AA

Acid-Base Theories and pH

Theoretical Acid–Base Definitions

Early Theories

  • Lavoisier's Oxygen Theory (1777):

    • Antoine Lavoisier discovered "vital air" (oxygen) and its role in combustion.

    • Burning sulfur and phosphorus in air produced acidic solutions.

    • Lavoisier believed oxygen was a common element in all acids and named it from the Greek words oxys (sour) and genes (born).

  • Challenges to Lavoisier's Theory:

    • Claude Louis Berthollet (1789) showed that hydrocyanic acid (HCN(aq)) did not contain oxygen.

    • Humphrey Davy discovered that muriatic (hydrochloric) acid consisted of hydrogen and chlorine, not oxygen.

Arrhenius Theory of Acids and Bases

  • Svante Arrhenius (1880s):

    • Developed a theory about electrolytes, explaining their electrical conductivity in solutions.

    • Electrolytes dissociate into ions when dissolved, allowing them to move freely and conduct electricity.

  • Arrhenius's Modifications:

    • Modified his theory to include acids and bases, as their solutions also conduct electricity, implying the presence of ions.

Definitions

  • Arrhenius Base:

    • An ionic compound that dissociates into cations and hydroxide ions (OH^{-}) in water.

    • Hydroxide ions give bases their characteristic properties.

    • Example: Calcium hydroxide dissociation: Ca(OH)_{2}(s) \rightarrow Ca^{2+}(aq) + 2OH^{-}(aq)

  • Arrhenius Acid:

    • A molecular compound that ionizes to produce hydrogen ions (H^{+}) in water.

    • Hydrogen ions give acids their characteristic properties.

    • Example: Nitric acid ionization: HNO{3}(aq) \rightarrow H^{+}(aq) + NO{3}^{-}(aq)

Ionization vs. Dissociation

  • Dissociation:

    • Water molecules pull apart the positive and negative ions of an ionic hydroxide (or any soluble ionic compound).

    • Ions already exist and are separated.

  • Ionization:

    • Formation of ions from uncharged molecules.

    • Previously uncharged entities become ionized.

  • Summary: Ionic compounds dissociate in water; molecular compounds ionize.

Neutralization Reactions

  • Hydrochloric Acid and Sodium Hydroxide:

    • Hydrochloric acid completely ionizes: HCl(aq) \rightarrow H^{+}(aq) + Cl^{-}(aq)

    • Sodium hydroxide completely dissociates: NaOH(s) \rightarrow Na^{+}(aq) + OH^{-}(aq)

    • Total ionic equation: H^{+}(aq) + Cl^{-}(aq) + Na^{+}(aq) + OH^{-}(aq) \rightarrow H_{2}O(l) + Na^{+}(aq) + Cl^{-}(aq)

    • Spectator ions (Na^{+} and Cl^{-}) are removed.

    • Net ionic equation: H^{+}(aq) + OH^{-}(aq) \rightarrow H_{2}O(l)

  • General Neutralization Reaction:

    • Simplified form: H^{+}(aq) + OH^{-}(aq) \rightarrow H_{2}O(l)

  • Hydrochloric Acid and Aluminum Hydroxide:

    • Balanced chemical equation: 3HCl(aq) + Al(OH){3}(aq) \rightarrow 3H{2}O(l) + AlCl_{3}(aq)

    • Total ionic equation: 3H^{+}(aq) + 3Cl^{-}(aq) + Al^{3+}(aq) + 3OH^{-}(aq) \rightarrow 3H_{2}O(l) + Al^{3+}(aq) + 3Cl^{-}(aq)

    • Net ionic equation: 3H^{+}(aq) + 3OH^{-}(aq) \rightarrow 3H_{2}O(l)

    • Simplified net ionic equation: H^{+}(aq) + OH^{-}(aq) \rightarrow H_{2}O(l)

Limitations of Arrhenius Theory

  • Some substances with basic properties do not contain hydroxide ions in their formula (e.g., ammonia, NH_{3}(aq)).

Importance of Competing Theories

  • Competing theories push scientists to refine and test their theories to explain more examples.

Strong and Weak Acids

  • Strength vs. Concentration:

    • Strength refers to the degree of ionization or dissociation.

    • Concentration refers to the quantity of solute in a solution.

  • Strong Acid:

    • A substance that ionizes completely in water.

    • Example: Hydrochloric acid, HCl(aq) \rightarrow H^{+}(aq) + Cl^{-}(aq)

    • Good conductor of electricity due to many ions.

  • Weak Acid:

    • A substance that only partially ionizes in water.

    • Example: Ethanoic acid, HC{2}H{3}O{2}(aq) \rightleftharpoons H^{+}(aq) + C{2}H{3}O{2}^{-}(aq)

    • Weak electrolyte; does not conduct electricity as well as strong acids.

  • General Acid Ionization:

    • HA(aq) \rightarrow H^{+}(aq) + A^{-}(aq)

    • Strong acids ionize almost completely.

    • Weak acids ionize only partially (e.g., about 2%).

Examples of Strong and Weak Acids

  • Strong Acids:

    • Hydrochloric acid (HCl(aq)): stomach acid, steel cleaning.

    • Phosphoric acid (H{3}PO{4}(aq)): rust remover, ingredient in pop.

    • Nitric acid (HNO_{3}(aq)): fertilizer and rocket fuel production.

    • Perchloric acid (HClO_{4}(aq)): bleaching agent.

    • Sulfuric acid (H{2}SO{4}(aq)): car batteries, detergent and plastic production.

  • Weak Acids:

    • Ethanoic acid (HC{2}H{3}O_{2}(aq)): vinegar, pickled vegetables.

    • Methanoic acid (HCO_{2}H(aq)): produced by ants.

    • Carbonic acid (H{2}CO{3}(aq)): makes rain naturally acidic, ingredient in pop.

pH and Acidity

  • pH Scale:

    • Numerical scale from 0 to 14 to measure acidity or basicity.

    • pH < 7: Acidic

    • pH > 7: Basic

    • pH = 7: Neutral

  • pH Change and Acidity:

    • One pH unit change represents a tenfold change in acidity or basicity.

    • Example: Lemon juice (pH 2) is ten times more acidic than pop (pH 3).

    • Two pH unit change represents a hundredfold change in acidity or basicity.

    • Example: Lemon juice (pH 2) is 100 times more acidic than tomatoes (pH 4).

  • Examples:

    • Concentrated sodium hydroxide (pH 14) is 10,000,000 times more basic than water (pH 7).

  • Environmental Impact:

    • Living things can tolerate only small pH changes.

    • Freshwater fish thrive in pH 5.5 to 7.5.

    • Clean rain has a pH of about 5.6 due to carbonic acid.

    • Acid precipitation (pH 4.6) is 10 times more acidic than normal rain, devastating aquatic life.

  • Beneficial Acidic Environments:

    • Slightly acidic shampoo (or lemon rinse) closes hair cuticle plates, making hair smoother.