Acids, Bases & the pH Scale Review

Definitions and Basic Concepts

  • A solution that is a base contains more hydroxide ions than hydrogen ions.

  • Strong acid: An acid that ionizes completely in aqueous solution, yielding a higher concentration of hydrogen ions (H+).

  • Weak acid: An acid that only ionizes partially in water and yields fewer hydrogen ions.

Important Ions in Solutions

  • When water bonds to a hydrogen ion in an aqueous solution, it forms a hydronium ion (H3O+).

  • In aqueous solutions:

    • Hydroxide ion (OH-) is produced by bases.

    • Hydronium ion is formed when water interacts with hydrogen ions.

Acid-Base Reactions

  • During a neutralization reaction, the acid and base react until the number of hydrogen ions equals the number of hydroxide ions. This is known as the equivalence point.

  • Example of a chemical reaction representation:

    • HA + H2O ⇌ A- + H3O+

    • Here, HA represents the acid, and A- is its conjugate base.

Identifying Acids and Bases in Chemical Reactions

  • In the reactions below, the substances acting as acids and bases can be identified:

    • a. HF + H2O ⇌ F- + H3O+

    • Acid: HF

    • Base: H2O

    • b. HNO2 + H2O ⇌ NO2- + H3O+

    • Acid: HNO2

    • Base: H2O

    • c. NH4+ + H2O ⇌ NH3 + H3O+

    • Acid: NH4+

    • Base: H2O

Impact of Strength on pH

  • Strong acids lead to a low pH because they dissociate completely in water, resulting in a higher concentration of hydrogen ions and thus a lower pH value.

  • If the pH of a solution changes by 2 units (for example, from 3 to 5), the hydrogen ion concentration changes by a factor of 100 times (this is derived from the formula: ( [H^+] ext{ is ten times more acidic for each unit decrease in pH} )).

Products of Neutralization Reactions

  • The products of a neutralization reaction between hydrochloric acid (HCl) and sodium hydroxide (NaOH) are:

    • NaCl (sodium chloride, salt) and H2O (water).

pH Scale and Acid-Base Classification

  1. Labeling of the pH Scale:

    • Neutral Solutions: pH = 7

    • Strong Acids: pH < 3

    • Weak Acids: pH = 3 to 6

    • Weak Bases: pH = 8 to 10

    • Strong Bases: pH > 12

  2. Bronsted-Lowry Definitions:

    • Acid: A proton (H+) donor.

    • Base: A proton (H+) acceptor.

  3. Identifying Non-Acids:

    • Example: Ca(OH)2 is not an acid; it is a base.

  4. Relationship between pH and Acidity:

    • If a solution's pH increases by 2 units, it becomes 100 times less acidic.

  5. Strength of Acids and Bases:

    • The strength of an acid or base is directly proportional to its percent ionization.

    • As bases increase in strength, their conductivity increases due to higher concentrations of hydroxide ions.

Properties of Acids and Bases

  • Acids:

    • Sour taste

    • Cause itching and burning

    • Chemical examples: HCl (Hydrochloric acid), H2SO4 (Sulfuric acid), H3PO4 (Phosphoric acid).

  • Bases:

    • Bitter taste

    • Feel slippery or soapy

    • Chemical examples: NaOH (Sodium hydroxide), KOH (Potassium hydroxide), LiOH (Lithium hydroxide).

Understanding Solutions and Their Classification

  • Solutions can be classified as:

    • Acidic: Solutions with pH < 7.

    • Basic/Alkaline: Solutions with pH > 7.

    • Neutral: Solutions with pH = 7.

Practical Examples

  1. The taste of acids is often described as sour (e.g., lemon juice).

  2. Bases are often used in products like hand washing soap and have a slippery feel.

Summary of pH Values for Common Substances

  • Battery Acid: pH 0 (Strong Acid)

  • Lemon: pH 2-3 (Weak Acid)

  • Milk: pH 6-7 (Neutral)

  • Soap: pH 9-10 (Weak Base)

  • Drain Cleaner: pH 13-14 (Strong Base)

Final Notes

  • Label substances on the pH scale and identify their properties accurately based on pH, taste, and reactivity.

  • Understand the relationship between pH, hydrogen ion concentration, and the strength of acids and bases in solution.