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 ScalePure 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 lactic- acid-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

NH3H2O NH4+ + OH-

+

BaseAcid Conjugate Base of Water

Conjugate Acid of Ammonia

strong baseweak base weaker acid

than waterstronger acid than ammonium ion

H2CO3 + H2O H3O+

+ HCO3-

Acid Base Conjugate Acid of water

Conjugate Baseof Carbonic Acid

weaker acidthan hydronium

weaker base than bicarbonateion

stronger acidthan carbonic acid

stronger base than water

• 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