Acids and Bases. i English Language Where do we use the words acid or acidic?
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Transcript of Acids and Bases. i English Language Where do we use the words acid or acidic?
Acids and Bases
i
English Language
Where do we use the words acid or acidic?
Examples of terms with “Acid”
Examples of terms with “Acid”
Battery acid Citric acidAcetylsalicylic acid Acid rain***DNA/RNA nucleic acid Antacids***Acid indigestion**** Phosphoric acidHydrochloric acid Dropping Acid***LSD*** Acid tongue***Acid test
**** = negative connotation
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• Acids taste sour–Citric acid in lemons
• React with bases to form salt and water–Hydrochloric acid and
sodium hydroxide react to form NaCl and water.
• Acids turn 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
• Bases turn litmus paper blue.• Bases are found in many
cleaning products.• Burn skin and damage
gastrointestinal tract on contact
Common Laboratory Bases
Acid-Base DefinitionsArrhenius
• Acids produce hydrogen ions (H+) in water solution.
• Bases produce hydroxide ions (OH-) in solution.
• But Arrhenius’ definitions do not apply in all cases.
• Don’t always have water present
Acid-Base DefinitionsBrønsted-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.
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 Hydrochloric acida strong acid
CH3CO2H
Acetic acidA weak acid
Strong and Weak• In the same manner, bases are considered either strong or weak.
Common Acids
Strong Acids:HCl Hydrochloric AcidH2SO4 Sulfuric Acid
Weak Acids:CH3COOH Acetic Acid
Most acids are weak acids
Common Bases
Strong Bases:NaOH Sodium hydroxideKOH Potassium hydroxide
Most Group IA and IIA hydroxides
Weak Bases:NH3 Ammonia
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 M.
The pH Scale• Pure water with H3O+ concentration of 1 x 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.
pH Scale
pH scale
Range from 0 to 14
pH = -log [H3O+]
pH < 7 acidic
pH = 7 neutral
pH > 7 basic
pH ScaleTypical values range from 0 to 14
pH = 7 – neutralpH > 7 – basicpH < 7 – acidic
The pH Scale
Neutralization Reactions
Also known as Acid-Base Reactions
H3O+ + OH- → 2 H2OAcid + Base → Water
When another cation and anion are present
HCl + NaOH → NaCl + H2OAcid + Base → Salt + Water
Neutralization Reaction
Acid and Base react with each other to form water and a salt
Frequently use a pH indicator to show when end point has been reached
End point is pH at which [H3O+] = [OH-]
and color of indicator changes
Example:
Add NaOH (a base) to a solution of acid that contains the pH indicator phenolphthalein
Phenolphthalein is colorless in acidic solution and is pink in basic solution
When enough NaOH is added so that it neutralizes all of the acid, any additional NaOH makes the solution basic and it turns pink
Fig. 9.6
pH indicator (acid-base indicator): substance that turns color when the H3O+ (acid) concentration changes
pH indicators may be extracted from many natural products
lichens, apple skins, blueberries, red cabbage, etc.
pH indicators may be in many forms: embedded in paper (pH paper)
liquid
Fig. 9.3a
Common Acids: Citric AcidNaturally 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.
Common Bases: Household Products
• 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+3 ions
(Mylanta): constipating effect to balance
• Ammonia and sodium hydroxide– Household cleaning products
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
Described by Robert Angus Smith 1872
Increased rapidly with onset of Industrial Revolution
Now defined as rain with a pH < 5.6
Look at water
CO2 from air dissolves in water
H2O + CO2 ↔ H2CO3 ↔ HCO3- + H3O+
pH of pure water = 7.0pH of water with dissolved CO2 = 5.6
Acid Rain Extent
Much of rainfall in US and northern Europe is pH 4-4.5
Some rainfall in isolated pockets is as low as pH 1.5
Acid Rain: Fossil Fuel Combustion
• SO2 and NO2 formed during fossil fuel combustion combine with atmospheric water to form acid rain.
Acid Rain: Fossil Fuel Combustion
• 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 buildup of acid 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
Stomach Acid
Secreted by cells in wall of stomach
Lining of stomach protected by mucin (protein-sugar material) that absorbs stomach HCl and neutralizes it with bicarbonate
Esophagus not protected by mucin, so we feel heartburn
Antacids
Antacid:Base used to neutralize stomach acid
that causes heartburn
Reaction of antacid with acid removes excess H3O+ from stomach
Types of Antacids
Fall into definite categories:
Carbonates: MgCO3 or CaCO3
Bicarbonates: NaHCO3
Hydroxides: Mg(OH)2 Al(OH)3 others
Bicarbonates
Example: Alka-Seltzer
HCO3- + H3O+ → H2CO3 + HOH
H2CO3 → HOH + CO2(g)
carbonic acid carbon dioxide
Gas formed and person burps
Carbonates
Example: Tums
CO32- + H3O+ → HCO3
- + H2O
Acid is removed from stomach
Hydroxides
Example: Milk of Magnesia
OH- + H3O+ → 2 H2O
Acid removed from stomach
Hydrangea plants
Non-acidic soil Acidic soil
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.