Acid-Base Models Beyond Bronsted: Lewis, Solvents and Hard and Soft
Lewis acids accept electron pairs and Lewis bases donate them, including in reactions without proton transfer. Hard and soft classifications help predict relative binding preferences. The solvent-system model defines acids and bases using a solvent's characteristic ions.
What a learner can do afterwards
- Identifies the Lewis acid and base in a reaction where no proton moves
- Classifies specified Lewis acids and donor atoms as hard or soft and predicts relative binding preferences under comparable conditions
- Uses hard and soft preferences as one factor in explaining carbonate and sulfide mineral associations
- Applies the solvent-system definition to a named non-aqueous solvent
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A Lewis base supplies an electron pair for a bond; a Lewis acid accepts it. In NH3 + BF3, nitrogen supplies its lone pair to boron: NH3 is the base and BF3 is the acid. No proton transfers. Brønsted-Lowry acids donate protons and bases accept them. That definition also applies outside water.
Metal ions can accept ligand pairs. In Ag+ + 2NH3 -> [Ag(NH3)2]+, Ag+ accepts pairs from two NH3 bases. The neutral ligands add no charge: +1 + 2(0) = +1. Product charge does not identify every reactant's Lewis role. Follow the donated pairs.
Hard species resist polarisation; soft species are more easily polarised. Hard ions often have high charge density, but neutral bases can also be hard. Mg2+, Al3+ and H+ are hard acids; Ag+, Hg2+ and Pt2+ are soft acids. F-, O^2- and NH3 are hard bases; I- and S^2- are soft bases. For comparable partners, HSAB predicts hard-hard and soft-soft preferences, not a universal ranking of binding strengths.
Magnesite, MgCO3, pairs hard Mg2+ with carbonate's hard oxygen donors. Cinnabar, HgS, pairs soft Hg2+ with soft sulfide. These associations fit HSAB. The rule cannot predict mineral abundance or an ore deposit by itself: solubility, available elements and chemical conditions also matter. Magnesium occurs in other minerals too.
The solvent-system model starts with self-ionisation. Liquid ammonia has 2NH3 ⇌ NH4+ + NH2-. An acid increases its characteristic cation, NH4+; a base increases its characteristic anion, NH2-. Neutralisation gives NH4+ + NH2- -> 2NH3. Water instead uses H3O+ and OH-. Brønsted-Lowry and Lewis models can still describe reactions in either solvent.
Lewis roles follow electron pairs. HSAB predicts relative preferences. Solvent-system roles follow the solvent's characteristic ions.
Lesson, questions and flashcards written for this dataset.
Readings and flashcards written for this dataset. Video: Khan Academy, Acid-base definitions. Video reuse licence not confirmed.
More to read
Assigning Lewis roles from a new bond
Identify which species supplies the electron pair used in a new bond. That supplier is the Lewis base; the acceptor is the Lewis acid. The electrons become shared in the product, so donation need not mean the release of free electrons.
For NH3 reacting with BF3, nitrogen supplies a lone pair to boron. This makes NH3 the base and BF3 the acid. No proton changes its bonding partner.
The same test applies to a metal complex. In Ag+ + 2NH3 -> [Ag(NH3)2]+, each ammonia ligand supplies a pair and the silver ion accepts it. The charges add separately: +1 + 0 + 0 = +1. The product's charge cannot replace the pair-donation test.
Two descriptions of acid-base reactions
A Brønsted-Lowry classification asks whether a proton transfers. The donor is the acid and the acceptor is the base. This definition does not require water as the solvent.
A Lewis classification asks which species supplies and accepts a bonding pair. When ammonia accepts a proton to form NH4+, nitrogen supplies the pair for the new N-H bond. Ammonia is therefore a base in both descriptions.
When ammonia bonds to BF3, nitrogen again supplies a pair, but no proton transfers. Lewis roles describe that reaction without a proton-transfer step.
Name the model being used. Moving to a different solvent does not by itself make Brønsted-Lowry chemistry unavailable.
What an HSAB prediction can tell you
Polarisation is distortion of an electron distribution. Hard species resist that distortion; soft species are more readily distorted. Ionic charge density is a useful clue, but it is not the definition. Neutral ammonia can be a hard base.
For comparable partners, hard acids tend to favour hard donors and soft acids tend to favour soft donors. Mg2+ and Al3+ are hard acids; Ag+ and Hg2+ are soft. Oxygen donors and fluoride are hard bases; iodide and sulfide are soft.
Magnesite, MgCO3, combines Mg2+ with carbonate's oxygen donors. Cinnabar, HgS, combines Hg2+ with sulfide. These associations agree with HSAB preferences. They do not establish why a deposit formed at a particular place or how much mineral it contains. Element availability, solubility and chemical conditions must also be considered.
Using ammonia's characteristic ions
The solvent-system model begins with a solvent reacting with itself. For liquid ammonia, write 2NH3 ⇌ NH4+ + NH2-. The characteristic positive ion is ammonium, NH4+; the characteristic negative ion is amide, NH2-.
A solute that increases NH4+ is an acid in this model. A solute that increases NH2- is a base. Their neutralisation can be represented by NH4+ + NH2- -> 2NH3. Both atoms and total charge balance.
Water uses H3O+ and OH- as its characteristic ions instead. Changing these ions changes the solvent-system classification, not the meanings of proton donation or electron-pair acceptance. Brønsted-Lowry and Lewis descriptions can still be applied where the reaction fits them.
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