Acids and bases are not defined in just one way because different reactions show different patterns. The Arrhenius theory explains acids and bases in water, the Bronsted-Lowry theory focuses on proton transfer, and the Lewis theory focuses on electron-pair transfer. Learning all three theories helps students classify reactions more accurately and see how chemistry connects across solutions, gases, and molecular structures.
The three theories form a useful ladder of generality. Arrhenius acids produce H+ in aqueous solution and Arrhenius bases produce OH-, while Bronsted-Lowry acids donate H+ and bases accept H+. Lewis acids accept an electron pair and Lewis bases donate an electron pair, so this theory can describe reactions that do not involve hydrogen ions at all.
Key Facts
- Arrhenius acid: produces H+ or H3O+ in water, such as HCl(aq) + H2O(l) -> H3O+(aq) + Cl-(aq).
- Arrhenius base: produces OH- in water, such as NaOH(aq) -> Na+(aq) + OH-(aq).
- Bronsted-Lowry acid: proton donor; Bronsted-Lowry base: proton acceptor.
- Conjugate acid-base pairs differ by exactly one H+, such as NH4+ and NH3.
- Lewis acid: electron-pair acceptor; Lewis base: electron-pair donor.
- Acid-base neutralization in water often forms salt and water, such as HCl(aq) + NaOH(aq) -> NaCl(aq) + H2O(l).
Vocabulary
- Arrhenius acid
- An Arrhenius acid is a substance that increases the concentration of H+ or H3O+ ions when dissolved in water.
- Arrhenius base
- An Arrhenius base is a substance that increases the concentration of OH- ions when dissolved in water.
- Bronsted-Lowry acid
- A Bronsted-Lowry acid is a substance that donates a proton, H+, to another particle.
- Conjugate acid-base pair
- A conjugate acid-base pair consists of two species that differ by one proton, H+.
- Lewis base
- A Lewis base is a substance that donates an electron pair to form a covalent bond.
Common Mistakes to Avoid
- Calling every hydrogen-containing compound an acid. A substance is acidic only if it can donate H+ or increase H3O+ under the conditions being considered.
- Forgetting that Arrhenius definitions apply only to aqueous solutions. A reaction outside water may still be Bronsted-Lowry or Lewis acid-base chemistry, but it is not automatically Arrhenius.
- Mixing up conjugate acids and conjugate bases. After an acid donates H+, it becomes its conjugate base, and after a base accepts H+, it becomes its conjugate acid.
- Thinking Lewis acids must contain hydrogen. Lewis acids accept an electron pair, so species such as BF3 or Al3+ can act as acids even without any H atoms.
Practice Questions
- 1 Classify each species as an Arrhenius acid, Arrhenius base, or neither in water: HNO3, KOH, CH4, Ba(OH)2. How many are Arrhenius bases?
- 2 In the reaction NH3(aq) + H2O(l) -> NH4+(aq) + OH-(aq), identify the Bronsted-Lowry acid, base, conjugate acid, and conjugate base. If 0.25 mol of NH3 reacts completely, how many moles of NH4+ form?
- 3 BF3 reacts with NH3 to form F3B-NH3. Explain which reactant is the Lewis acid and which is the Lewis base, using electron-pair donation and acceptance.