Properties & Theories of Acids & Bases

Key Terms

• Amphoteric: Substances capable of acting as either acids or bases depending on the reaction.
• Arrhenius Theory of Acids and Bases: A theory that defines acids as substances that increase the concentration of $H^+$ ions in aqueous solutions, and bases as substances that increase the concentration of $OH^-$ ions in aqueous solutions.
• Brønsted-Lowry Theory of Acids and Bases: A broader theory that defines acids as proton donors and bases as proton acceptors.
• Conjugate Acid: The species formed when a base gains a proton.
• Conjugate Base: The species that remains after an acid donates a proton.
• Conjugate Acid-Base Pair: A pair consisting of an acid and its conjugate base, or a base and its conjugate acid.
• Hydronium Ion: The ion formed when $H^+$ attaches to water, represented as $H_3O^+$.

Observable Properties of Acids and Bases

• Observable properties are utilized as operational definitions for acids and bases.
• These properties include macroscopic characteristics and behaviors.
• The focus is on observable rather than unobservable properties or causes.

The Arrhenius Theory of Acids and Bases

• Definitions:
    - Acid: A substance that dissociates in water to produce one or more hydrogen ions ($H^+$).
    - Base: A substance that dissociates in water to produce one or more hydroxide ions ($OH^-$).
• Examples of Acids Dissociating in Water:
    - HBr(aq)
  ightarrow H^+(aq) + Br^-(aq)
    - H_2SO_4(aq)
  ightarrow H^+(aq) + HSO_4^-(aq)
    - HClO_4(aq)
  ightarrow H^+(aq) + ClO_4^-(aq)
• Examples of Bases Dissociating in Water:
    - LiOH(aq)
  ightarrow Li^+(aq) + OH^-(aq)
    - KOH(aq)
  ightarrow K^+(aq) + OH^-(aq)
    - Ba(OH)_2(aq)
  ightarrow Ba^{2+}(aq) + 2 OH^-(aq)

Acid-Base Neutralization Reaction

• An acid reacts with a base, resulting in the formation of an ionic compound and water.
• Complete Ionic Equation:
    H^+(aq) + Cl^-(aq) + Na^+(aq) + OH^-(aq)
  ightarrow Na^+(aq) + Cl^-(aq) + H_2O(l)

Limitations of the Arrhenius Theory

• When an acid dissociates, the $H^+$ ion interacts with water molecules and becomes hydrated, forming a hydronium ion ($H_3O^+$).
• The theory does not account for non-aqueous reactions or reactions involving substances without hydrogen or hydroxide ions, such as ammonia ($NH_3$).
    - $NH_3$ acts as a base and produces hydroxide ions when it reacts with water, but the theoretical intermediate (ammonium hydroxide) is not directly observable in experiments.
• Arrhenius theory assumes all acid-base reactions occur in aqueous solutions, which limits its applicability to other solvents.
• Some compounds, such as $HPO_4^{2-}$, appear to produce $H^+$ ions but exhibit basic properties upon testing.
• Overall, while useful for understanding neutralization reactions, the Arrhenius definition is incomplete.

The Brønsted-Lowry Theory of Acids and Bases

• This theory posits that acid-base reactions are characterized by the transfer of protons ($H^+$).
• Definitions:
    - Acids must contain hydrogen.
    - Any negatively charged ion can function as a Brønsted-Lowry base.
• Not limited to aqueous solutions; broader application as solvents other than water are considered.

Example of Brønsted-Lowry Theory

• Water can act as a base when it reacts with an acid such as $HCl$ to form a hydronium ion.

Conjugate Acid-Base Pairs

• These pairs consist of two molecules or ions that are related through the transfer of a proton.
• Conjugate Base of an Acid: The species that remains after the acid donates a proton.
• Conjugate Acid of a Base: The species formed when the base accepts a proton.
• Characteristics:
    - Every acid has a conjugate base that is more negatively charged than the acid (having lost a proton).
    - Every base has a conjugate acid that is less negatively charged than the base (having gained a proton).

Sample Problem 1

• Identify the acid, base, conjugate acid, and conjugate base for specific reactions (values to be filled).

Amphoteric Substances

• Substances that can act as either an acid (donating protons) or a base (accepting protons).
• The prefix "ampho-" comes from the Greek word "amphō," meaning "both."

Sample Problem 2

• Determine in which reaction $H_2PO_4^-$ acts as a base and where it acts as an acid:
    1. H_2PO_4^-(aq) + H_2O(l)
  ightarrow H_3PO_4(aq) + OH^-(aq)
    2. H_2PO_4^-(aq) + H_2O(l)
  ightarrow HPO_4^{2-}(aq) + H_3O^+(aq)

Summary Table: Comparing Theories