Acids and Bases: Properties, Theories, and Applications

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Acids and Bases

Experimental Definitions

Acids and bases are fundamental chemical substances with distinct properties and behaviors. Their identification in laboratory and everyday contexts relies on observable characteristics.

Acids:
- Taste sour.
- Turn litmus paper red.
- React with active metals to release hydrogen gas.
- React with bases to form water and a salt.
Bases:
- Taste bitter.
- Turn litmus paper blue.
- Feel slippery to the touch.
- React with acids to form water and a salt.

Examples of acids and bases in household products

Arrhenius Theory

The Arrhenius theory provides a classical definition of acids and bases based on their behavior in aqueous solutions.

Acid: A molecular substance that ionizes in water to produce hydrogen ions ().
Base: A substance that produces hydroxide ions () in water.

Common Acids and Bases

Name	Formula	Acid/Base Strength	Common Uses/Notes
Sulfuric acid	H2SO4	Strong	Battery acid, fertilizer manufacturing
Hydrochloric acid	HCl	Strong	Cleaning metals, bricks
Phosphoric acid	H3PO4	Moderate	Colas, rust removers
Lactic acid	C3H6O3	Weak	Yogurt, lotions
Acetic acid	CH3COOH	Weak	Vinegar
Boric acid	H3BO3	Very weak	Eyewash, insecticide
Hydrocyanic acid	HCN	Very weak	Plastics manufacture, toxic

Name	Formula	Classification	Common Uses/Notes
Sodium hydroxide	NaOH	Strong	Soap making, acid neutralization
Potassium hydroxide	KOH	Strong	Liquid soap, biodiesel
Lithium hydroxide	LiOH	Strong	Batteries
Calcium hydroxide	Ca(OH)2	Strong	Plaster, cement, soil neutralizer
Magnesium hydroxide	Mg(OH)2	Strong	Antacid, laxative
Ammonia	NH3	Weak	Cleaning, fertilizer

Neutralization

Neutralization is the reaction between an acid and a base, resulting in the formation of water and a salt. This process is fundamental in both laboratory and industrial chemistry.

General equation:

Neutralization reaction: NaOH + HCl produces NaCl and H2O

Limitations of the Arrhenius Theory

While useful, the Arrhenius theory has several limitations:

ions do not exist freely in water; they form hydronium ions ().
Does not explain the basicity of ammonia and similar compounds.
Applies only to aqueous solutions.

Brønsted-Lowry Theory

The Brønsted-Lowry theory expands the definition of acids and bases:

Acid: Proton donor.
Base: Proton acceptor.

Example reactions:

Brønsted-Lowry acid-base proton transfer Ammonia accepting a proton from water

Salts

Salts are ionic compounds formed from cations other than hydrogen and anions other than hydroxide. They are often products of neutralization reactions and have various uses in medicine and industry.

Name	Formula	Uses
Silver nitrate	AgNO3	Germicide, antiseptic
Stannous fluoride	SnF2	Toothpaste additive
Calcium sulfate	CaSO4	Plaster casts
Magnesium sulfate	MgSO4	Laxative, foot baths
Potassium permanganate	KMnO4	Cauterizing agent, antiseptic
Ferrous sulfate	FeSO4	Treats anemia
Zinc sulfate	ZnSO4	Skin treatment
Barium sulfate	BaSO4	Contrast for X-rays
Mercurous chloride	Hg2Cl2	Laxative (no longer used)

Anhydrides

Anhydrides are compounds formed by removing water from acids or bases. Nonmetal oxides are acidic anhydrides, while metal oxides are basic anhydrides.

Acidic anhydride example:
Basic anhydride example:

Basic anhydride reaction: O2- with H2O forms OH-

Strong and Weak Acids and Bases

Ionization in Water

Acids and bases are classified as strong or weak based on their degree of ionization in water.

Strong acids: Ionize completely in water. Example:
Weak acids: Ionize only partially. Example:
Strong bases: Dissociate completely. Example:
Weak bases: Ionize partially. Example:

Ammonia and water forming ammonium and hydroxide ions

Neutralization

Acid-Base Reactions

Neutralization reactions are essential for determining the amount of acid or base in a solution and for practical applications such as antacid use.

General equation:
Produces water and a salt.

Titration process showing neutralization

The pH Scale

Definition and Calculation

The pH scale quantifies the acidity or basicity of a solution, ranging from 0 (very acidic) to 14 (very basic).

pH formula:
pH 7 is neutral; values below 7 are acidic, above 7 are basic.

pH scale with common substances

Relationship Between pH and Hydrogen Ion Concentration

pH	H+ Concentration (mol/L)
0	1 × 100
1	1 × 10-1
2	1 × 10-2
...	...
14	1 × 10-14

Approximate pH Values of Common Solutions

Solution	pH
Hydrochloric acid (4%)	0
Gastric juice	1.6–1.8
Soft drink	2.0–4.0
Lemon juice	2.1
Vinegar (4%)	2.5
Urine	5.5–7.0
Rainwater	5.6
Saliva	6.2–7.4
Milk	6.3–6.6
Pure water	7.0
Blood	7.4
Fresh egg white	7.6–8.0
Bile	7.8–8.6
Milk of magnesia	10.5
Washing soda	12.0
Sodium hydroxide (4%)	13.0

Buffers and Conjugate Acid–Base Pairs

Conjugate Acid–Base Pairs

Conjugate acid–base pairs differ by one proton. For example, chloride ion (Cl-) is the conjugate base of hydrochloric acid (HCl).

Buffer Solutions

Buffer solutions contain a weak acid and its conjugate base, allowing them to maintain a nearly constant pH when small amounts of acid or base are added.

Acid Rain

Formation and Effects

Acid rain is caused by nonmetal oxides in the air reacting with water to form acidic solutions. Rain with a pH less than 5.6 is considered acid rain. Major contributors include carbon dioxide (CO2), sulfur dioxide (SO2), nitrogen monoxide (NO), and nitrogen dioxide (NO2).

Antacids and Acid Reducers

Antacids: A Basic Remedy

Antacids are substances used to neutralize excess stomach acid and relieve symptoms of hyperacidity. Excessive use can lead to alkalosis, an increase in blood pH.

Substance	Formula	Product(s)
Sodium bicarbonate	NaHCO3	Baking soda
Calcium carbonate	CaCO3	Tums®
Aluminum hydroxide	Al(OH)3	Amphojel®
Magnesium hydroxide	Mg(OH)2	Milk of magnesia

Common antacid products

Acid Reducers

Acid reducers are medications that decrease acid production in the stomach, increasingly used instead of antacids.

Acid reducer medications

Acids and Bases in Industry and at Home

Industrial and Household Uses

Acids and bases are widely used in industry and at home. Sulfuric acid is the most produced chemical in the U.S., used in fertilizers and batteries. Hydrochloric acid is used for cleaning and rust removal. Lime (CaO) is produced by heating limestone and is used in cement and soil treatment. Sodium hydroxide (lye) is used in soap making and cleaning. Ammonia is used in fertilizers and cleaning products.

Applying slaked lime to soil

Concentrated acids and bases are corrosive and hazardous to health, capable of denaturing proteins in living cells. The human body maintains proper pH in tissues, blood, and fluids through various mechanisms.