UGEB2380
The Chemistry of Life
Academic Year 2023-24
Dr. Sam CK HAU (Department of Chemistry)
Acids and Bases
Water Chemistry
• The oceans may seem remote
from our living environment
• But this watery world is greatly
a ected by our activities and
the pollutants we generate
and release into it
ff
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Acids, Bases and Neutralisation
Acid
• Latin “Acidus” - sour, or having
a sharp taste
• Turn litmus paper red
• React with certain metals, like
magnesium or zinc to liberate
hydrogen gas bubbles
• Many common foods like lemon
or vinegar contain acid and
taste distinctly acid
4
Litmus paper - an absorbent paper strip containing a natural
dye that turns red in acids and blue in bases
Acids, Bases and Neutralisation
Bases
• Turn litmus paper blue
• Feel slippery
• React with acids to form salts
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Acids, Bases and Neutralisation
Vitamin C is also
known as ascorbic
acid, C6H8O6
Vinegar is a dilute
solution of acetic acid,
CH3COOH
Lemons are a good
source of citric acid,
C6H8O7
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Toilet bowl cleaner
contain hydrochloric
acid
Acids, Bases and Neutralisation
Drain and oven
cleaners can contain
sodium hydroxide
Baking soda is also
known as NaHCO3
Calcium carbonate,
CaCO3, use to combat
excess stomach acid
and relieve heartburn
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Definition of Acids and Bases
Arrhenius definition (1887)
• Acid as any substances that produce hydrogen ions, H+, in water;
Bases as any substances that produce hydroxide ions, OH ˉ in water
• E.g. when hydrochloric acid (HCl) dissolves in water, it dissociation into
hydrogen ion (H+) and chloride (Cl ˉ )
• Restricted to aqueous, or water-based solutions
Definition of Acids and Bases
Brønsted-Lowry definition (1923)
• Acid as any substances that donate hydrogen ions, H+, in water;
Bases as any substances that accept hydrogen ions, H+ in water
• E.g. when hydrochloric acid (HCl) donates a proton (H+), to the base,
(NH3), to form ammonium ion (NH4+) and chloride (Cl ˉ )
• Apply to solutions of any kinds, both aqueous and non-aqueous
Neutralisation Reactions
• A reaction between an acid and a base to produce a salt and water
HCl + NaOH → NaCl + H2O
• The H of the HCl and the OH of the NaOH combine to form water H2O;
a combination of Cl and Na to produce NaCl
• Neutralisation can be found in a variety of practical everyday uses
Use vinegar to clean a shower-head
2CH3CO2H + CaCO3
→ Ca(CH3CO2)2 + H2O + CO2
Epsom salt as the bath salts
H2SO4 + Mg(OH)2 → MgSO4 + 2H2O
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Conjugate Acid-Base Pairs
• A pair of chemical species differ by the presence of a proton (H+) is an
acid-base conjugate pair
Carbonic Acid
H3O+ + HCO3
H2CO3 + H2O
Conjugate
Conjugate Base
Base
Carbonate ion
Bicarbonate
ion
Acetic Acid
H3O+ + CH3CO2
CH3COOH + H2O
Conjugate Base
Acetate ion
Ammonia
OH + NH4+
NH3 + H2O
Conjugate Acid
Ammonium ion
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The pH Scale
Amphoteric Water
• Amphoteric - able to act as either an acid or a base
• Water ionises and then re-form itself, the amount of ionised products is
exceedingly small
Brønsted-Lowry
De nition
Arrhenius De nition
H
O
H
H+
+
proton
HO
hydroxide ion
H
H
H
fi
fi
Hydronium
ion
H
+
H
O
H
H
+ OH
H
Hydronium
ion
H 2O
H
O
O
H
O
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The pH Scale
Amphoteric Water
• The simultaneous ionisation and recombination of water is a dynamic
equilibrium
• Dynamic equilibrium - a state in which the rate of forward reaction
equals the rate of the reverse reaction, so that the concentrations of all
chemical species remain constant
Compound A
Compound B
3A→3B
3B→3A
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The pH Scale
Amphoteric Water
• At any instant, countless covalent water molecules to hydronium and
hydroxide ions; at the same instant, equal amount of hydronium and
hydroxide ions recombine to generate water
• The concentrations of all the transient hydronium and hydroxide ions at
equilibrium are fixed at any given temperature and equal to each other
H 2O
H3O+ + HO-
Concentration of hydronium ion H3O+
[H3O+] = 1 x 10-7 M,
Equally, [OH ˉ ] = 1 x 10-7 M
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The pH Scale
• In 1909, Danish biochemist Søren Sørensen proposed the
concentration of [H+] be treated as exponential values
• pH value = “power of the hydrogen (hydronium) ions”, as a symbol of
acidity
• pH - a measure of acidity, and defined as the negative logarithm of the
hydronium ion concentration
pH = –log[H3O+]
H 2O
H3O+ + HOConcentration of hydronium ion H3O+
at 25℃
[H3O+] = 1 x 10-7 M,
Equally, [OH ˉ ] = 1 x 10-7 M
pH = –log[H3O+] = –log(1 x 10-7)
=7
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The pH Scale
• In water solution at 25℃,
[H3O+] x [OH ˉ ]
= 1 x 10-14
i.e. pH + pOH = 14
• Increase the [H3O+] of a
solution by adding acid
• Adding base to water
increase [OH ˉ ] but lowers
[H3O+]
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The pH Scale
• Measuring pH and acid-base indicators
A pH meter very accurately measures
hydronium ion concentration and can be used
to record the pH of a sample of yogurt
Universal test strips are saturated with acidbase indicators that turn various colours as the
pH changes, but are less accurate than pH
meters
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Strong vs. Weak Acids
• Water generates a very small concentration of hydronium ions from
very slight ionisation
• In contrast, the ionisation of hydrochloric acid (HCl) in water produces a
relatively large number of hydronium and chloride ions
• This is a characteristic of a strong acid
H Cl
H 2O
Hydronium
ion
H3O+ + Cl
Hydrochloric acid
Chloride ion
Before dissolving in
water, HCl has not
ionised, so consist of
only acid molecules
and no ions
After dissolving in
water, HCl is
completely ionised
HCl H3O+ Cl
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HCl H3O+ Cl
Strong vs. Weak Acids
• In contrast, acetic acid ionises reversibly and partially in water
HO
O
C
CH3
+ H 2O
O
O
C
CH3
+ H3O+
Acetate
Acetic Acid
Before dissolving in
water, acetic acid has
not ionised, so consist
of only acid molecules
and no ions
After dissolving in
water, at equilibrium,
only a fraction of the
has ionised.
