Arrhenius - Chemistry

1. Fundamental Concepts

Arrhenius is an early acid-base theory defining acids and bases based on their behavior in aqueous solution (water):
- Acid: Substance that increases H⁺ (hydrogen ion) concentration in water.
- Base: Substance that increases OH⁻ (hydroxide ion) concentration in water.
- Neutralization: Acid + Base → Salt + Water (H⁺ + OH⁻ → H₂O).

2. Key Concepts

Only applies to water solutions (major limitation).
H⁺ in water actually exists as hydronium ion (H₃O⁺).
Strong acids/bases dissociate completely; weak acids/bases dissociate partially.
Does not explain basic substances without OH⁻ (e.g., NH₃).
Forms foundation for later theories (Brønsted-Lowry, Lewis).

3. Examples

Easy

Which is an Arrhenius acid?

A) NaOH
B) HCl
C) NaCl

Answer: B (HCl releases H⁺ in water)

Medium

Write the dissociation equation for Arrhenius base KOH in water.

Answer: KOH(aq) → K⁺(aq) + OH⁻(aq)

Hard

Explain why NH₃ is not an Arrhenius base but is basic in water, using Arrhenius rules.

Answer: NH₃ has no OH⁻ group and does not directly release OH⁻; it forms OH⁻ indirectly by reacting with water, so it is not classified under Arrhenius definition.

4. Problem-Solving Techniques

Check if the substance releases H⁺ (acid) or OH⁻ (base) directly in water.
Write dissociation equations to identify ions produced.
Distinguish strong (full dissociation) vs. weak (partial) using ion concentration.
Neutralization problems: balance H⁺ and OH⁻ to form H₂O.
Remember the limitation: only valid in water; substances without OH⁻ are not Arrhenius bases.