Acids and Bases: Properties, Definitions, and Reactions

Acids and Bases

Introduction to Acids and Bases

Acids and bases are two fundamental categories of compounds in chemistry, each with distinct properties and important roles in both laboratory and everyday contexts. Their behavior is central to many chemical reactions, including those in biological and industrial processes.

• Acids are substances that typically taste sour, dissolve many metals, and turn blue litmus paper red.

• Bases are substances that taste bitter, feel slippery, and turn red litmus paper blue.

Properties and Examples of Acids

• Sour taste: Many foods, such as lemons and candies like Sour Patch Kids, taste sour due to the presence of acids like citric and tartaric acid. These acids release H+ ions, which interact with proteins on the tongue to produce the sour sensation.

• Reaction with metals: Acids can dissolve many metals, producing hydrogen gas and a salt. However, some metals, such as gold, are resistant to acid attack.

• Litmus test: Acids turn blue litmus paper red.

Common Acids

• Hydrochloric acid (HCl): Found in laboratories and the stomach, used for cleaning metals and processing foods.

• Sulfuric acid (H2SO4): Widely used in industry, especially in fertilizers and batteries.

• Nitric acid (HNO3): Used in manufacturing fertilizers, explosives, and dyes.

• Acetic acid (HC2H3O2): The main component of vinegar, a carboxylic acid.

• Carboxylic acids: Organic acids containing the –COOH group, found in many biological substances (e.g., citric acid in lemons, malic acid in apples).

Properties and Examples of Bases

Bases are less common in foods due to their bitter taste, which may serve as a natural warning against toxins.

• Bitter taste: Many bases taste bitter; for example, caffeine in coffee is a base.

• Slippery feel: Bases react with oils on the skin to form soap-like substances, giving a slippery sensation.

• Litmus test: Bases turn red litmus paper blue.

Common Bases

• Sodium hydroxide (NaOH): Used in drain cleaners and soap manufacturing.

• Potassium hydroxide (KOH): Used in similar applications as NaOH.

• Sodium bicarbonate (NaHCO3): Baking soda, used as an antacid.

Definitions of Acids and Bases

Arrhenius Definition

The Arrhenius definition is one of the earliest and simplest ways to classify acids and bases:

• Acid: Produces H+ ions in aqueous solution.

• Base: Produces OH− ions in aqueous solution.

Brønsted–Lowry Definition

The Brønsted–Lowry definition expands the concept of acids and bases beyond aqueous solutions:

• Acid: Proton (H+) donor.

• Base: Proton (H+) acceptor.

This definition explains acid–base behavior in a wider range of chemical environments.

Conjugate Acid–Base Pairs

In Brønsted–Lowry acid–base reactions, acids and bases always occur in pairs called conjugate acid–base pairs.

• Conjugate acid: The species formed when a base gains a proton.

• Conjugate base: The species formed when an acid loses a proton.

Reactions of Acids and Bases

Neutralization Reactions

When an acid reacts with a base, the H+ from the acid combines with the OH− from the base to form water. The other ions form a salt.

• Net ionic equation: $\mathrm{H^+ (aq) + OH^- (aq) \rightarrow H_2O (l)}$

Gas Evolution Reactions

Acids react with carbonates or bicarbonates to produce water, carbon dioxide gas, and a salt.

Acids Reacting with Metals

Acids react with many metals to produce hydrogen gas and a dissolved salt containing the metal ion.

Acids Reacting with Metal Oxides

Acids react with metal oxides to produce water and a dissolved salt.

Bases Reacting with Aluminum

Some metals, such as aluminum, can dissolve in strong bases like NaOH, producing hydrogen gas. This is important in industrial and household contexts.

Acid–Base Titration

Quantifying Acid or Base in Solution

Titration is a laboratory technique used to determine the concentration of an unknown acid or base by reacting it with a solution of known concentration.

• The equivalence point is reached when the amount of acid equals the amount of base, as indicated by a color change from an indicator.

Strong and Weak Acids and Bases

Strong Acids

• Completely ionize in solution, producing a high concentration of H+ (or H3O+) ions.

• Examples: HCl, HBr, HI, HNO3, HClO4, H2SO4 (first proton only).

Weak Acids

• Only partially ionize in solution; most molecules remain intact.

• Examples: HF, acetic acid, formic acid, carbonic acid, phosphoric acid.

Strong Bases

• Completely dissociate in solution to produce OH− ions.

• Examples: NaOH, KOH, Ba(OH)2, Sr(OH)2.

Weak Bases

• Partially react with water to produce OH− ions; most molecules remain unreacted.

• Examples: Ammonia (NH3), organic amines.

Water: Acid and Base in One (Amphoteric)

Water can act as both an acid and a base, a property called amphoterism. Water self-ionizes to a very small extent:

• At 25°C: [H3O+] = [OH−] = 1.0 × 10−7 M

• Ion product constant: $\mathrm{K_w = [H_3O^+][OH^-] = 1.0 \times 10^{-14}}$

pH and pOH Scales

The pH scale is a logarithmic measure of acidity:

• pH < 7: acidic

• pH = 7: neutral

• pH > 7: basic

Calculating pH and [H3O+]

• pH is calculated as: $\mathrm{pH = -\log [H_3O^+]}$

• [H3O+] can be found from pH: $\mathrm{[H_3O^+] = 10^{-pH}}$

Buffers

Buffers are solutions that resist changes in pH when small amounts of acid or base are added. They contain significant amounts of both a weak acid and its conjugate base.

• Example: Acetic acid and sodium acetate buffer system.

• Biological importance: Human blood is buffered to maintain a pH between 7.36 and 7.40.

Chemistry and Health: Acid Rain and Antifreeze Poisoning

• Acid rain: Caused by sulfur and nitrogen oxides reacting with water to form acids, which can damage buildings and ecosystems.

• Antifreeze poisoning: Ethylene glycol is metabolized to glycolic acid, which can overwhelm the body's buffer system and lower blood pH to dangerous levels.