Nivaldo J. Tro

http://www.cengage.com/chemistry/tro

Mark Erickson • Hartwick College

Chapter 13Acids and Bases:

The Molecules Responsible for Sour and Bitter

Acids

• Sourness in foods is caused by acids, molecules that release protons.

• The protons or hydrogen ions react with proteins on the tongue.

• Acids and their chemical opposites, bases, are all around us.

• We eat them, smell them, and use them in everyday products.

Properties of Acids• Acids dissolve metals

– But not as rapidly as James Bond movies depict

• Acids taste sour– Citric acid in lemons

• React with bases to form salt and water

– HCl + NaOH → H2O + NaCl

• Acids turn blue litmus paper red.

Common Laboratory Acids

Properties of Bases• Bases feel slippery.

• Bitter taste

• React with acids to form water and salt in neutralization reactions– NaOH + HCl → H2O + NaCl

• Bases turn red litmus paper blue.

• Bases are found in many cleaning products.

• Burn skin and damage gastrointestinal tract on contact

Common Laboratory Bases

Concept Check 13.1

• Write a chemical equation to show the neutralization of hydrochloric acid (HCl) by sodium hydroxide (KOH).

Concept Check 13.1 Solution

• All neutralization equations have the form

acid + base → H2O + salt

where the salt is composed of the anion from the acid and the cation from the base. So in this case we write the balanced chemical equation:

HCl + KOH → H2O + KCl

Acids-Bases 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?

Brønsted-Lowry

• Broader definition

• Works in solutions that do not contain water

• Focuses on the transfer of protons (H+ ions)

• In the Brønsted-Lowry definition, acids are proton donors and bases are proton acceptors.

Concept Check 13.2

• Identify the Brønsted–Lowry acid and base in the following reaction:

HF + NH3 → NH4+ + F-

Concept Check 13.2 Solution

• Because HF is the proton donor, it is the acid. NH3 is accepting the proton, making it the base.

HF + NH3 → NH4+ + F−

• Using arrows and Lewis structures to describe the proton transfer, we have:

H FN

H

H

H

FN

H

H

H

H ++

Strong and Weak Acids• Acids that completely

dissociate, like HCl, are strong acids.

• Acids in an aqueous environment, which primarily remain undissociated, are weak acids. A double arrow indicates that the dissolution does not go to completion.

HCl + H2O H3O+ + Cl

CH3COOH + H2O H3O+ + CH3COO

Strong and Weak• 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 × 10-7 M.

2H2O H3O+ + OH

The pH Scale

• Pure water with H3O+ concentration of 1.0 × 10-7 M 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 one unit on the pH scale, H3O+ changes by a factor of 10.

The pH Scale

Concept Check 13.3

The ideal pH of a swimming pool is 7.2. You measure the pH of your pool to be 7.9. What should you add, acid or base, to restore your pool to the ideal pH?

Concept Check 13.3 Solution

A pH of 7.9 is too basic compared to the ideal pH of 7.2. Adding the right amount of acid will drop the pH to 7.2.

Common Acids: Citric Acid

Naturally occurring citric acid and other acids in acidic foods offer spoilage resistance.

Common Acids: Lactic Acid• Fermentation with lactic acid-forming bacteria helps

preserve low-acid foods like cucumbers and cabbage.

• This technique is called pickling.

Common Acids: 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.

Common Acids: 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.

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.

N

H

CH2CH2CH3

Coniine(toxic alkaloid from hemlock)

basicnitrogen

Common Bases: Household Products

• Sodium bicarbonate– Taken directly or as Alka-Seltzer– Baking

• Calcium carbonate– Active ingredient in Tums

• Magnesium hydroxide– Milk of Magnesia: laxative effect– In combination with Al+3 ions (Mylanta):

constipating effect to balance• Ammonia and sodium hydroxide

– Household cleaning products

Concept Check 13.4

• Describe the antacid action of sodium bicarbonate (NaHCO3) with a chemical equation. Is sodium bicarbonate acting as an acid or a base?

Concept Check 13.4 Solution

• Sodium bicarbonate is a weak base. It neutralizes the HCl present in the stomach.

HCl + NaHCO3 → H2CO3 + NaCl

• H2CO3 (carbonic acid) is quickly decomposes to carbon dioxide and water.

H2CO3 → H2O + CO2 (burp)

The Chemistry of 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 geology, such as

limestone, can serve to neutralize the acid. • Rapid acidification occurs when neutralization is not

possible.

Acid Rain: The Effects• Lakes and streams

– Approximately 2000 lakes and streams in the eastern U.S. have lowered pH.

– Some aquatic species cannot survive.

– Emissions from U.S. coal-fired power plants have also contributed to the acidification of Canadian lakes.

Acid Rain: The Effects• Building materials

– Acids dissolve stone, marble, and 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 or

remove 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

Chapter SummaryMolecular Concept

• What describes an acid?

• What describes a base?

• pH

• Acid rain

Societal Impact

• Acids and bases are a part of our daily lives, including their presence in a number of the foods we eat.

• Acid rain is a significant problem in the U.S. and Canada.

• Legislation (the Clean Air Act) has been passed to help the problem of acid rain.