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 behaviors. Their study is essential for understanding chemical reactions, biological processes, and industrial applications.

Properties of Acids

• Sour Taste: Acids typically have a sour taste, as experienced with citric acid in lemons or tartaric acid in candies.

• Reaction with Metals: Many acids can dissolve metals, producing hydrogen gas and a salt.

• Litmus Test: Acids turn blue litmus paper red.

Examples of Common Acids

• Hydrochloric Acid (HCl): Found in stomach acid and used in industry for cleaning metals and processing foods.

• Sulfuric Acid (H2SO4): Widely used in fertilizer and battery production.

• Nitric Acid (HNO3): Used in manufacturing fertilizers and explosives.

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

• Carboxylic Acids: Organic acids containing the carboxyl group (–COOH), found in many biological substances.

Properties of Bases

• Bitter Taste: Bases often taste bitter, which is a natural deterrent against consuming potentially toxic substances.

• Slippery Feel: Bases feel slippery because they react with oils on the skin to form soap-like substances.

• Litmus Test: Bases turn red litmus paper blue.

Examples of Common Bases

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

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

• Sodium Bicarbonate (NaHCO3): Commonly known as baking soda, used as an antacid.

Definitions of Acids and Bases

Arrhenius Definition

• Acid: Produces H+ ions in aqueous solution.

• Base: Produces OH− ions in aqueous solution.

Limitations: The Arrhenius definition does not account for acid–base behavior in nonaqueous solutions or substances that do not contain OH− but act as bases.

Brønsted–Lowry Definition

• Acid: Proton (H+) donor.

• Base: Proton (H+) acceptor.

This definition is broader and includes more substances, such as ammonia (NH3), which acts as a base by accepting a proton from water.

Amphoteric Substances

Some substances, such as water, can act as either an acid or a base. These are called amphoteric substances.

Conjugate Acid–Base Pairs

Any two substances related by the gain or loss of a proton are called a conjugate acid–base pair.

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.

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

Reactions of acids with carbonates or bicarbonates 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 and a soluble aluminate salt.

Acid–Base Titration

Principle of Titration

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.

Example Calculation

Given: 10.00 mL HCl solution, 12.54 mL of 0.100 M NaOH required to reach equivalence point. Find the concentration of HCl.

• Balanced equation: $\mathrm{HCl(aq) + NaOH(aq) \rightarrow H_2O(l) + NaCl(aq)}$

• Solution map and calculation:

Strength of Acids and Bases

Strong and Weak Acids

• Strong acids completely ionize in solution (e.g., HCl, HNO3, H2SO4).

• Weak acids only partially ionize (e.g., HF, acetic acid).

Strong and Weak Bases

• Strong bases completely dissociate in solution (e.g., NaOH, KOH).

• Weak bases only partially react with water to produce OH− (e.g., NH3).

Water: Acid and Base in One

Self-Ionization of Water

Water can act as both an acid and a base, undergoing self-ionization to produce equal concentrations of H3O+ and OH− at 25°C:

• $\mathrm{[H_3O^+] = [OH^-] = 1.0 \times 10^{-7}~M}$

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

The pH and pOH Scales

Definition and Calculation

• pH: $\mathrm{pH = -\log [H_3O^+]}$

• pOH: $\mathrm{pOH = -\log [OH^-]}$

• At 25°C: pH + pOH = 14

• pH < 7: acidic; pH > 7: basic; pH = 7: neutral

Buffers

Definition and Function

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. Buffers are crucial in biological systems, such as human blood, to maintain a stable pH.

Chemistry and Health: Acid Rain and Antifreeze Poisoning

Acid Rain

Acid rain results from sulfur and nitrogen oxides in the atmosphere reacting with water to form acids, which can damage buildings and ecosystems.

Antifreeze Poisoning

Ethylene glycol (antifreeze) is metabolized to glycolic acid in the body, which can overwhelm the blood's buffering system and lead to dangerously low blood pH.