Acetic Acetate H3O+
acid
Acetic Acetate H3O+
acid
• Incomplete ionisation is characteristic of a weak acid
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Strong vs. Weak Acids
In summary,
• Strong acid - an acid ionises completely in water to form an equivalent
amount of hydronium ions
• Weak acid - an acid ionises partially in water
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Strong vs. Weak Bases
• Weak bases are similar to weak acids - ionises reversibly and partially in
water
• Bases accept protons from water as opposed to donating them
H
H N H + OH
H
H N H + H 2O
H
Ammonia
Ammonium ion
Before dissolving in
water, ammonia has
not ionised, so consist
of only base molecules
and no ions
After dissolving in
water, at equilibrium,
only a fraction of the
base has ionised.
NH3
OH
NH4+
NH3
OH
NH4+
• Strong bases like NaOH are similar to strong acids that dissociate
completely in water
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Acids and Bases in Everyday Life
• Strongest acid that we encountered - gastric juices secreted by our
stomach containing HCl
• The digestion of proteins requires an acidic environment
• Stomach enzyme pepsin - cut large proteins into smaller ones but
works best at a pH of about 1.5 - 2.5, and not effective when pH at 4 to
5
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Acids and Bases in Everyday Life
• To maintain our acidic
digestive environment, our
foods are generally acidic to
neutral, with a few in the
weak basic category.
• Citrus fruits, vinegar, pickles,
soft drinks, wines, and
tomatoes are generally run in
a pH range of 2.0 to 4.5
• Cow’s milk is slightly acidic
(6.4~6.8); Fresh eggs are
slightly basic (7.5~8.0)
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Acids and Bases in Everyday Life
• Many of the acids present in
our foods are carboxylic
acids
O
C
HO
OH
Carboxyl group
simplest carboxylic acid
acid, occurs in green leafy
veggies like spinach
• Citric acid - tricarboxylic
acid, the principal acid of the
citrus fruits
C
O
OH
Oxalic Acid
• Acetic acid (Vinegar) is the
• Oxalic acid - dicarboxylic
O
C
COOH
H 2C
HO C COOH
H 2C
COOH
Citrus Acid
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𝜶𝛃
Acids and Bases in Everyday Life
• Citrus acid - an example of alpha-hydroxy acids (AHAs)
Carboxyl group
COOH
OH
O
H C
OH
C
OH
Alpha (𝜶) carbon
Salicylic acid
Hydroxy group
• Widely used in a variety of skin and facial creams to improve skin
texture and reduce wrinkles
• The greater the AHA content, the stronger the exfoliating (去⾓質) effect
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Acids and Bases in Everyday Life
Antacids
• An acidic stomach is essential to good health, but excess stomach
acidity can cause uncomfortable and dangerous
• Contains weak bases to neutralise excess stomach acid
• Sodium bicarbonate (NaHCO3, pH ~8.4), Calcium carbonate (CaCO3,
pH ~9.4), Magnesium hydroxide (Mg(OH)2, pH ~10.5)
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Acids and Bases in Everyday Life
Buffers
• Living organism are generally sensitive to changes in pH and have
buffers to help maintain a stable pH
• A combination of a weak acid and its conjugate base that resist
changes in pH
• Eg. Blood is buffered to maintain a pH of about 7.4, which is slightly
basic by the presence of carbonic acid and bicarbonate ion
Added OHˉ
H2CO3
HCO3 + H2O
Carbonic Acid
Bicarbonate ions
neutralise added acid,
H+
Carbonic acid neutralise
added base, OHˉ
Conjugate Base
Bicarbonate ion
Added H+
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Acids in our Environment
Acid Rain
• All rainfall is naturally acidic as the water absorbs atmospheric carbon
dioxide (CO2) to form carbonic acid (H2CO3)
• With a high enough concentration of CO2, the pH of rainwater can fall
as low as 5.6
• When pH < 5.6 due to other acidic components → acid rain
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Acids in our Environment
Acid Rain
• Generate by the absorption of a
variety of atmospheric pollutants,
by both natural and human
activities
• Volcanic eruption and burning
fossil fuels - Sulphur dioxide
(SO2)
→ Sulphuric acid (H2SO4), a
strong acid
• Lighting and bacterial action,
also combustion of fossil fuels in
cars and trucks - Nitrogen oxides
(NOx) → Nitric acid (HNO3), a
strong acid
• Produce a rainfall with a pH as
low as vinegar (pH ~2)
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Acids in our Environment
Ocean Acidification
• The oceans absorb a
significant amount (more
than a quarter) of the CO2
generated by burning the
fossil fuels
• The CO2 then dissolves
and converts into carbonic
acid (H2CO3)
• The ocean pH is predicted
to drop from 8.1 to 7.8 in
next century
• Can harm the ocean life in
a variety of ways,
including the slowing of
growth or decay the shells
of marine organisms
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