Chapter 13
Acids and Bases: The Molecules
Responsible for Sour and Bitter
Introduction
• Sourness in foods is
caused by acids,
molecules that
release protons.
• The chemical
opposite, bases, are
all around us.
Properties of Acids
• Acids do dissolve
metals, but not with
the speed alluded to
in movies or
television.
Properties (continued)
• Acids have the ability to react with bases to form
water and a salt through neutralization reactions.
• Acids turn litmus paper red.
– Bases turn litmus paper blue.
• Acids are considered to be dangerous materials.
– Dissolve clothing, burn skin, damage gastrointestinal
tract, kill
Properties of Bases
• Slippery feel
• Bitter taste
• Ability to react with acids
to form water and a salt
in neutralization
reactions
• Bases turn litmus paper
blue.
Properties (continued)
• Bases are found in many cleaning
products.
• Burn skin and damage gastrointestinal
tract on contact
Molecular Definitions
• Arrhenius
– Acids produce hydrogen ions (H+) in
solution.
– Bases produce hydroxide ions (OH-) in
solution.
– But Arrhenius’ definitions do not apply
in all cases.
• What about ammonia?
Definition (continued)
• Brønsted-Lowry
– Broader definition
– Works in solutions that do not contain water
– Focuses on the transfer of protons (H+ ions)
– Both B-L equations here, the overall ones with HCl
and then NH3 as reactants
– In the Bronsted-Lowry definition, acids are proton
donors and bases are proton acceptors.
Strong and Weak
• Acids that completely dissociate, like HCl,
are strong acids.
• Acids in an aqueous environment, that in
large part remain undissociated, are weak
acids. A double arrow indicates that the
dissolution does not go to completion.
In the same manner, bases are considered
either strong or weak.
H3O+ Concentration
• The acidity of a solution is normally
specified by the concentration of H3O+ in
moles per liter of solution, M
– Strong acids: acid concentration equals
concentration of H3O+
– Weak acids: acid concentration is greater
than concentration of H3O+
• Pure water has a H3O+ concentration of
1 x 10-7.
The pH Scale
Pure water with H3O+ concentration of 1 x
10-7 has a pH of 7.
• H3O+ concentration greater than that of
pure water is termed acidic.
– pH values less than 7
• H3O+ concentration less than that of pure
water is termed basic.
– pH values greater than 7
For every change of 1 unit on the pH scale, [H3O+ ]
changes by a factor of 10
Common Acids: Citric
• Naturally occurring citric
and other acids in acidic
foods offer spoilage
resistance.
Lactic Acid
• Fermentation with lacticacid-forming bacteria
helps preserve low acid
foods like cucumbers and
cabbage.
• This technique is called
pickling.
Acetic Acid
• Vinegar is a solution of acetic acid in
water.
• Vinegar is from the French vin aigre,
meaning sour wine. Oxygen will convert
ethanol in wine to acetic acid.
Salicylic Acid
• Salicylic acid is a precursor to
acetylsalicylic acid, aspirin, the most
widely used of all drugs.
Wine
• All wines contain 0.60 to 0.80% acid
content by volume.
– From grapes and from fermentation
• The balance of these acids determines the
quality of the wine.
Common Bases
• Bases have a bitter taste.
– Evolutionary adaptation that warns against
(often poisonous) alkaloids?
• Active ingredient in antacids
– These are substances that dissociate in water
to form a metal ion and a base.
Common Bases (continued)
• Sodium bicarbonate
– Taken directly or as Alka-Seltzer
• Calcium carbonate
– Active ingredient in Tums
• Magnesium hydroxide
– Milk of Magnesia – laxative effect
– In combination with Al ions (Mylanta):
constipating effect to balance
• Ammonia and sodium hydroxide
– Household cleaning products
Baking
• Baking powder is used to produce carbon
dioxide gas pockets in dough, making the
baked product lighter and fluffier.
– sodium bicarbonate
– sodium aluminum sulfate
– calcium acid phosphate
• Acidic salts combine with the basic salts
making carbon dioxide and water.
– Warm carbon dioxide gas expands during
baking.
Yeast produces CO2 and ethanol for a
similar effect with breads.
Acid Rain: Fossil Fuel Combustion
• SO2 and NO2 formed during fossil fuel combustion
combine with atmospheric water to form acid rain.
• Unpolluted rain is slightly acidic due to atmospheric
carbon dioxide.
• Most acidic rainfall occurs in the northeastern U.S.
Acid Rain: The Effects
• The environment into which acid rain falls
determines its fate.
• In some cases naturally occurring
geography can serve to neutralize the
acid.
• Rapid acidification occurs when
neutralization is not possible.
• Lakes and Streams
– Approx. 2000 lakes and streams in the
eastern U.S. have elevated pH.
– Some aquatic species cannot survive.
– U.S. emissions have contaminated Canadian
lakes.
• Building Materials
– Acids dissolve stone, marble, paint
– Rusting of steel is accelerated
• Forests and Reduced Visibility
– Trees cannot grow and fend off disease.
– Sulfate aerosols account for 50% of visibility
problems in the eastern U.S.
Clean Air Act Amendments
• Cut SO2 emissions to half of 1980 levels
by 2010
– Use low-sulfur coal, rremove sulfur before
burning
– Use flue gas scrubbers
– Conservation and efficiency of customers
• SO2 emission allowances
– Can be traded among utilities but congress
reduces number of allowances as per
regulations
• Conjugate acid and base pairs
• An acid reacts with a base to form the conjugate
base of the acid and the conjugate acid of the
base
• - the two substances only differ by a hydrogen ion,
H+
• Examples
stronger acid weak base
t han ammonium ion
H2O
Acid
+
NH3
Base
weaker acid
t han water strong base
+
NH4 + OH
Conjugat e Acid Conjugat e Base
of Ammonia
of Water
weaker base
stronger acid
weaker acid
t han bicarbonat e t han carbonic acid stronger base
t han hydroniumion
t han water
+
H2CO3
Acid
+
H2O
Base
H3O
+
HCO3
Conjugat e Acid Conjugat e Base
of water
of Carbonic Acid
• ion product for water (Kw) - the product of
hydronium and hydroxide ion concentrations in
pure water
•
Kw = Ka[H2O]
•
Kw = [H3O+] [OH-]
•
= [1.0x10-7 M][1.0x10-7 M]
•
= 1.0x10-14 M (at 25 °C)
• Kw is the same value for every aqueous solution
• acidic solution
[H3O+] > 10-7 M
[OH-] < 10-7 M
• neutral solution
[H3O+] = 10-7 M
[OH-] = 10-7 M
• basic solution
[H3O+] < 10-7 M
[OH-] > 10-7 M
pH
pH = - log [H3O+]
[H3O+] = 10-pH
Acidic solutions
(pH 0-6 or < 7)
[H3O+] > 1x10-7 M
Neutral solutions
(pH = 7)
[H3O+] = 1x10-7 M
Basic solutions
(pH 8-14 or > 7)
[H3O+] < 1x10-7 M