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Lesson #5 Polyprotic and Amphiprotic Substances

Chemistry 20: Acids & Bases Notes

Lesson #5: Polyprotic and Amphiprotic Substances

Monoprotic Acids

  • Definition: Monoprotic acids are defined as acids that contain only one hydrogen atom that can be donated to a base.

  • Characteristics:

    • Can react with water to produce hydronium ions only once.

    • Upon dissolution, they exclusively release a single hydrogen ion (H+) as a hydronium ion (H3O+).

    • General Formula: Represented as HA, where H is the ionizable hydrogen, and A represents the anion.

  • Examples:

    • HCl (Hydrochloric acid): A strong acid commonly used in laboratories and industries.

    • HCn (Hydrocyanic acid): A weak acid known for its toxicity.

  • Hydrolysis Reaction Example:

    • HCl(aq) + H2O(l) → H3O+(aq) + Cl-(aq)

Warm-up Activity

  • Use the Modified Arrhenius Theory to write hydrolysis equations for the following acids with water:

    1. HClO4(aq) + H2O(l) → H3O+(aq) + ClO4-(aq)

    2. H2SO4(aq) + H2O(l) → H3O+(aq) + HSO4-(aq) (notably, H2SO4 is a strong acid that can donate more than one proton)

    3. In the last equation, HSO4- is still an acid. Write its hydrolysis equation with water:

    • HSO4-(aq) + H2O(l) → H3O+(aq) + SO4^2-(aq)

Polyprotic Substances

  • Definition: Polyprotic substances are acids that can react with water more than once, undergoing multiple hydrolysis reactions to produce hydronium ions (H3O+) or hydroxide ions (OH-) with each successive reaction.

  • Strength Consideration: The strength of polyprotic acids decreases with each successive hydrolysis reaction.

    • First reaction occurs in more than 50% of cases (significant).

    • Second reaction occurs in less than 1% of cases (weak).

    • Third reaction occurs at a negligible rate (~0%) or is virtually undetectable.

Polyprotic Acids

  • Definition: These acids possess more than one hydrogen atom that can be donated during a reaction.

  • Example: H2SO4 (Sulfuric acid) is a strong polyprotic acid that can donate two protons, thus capable of undergoing two hydrolysis reactions.

  • Characteristics:

    • Most polyprotic acids are weak and only H2SO4 is classified as strong.

  • Examples from the Acid/Base Table:

    • H3PO4 (Phosphoric acid), H2SO3 (Sulfurous acid), H2S (Hydrosulfuric acid), C6H8O6 (Ascorbic acid).

  • Hydrolysis Reactions for Phosphoric Acid:

    • First Reaction: H3PO4(aq) + H2O(l) ⇌ H3O+(aq) + H2PO4-(aq) (< 50%) (First proton donation)

    • Second Reaction: H2PO4-(aq) + H2O(l) ⇌ H3O+(aq) + HPO4^2-(aq) (< 1%) (Second proton donation)

    • Third Reaction: HPO4^2-(aq) + H2O(l) ⇌ H3O+(aq) + PO4^3-(aq) (~0%) (Third proton donation)

  • Conclusion: Notably, only the first reaction is significant, which classifies phosphoric acid as a weak acid.

Polyprotic Bases (Polybasic)

  • Definition: Bases that can react with water multiple times to produce hydroxide ions (OH-).

  • Examples from Acid/Base Table:

    • HPO4^2-, SO4^2-, CO2-.

  • Example 1: Hydrolysis Reactions for Oxalic Acid

    • Example: Show hydrolysis reactions to demonstrate oxalic acid as a polyprotic acid:

    • HOOC-COOH(aq) + H2O(l) ⇌ H3O+(aq) + HOOC-COO-(aq) (< 50%)

    • HOOCCOO-(aq) + H2O(l) ⇌ H3O+(aq) + CO2^2-(aq)

    • Strongest Acid Identification: The first steps produce significant amounts of hydronium, confirming oxalic acid's polyprotic nature.

  • Example 2: Hydrolysis of Sodium Phosphate

    • Reaction Outline:

    • Na3PO4(aq) → 3 Na+(aq) + PO4^3-(aq)

    • PO4^3-(aq) + H2O(l) ⇌ OH-(aq) + HPO4^2-(aq) (≥ 50%) (first significant base reaction)

    • HPO4^2-(aq) + H2O(l) ⇌ OH-(aq) + H2PO4-(aq) (< 50%) (subsequent weaker base reaction)

  • Strongest Base Identification: PO4^3- is identified as the strongest base; it can accept multiple protons until neutralized.

Notes on Acids Ending in COOH

  • The hydrogen atom in the carboxyl group (COOH) is the only one that is donated to water, indicating its role as an acid.

  • Example:

    • CH3COOH (Acetic acid) is monoprotic (only the single H from COOH is donated), whereas H2C6H8O6 (Ascorbic acid) exhibits diprotic behavior (donates the two front hydrogens).

Amphiprotic Substances

  • Definition: Amphiprotic substances are those species capable of reacting with water to produce either hydronium (acting as acids) or hydroxide ions (acting as bases). They can function in both roles depending on the reaction context.

  • Requirements:

    • Must contain at least one hydrogen atom and is usually accompanied by a negative charge.

    • Exception: Water (H2O) is unique in that it can provide and accept protons, fulfilling the requirement of an amphiprotic substance.

  • Examples from the Acid/Base Table:

    • H2PO4-, HSO3-, HCO3-, HPO4^2-.

  • Example Equations:

    • HSO3-(aq) + H2O(l) ⇌ H3O+(aq) + SO3^2-(aq) (acting as an acid)

    • HPO4^2-(aq) + H2O(l) ⇌ OH-(aq) + H2PO4^-(aq) (acting as a base)

Conclusion

  • Complete related worksheets: Worksheet #6 (Polyprotic Substances), Worksheet #7 (Amphiprotic Substances), and Worksheet #8 (Modified Arrhenius Equations).