Overview
This lecture introduces acids and bases, covering their definitions, behaviors in water, classification as strong or weak, how to write their dissociation and reaction equations, and the principles of ionic and net ionic equations.
Acid Definitions and Behavior
- According to Arrhenius, an acid increases the hydrogen ion (H⁺) content of water.
- All Arrhenius acids contain hydrogen that can dissociate from the molecule.
- Acids in water ionize to give a hydrogen ion (proton) and a conjugate base (anion).
- In water, H⁺ immediately binds to H₂O, forming the hydronium ion (H₃O⁺).
- Modern definition: an acid increases hydronium ion concentration in water.
Strong vs. Weak Acids
- Strong acids dissociate 100% in water; examples: HCl, HNO₃, H₂SO₄, HClO₄, HBr, HI.
- Any acid not on the strong acid list is classified as a weak acid.
- Weak acids dissociate less than 100% and exist in dynamic equilibrium (some molecules dissociate, others recombine).
Writing Acid Dissociation Reactions
- Products: hydronium ion (H₃O⁺) and a conjugate base (anion).
- For strong acids, use a one-way arrow; for weak acids, use a double-headed (equilibrium) arrow with water above.
- To find the conjugate base, remove a hydrogen from the acid and add a negative charge.
Polyprotic Acids
- Polyprotic acids have more than one ionizable hydrogen atom (e.g., H₂SO₄, H₃PO₄).
- They lose protons one at a time; first dissociation is easier than the second.
- For equations, typically show only the first dissociation step.
Arrhenius Bases and Their Reactions
- An Arrhenius base increases the hydroxide ion (OH⁻) concentration in water.
- Strong bases: LiOH, NaOH, KOH, Ca(OH)₂, Sr(OH)₂, Ba(OH)₂, which dissociate completely in water.
- Bases with two OH⁻ (like Ca(OH)₂) produce two hydroxide ions on dissociation.
Acid-Base Reactions and Salt Formation
- Acid + base → water + salt (an ionic compound composed of the base’s cation and the acid’s anion).
- For polyprotic acids or bases with multiple OH⁻, form one water molecule per exchangeable H⁺ or OH⁻.
- Balance charges and molecules for correct salt formula.
Ionic and Net Ionic Equations
- Strong acids and bases are split into their ions in ionic equations; weak acids remain intact.
- Water is written as a liquid and not split.
- Spectator ions (appear unchanged on both sides) are omitted in the net ionic equation.
- For any strong acid + strong base: Net ionic equation is H₃O⁺ + OH⁻ → 2 H₂O.
Key Terms & Definitions
- Arrhenius Acid — Substance that increases H⁺ concentration in water.
- Hydronium Ion (H₃O⁺) — The ion formed when H⁺ bonds to H₂O in solution.
- Conjugate Base — The species remaining after an acid donates a proton.
- Strong Acid/Base — Completely dissociates in water.
- Weak Acid/Base — Partially dissociates, exists in equilibrium.
- Polyprotic Acid — Acid with more than one ionizable hydrogen atom.
- Salt — Ionic compound formed from acid-base reaction.
- Spectator Ion — Ion unchanged during a chemical reaction.
- Net Ionic Equation — Shows only species that actually participate in the reaction.
Action Items / Next Steps
- Memorize the six strong acids and six strong bases.
- Practice writing acid/base dissociation and reaction equations.
- Be able to identify and write net ionic equations for acid-base reactions.
- Review the process for determining conjugate bases and forming salts.