brønsted-Lowry theory
• According to the Brønsted-Lowry theory, a substance behaves as an acid when it
donates a proton to a base. A substance behaves as a base when it accepts a proton
from an acid. Thus:
o A Brønsted-Lowry acid is a proton (H+) donor
o A Brønsted-Lowry base is a proton (H+) acceptor
• Acids can be a combination of hydrogen ions (𝑯! ) and an anion. Examples include:
𝐻𝐶𝑙, 𝐻𝑁𝑂" , 𝐻𝐶# 𝐻" 𝑂#
• Bases can be a combination of hydroxide ions (𝑶𝑯$ ) and metal cations. Examples
include: 𝑁𝑎𝑂𝐻, 𝐾𝑂𝐻
o However, sometimes a hydrogen next to a metal signifies a base such as 𝑁𝑎𝐻
• Acid-Base Conjugate Pairs: A conjugate pair is two species which differ by a single
proton:
o The acid will become the conjugate base
o The base will become the conjugate acid
Conjugate-Base Pair Question:
Label the conjugate acid-base pairs in the following reactions:
𝐂𝐇𝟑 𝐂𝐎𝐎𝐇 + 𝐇𝟐 𝐎(𝐠) ⇌ 𝐂𝐇𝟑 𝑪𝑶𝑶$ + 𝐇𝟑 𝐎!
Conjugate pair 1: CH" COOH/CH" 𝐶𝑂𝑂$
𝐶𝐻" 𝐶𝑂𝑂𝐻 is the acid because it donates a H! , so CH" 𝐶𝑂𝑂$ is conjugate base
Conjugate pair 2: H# O/H" O!
H# O is the base because it accepts a H! , so H" O! is the conjugate acid
• To find the conjugate acid of a species, add one H!
• To find the conjugate base of a species, takeaway one H!
, • Before the Brønsted-Lory theory acids and bases were distinguished by their taste, acids
tasted sour
Amphiprotic
• Some substances can behave as either acids and bases, depending on what they react
with, and can therefore donate or receive protons. Such substances are said to be
amphiprotic
o Amphiprotic: A chemical species capable of accepting and donating protons,
thus able to act as a Brønsted-Lowry acid and a
base
• This means the species must be able to accept or donate
a proton to another species
• H2O is amphiprotic as it is able to donate a proton to form a OH$ or accept a proton to
form H" O! therefore acting as a Brønsted-Lowry acid or base
• HCO3-, HSO4-, H2PO4-, CH3COOH, NH3
Amphiprotic Species Question:
Write an equation to show hydrogen phosphate acting as and an acid and a base in
water.
Acting like an Acid: HPO' #$ + 𝐇𝟐 𝐎(𝐠) ⇌ 𝐏𝐎𝟒 𝟑$ + 𝐇𝟑 𝐎!
Acting like a Base: HPO' #$ + 𝐇𝟐 𝐎(𝐠) ⇌ 𝐇𝟐𝐏𝐎𝟒 $ + 𝐎𝐇$
Amphoteric
• Amphoteric: A species that can act as an acid or base, including reactions that do not
involve a proton
• Amphiprotic specifically relates to the Brønsted-Lowry acid-base theory, where the
emphasis is on the transfer of a proton. Amphoteric is a more general term that isn’t
just confined to proton transfer
• All amphiprotic species are also amphoteric but not all amphoteric species are
amphiprotic
