Acids and Bases

i English Language Where do we use the words acid or acidic?

Examples of terms with “Acid”

Battery acidCitric acid Acetylsalicylic acidAcid rain*** DNA/RNA nucleic acidAntacids*** Acid indigestion****Phosphoric acid Hydrochloric acidDropping 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 Definitions Arrhenius 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 Definitions 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.

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 acid a strong acid CH 3 CO 2 H Acetic acid A weak acid

Strong and Weak In the same manner, bases are considered either strong or weak.

Common Acids Strong Acids: HCl Hydrochloric Acid H 2 SO 4 Sulfuric Acid Weak Acids: CH 3 COOH Acetic Acid Most acids are weak acids

Common Bases Strong Bases: NaOH Sodium hydroxide KOHPotassium hydroxide Most Group IA and IIA hydroxides Weak Bases: NH 3 Ammonia

H 3 O + Concentration The acidity of a solution is normally specified by the concentration of H 3 O + in moles per liter of solution, M. – Strong acids: Acid concentration equals concentration of H 3 O + – Weak acids: Acid concentration is greater than concentration of H 3 O + Pure water has a H 3 O + concentration of 1 x M.

The pH Scale Pure water with H 3 O + concentration of 1 x M has a pH of 7. H 3 O + concentration greater than that of pure water is termed acidic. – pH values less than 7 H 3 O + 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, H 3 O + changes by a factor of 10.

pH Scale

pH scale Range from 0 to 14 pH = -log [H 3 O + ] pH < 7acidic pH = 7neutral pH > 7basic

pH Scale Typical values range from 0 to 14 pH = 7 – neutral pH > 7 – basic pH < 7 – acidic

The pH Scale

Neutralization Reactions Also known as Acid-Base Reactions H 3 O + + OH - → 2 H 2 O Acid + Base → Water When another cation and anion are present HCl+ NaOH→ NaCl + H 2 O Acid + 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 [H 3 O + ] = [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 H 3 O + (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 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.

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 CO 2 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 CO 2 from air dissolves in water H 2 O + CO 2 ↔ H 2 CO 3 ↔ HCO H 3 O + pH of pure water = 7.0 pH of water with dissolved CO 2 = 5.6

Acid Rain Extent Much of rainfall in US and northern Europe is pH Some rainfall in isolated pockets is as low as pH 1.5

Acid Rain: Fossil Fuel Combustion SO 2 and NO 2 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 SO 2 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 SO 2 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 acidthat causes heartburn Reaction of antacid with acid removes excess H 3 O + from stomach

Types of Antacids Fall into definite categories: Carbonates: MgCO 3 or CaCO 3 Bicarbonates: NaHCO 3 Hydroxides: Mg(OH) 2 Al(OH) 3 others

Bicarbonates Example: Alka-Seltzer HCO H 3 O + → H 2 CO 3 + HOH H 2 CO 3 → HOH + CO 2(g) carbonic acidcarbon dioxide Gas formed and person burps

Carbonates Example: Tums CO H 3 O + → HCO H 2 O Acid is removed from stomach

Hydroxides Example: Milk of Magnesia OH - + H 3 O + → 2 H 2 O Acid removed from stomach

Hydrangea plants Non-acidic soilAcidic soil

Summary Molecular 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.