Acids and Bases: Foundations, Properties, and Calculations

Acids and Bases

Introduction to Acids and Bases

Acids and bases are fundamental classes of compounds in chemistry, each with distinct properties and behaviors. Understanding their definitions, reactions, and calculations is essential for mastering introductory chemistry.

Properties of Acids

General Properties

• Sour taste: Many acids, such as citric acid in lemons, have a characteristic sour flavor.

• Reaction with metals: Acids can dissolve many metals, producing hydrogen gas.

• Litmus test: Acids turn blue litmus paper red due to their effect on the dye.

Example: The sour taste of candies like Sour Patch Kids is due to acids such as citric and tartaric acid, which release H+ ions that interact with taste receptors.

Common Acids

• Hydrochloric acid (HCl): Used in laboratories and industry, and is the main component of stomach acid.

• Sulfuric acid (H2SO4): Widely used in manufacturing fertilizers and batteries.

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

• Acetic acid (HC2H3O2): Responsible for the sour taste of vinegar; a carboxylic acid.

Carboxylic Acids

Carboxylic acids contain the –COOH group and are common in biological substances. Examples include citric acid (in citrus fruits) and malic acid (in apples and grapes).

Properties of Bases

General Properties

• Bitter taste: Bases often taste bitter, which is why they are less common in foods.

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

• Litmus test: Bases turn red litmus paper blue.

Example: Many cleaning products, such as ammonia and soap, are basic and feel slippery to the touch.

Common Bases

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

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

• Sodium bicarbonate (NaHCO3): 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.

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.

Conjugate Acid-Base Pairs

In Brønsted–Lowry reactions, acids and bases exist in pairs related by the gain or loss of a proton. The acid becomes its conjugate base after donating a proton, and the base becomes its conjugate acid after accepting a proton.

Reactions of Acids and Bases

Neutralization Reactions

When an acid reacts with a base, they neutralize each other, forming water and a salt. The general equation is:

$\mathrm{Acid\ +\ Base\ \rightarrow\ Water\ +\ Salt}$

For example, hydrochloric acid reacts with sodium hydroxide:

$\mathrm{HCl(aq)\ +\ NaOH(aq)\ \rightarrow\ H_2O(l)\ +\ NaCl(aq)}$

Acid Reactions with Metals and Metal Oxides

• With metals: Acids react with certain metals to produce hydrogen gas and a salt.

• With metal oxides: Acids react with metal oxides to produce water and a 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.

Titration Calculations

To calculate the unknown concentration, use the balanced equation and stoichiometry:

$\mathrm{M_1V_1 = M_2V_2}$ (for monoprotic acids and bases)

Strong and Weak Acids and Bases

Strong Acids and Bases

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

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

Weak Acids and Bases

• Weak acids: Only partially ionize in solution (e.g., HF, acetic acid).

• Weak bases: Only partially accept protons or produce OH− (e.g., NH3).

Self-Ionization of Water and the Ion Product Constant

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−:

$\mathrm{2H_2O(l) \rightleftharpoons H_3O^+(aq) + OH^-(aq)}$

Ion Product Constant for Water (Kw)

At 25°C, $K_w = [H_3O^+][OH^-] = 1.0 \times 10^{-14}$

In a neutral solution, $[H_3O^+] = [OH^-] = 1.0 \times 10^{-7}$ M.

The pH Scale

Definition and Calculation

The pH scale expresses the acidity or basicity of a solution based on the concentration of H3O+ ions:

$\mathrm{pH = -\log_{10}[H_3O^+]}$

At 25°C:

• pH < 7: Acidic solution

• pH = 7: Neutral solution

• pH > 7: Basic solution

Relationship Between pH and [H3O+]

A decrease of 1 pH unit corresponds to a tenfold increase in [H3O+].

Buffers

Definition and Function

A buffer is a solution containing significant amounts of both a weak acid and its conjugate base. Buffers resist changes in pH when small amounts of acid or base are added. Human blood is an example of a natural buffer system.

Chemistry and Health Applications

Alkaloids

Alkaloids are organic bases found in plants, often with medicinal or toxic properties (e.g., caffeine, morphine, coniine).

Antifreeze Poisoning

Ethylene glycol, found in antifreeze, is metabolized to glycolic acid, which can overwhelm the body's buffer system and cause dangerous drops in blood pH.

Summary Table: Strong Acids and Bases

Strong Acids	Strong Bases
HCl (hydrochloric acid)	NaOH (sodium hydroxide)
HBr (hydrobromic acid)	KOH (potassium hydroxide)
HI (hydroiodic acid)	Ba(OH)2 (barium hydroxide)
HNO3 (nitric acid)	Ca(OH)2 (calcium hydroxide)
HClO4 (perchloric acid)	Sr(OH)2 (strontium hydroxide)
H2SO4 (sulfuric acid, first proton)	LiOH (lithium hydroxide)

Key Equations

• $\mathrm{pH = -\log_{10}[H_3O^+]}$

• $\mathrm{[H_3O^+] = 10^{-pH}}$

• $\mathrm{K_w = [H_3O^+][OH^-] = 1.0 \times 10^{-14}}$ at 25°C

• $\mathrm{pOH = -\log_{10}[OH^-]}$

• $\mathrm{pH + pOH = 14}$ at 25°C

Learning Objectives

• Identify and describe properties of common acids and bases.

• Apply Arrhenius and Brønsted–Lowry definitions to classify acids and bases.

• Write equations for neutralization and other acid-base reactions.

• Perform titration calculations to determine unknown concentrations.

• Distinguish between strong and weak acids/bases and their electrolytic properties.

• Calculate pH, pOH, [H3O+], and [OH−].

• Explain the function of buffers and their importance in biological systems.