Overview
This lecture covers the behavior of substances in aqueous (water-based) environments, focusing on electrolytes, acids, and conductivity in solutions.
Electrolytes and Conductivity
- To conduct electricity, a substance needs charged particles that can move freely.
- Electrolytes are substances that conduct electricity when dissolved in water.
- Ionic compounds like sodium chloride (NaCl) are electrolytes because they dissociate into ions in water.
- In solid form, ions in NaCl are fixed and do not conduct electricity, but in water, they move freely.
Writing Dissociation Equations
- Dissolving ionic compounds in water forms separate ions; write equations showing each ion with its charge and (aq) for aqueous.
- Example: Na₂SO₄ (s) → 2 Na⁺ (aq) + SO₄²⁻ (aq).
- Balance dissociation equations by adjusting coefficients, not subscripts.
Conductivity and Number of Particles
- Conductivity increases with more ions in solution.
- For example, 1 mole of Fe(NO₃)₃ yields 4 moles of ions (Fe³⁺ + 3 NO₃⁻), making it a strong conductor.
Non-Electrolytes
- Non-electrolytes do not conduct electricity in water; covalent compounds (like ethanol) usually do not dissociate into ions.
Strong and Weak Electrolytes
- Strong electrolytes dissociate completely in water (e.g., most ionic compounds and strong acids).
- Weak electrolytes partially dissociate in water; only some molecules ionize.
Acids, Hydronium, and Reactions
- Acids increase hydrogen ion (H⁺) content in water; H⁺ quickly forms hydronium (H₃O⁺).
- Acid reaction: HCl + H₂O → H₃O⁺ + Cl⁻.
Strong vs. Weak Acids
- Strong acids dissociate fully; memorize the six strong acids: perchloric (HClO₄), nitric (HNO₃), sulfuric (H₂SO₄), hydrochloric (HCl), hydrobromic (HBr), hydroiodic (HI).
- Weak acids only partially dissociate and reach equilibrium (e.g., HF).
Equilibrium and Weak Acids
- Weak acid reactions do not go to completion; instead, the ratio of dissociated to undissociated molecules stays constant at equilibrium.
Polyprotic Acids
- Polyprotic acids have more than one acidic hydrogen (e.g., H₂CO₃).
- Usually, only the first hydrogen ionizes significantly; the second one ionizes much less.
Conductivity and Type of Substance
- Solutions of strong electrolytes like CaBr₂ conduct better than weak acids like H₂CO₃ at equal concentrations.
Key Terms & Definitions
- Electrolyte — substance that conducts electricity in water by forming ions.
- Non-electrolyte — substance that does not conduct electricity in water.
- Strong electrolyte — dissociates completely into ions in water.
- Weak electrolyte — only partially dissociates in water.
- Acid — substance that increases hydronium ion (H₃O⁺) content in water.
- Strong acid — acid that fully dissociates in water.
- Weak acid — acid that partially dissociates in water.
- Polyprotic acid — acid with more than one ionizable hydrogen atom.
- Equilibrium — state where the rates of forward and reverse reactions are equal.
Action Items / Next Steps
- Memorize the six strong acids.
- Practice writing and balancing dissociation equations for ionic compounds and acids.
- Prepare for lab experiments on conductivity and electrolytes.