Acid-Base Models Beyond Bronsted: Lewis, Solvents and Hard and Soft · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Acids without protons

Science · Chemistry · ages 19-20
Name ______________________   Date ____________

  1. NH3 reacts with BF3 without proton transfer. Which reactant is the Lewis acid?

    • Ammonia, because supplying its lone pair makes it the electron-pair acceptor
    • Both reactants, because every species forming a new bond is a Lewis acid
    • Boron trifluoride, because it accepts ammonia's electron pair

  2. Which statement defines a Lewis acid?

    • It donates a proton to another species in a reaction
    • It accepts an electron pair from another species
    • It supplies hydroxide ions when dissolved in water

  3. For comparable partners, HSAB predicts a preference for hard acid-hard base and soft acid-soft base binding.

    Circle one:   True   False

  4. A metal ion is a soft Lewis acid. With other factors comparable, which anion does HSAB predict it will prefer?

    • Fluoride (F-)
    • Oxide (O^2-)
    • Iodide (I-)

  5. Magnesite has formula MgCO3. Which interpretation fits HSAB?

    • Hard Mg2+ pairs with carbonate's hard oxygen donors
    • Soft Mg2+ pairs with carbonate through soft carbon donors
    • Hard Mg2+ can occur only in minerals containing carbonate

  6. Liquid ammonia self-ionises: 2NH3 ⇌ NH4+ + NH2-. A solute increases NH4+ concentration. How does the solvent-system model classify it?

    • A base, because it increases the characteristic cation
    • An acid, because it increases the characteristic cation
    • An acid, because it increases the characteristic anion

  7. HSAB alone can predict the relative amounts of magnesite and cinnabar in an ore deposit.

    Circle one:   True   False

  8. In Zn2+ + 4NH3 -> [Zn(NH3)4]2+, Lee calls NH3 the acid because the product is positive. Which correction fits both pair donation and charge?

    • Zn2+ accepts pairs; neutral NH3 donates them; total charge stays +2
    • NH3 accepts pairs from Zn2+; all four ligands become positive; total charge becomes +4
    • Both reactants donate pairs; a positive product makes both reactants Lewis acids
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Acids without protons W1-mt_jarui-9yfx-s1

  1. Boron trifluoride, because it accepts ammonia's electron pair · NH3 supplies the nitrogen lone pair. BF3 accepts it, so BF3 is the Lewis acid.
  2. It accepts an electron pair from another species · A Lewis acid accepts an electron pair. Proton donation and hydroxide production are features of other acid-base definitions.
  3. True · Matching hardness is the qualitative HSAB preference. The qualification does not promise a universal ranking of binding strengths.
  4. Iodide (I-) · Iodide is soft. Fluoride and oxide are hard, so HSAB predicts a relative preference for iodide.
  5. Hard Mg2+ pairs with carbonate's hard oxygen donors · Mg2+ is hard, and carbonate binds through hard oxygen donors. This pairing does not make carbonate the only possible magnesium mineral.
  6. An acid, because it increases the characteristic cation · NH4+ is ammonia's characteristic cation. A solute that increases it is an acid in the solvent-system model.
  7. False · HSAB helps interpret partner preferences. Mineral amounts also depend on available elements, solubility and chemical conditions.
  8. Zn2+ accepts pairs; neutral NH3 donates them; total charge stays +2 · NH3 supplies nitrogen lone pairs and is the base. Zn2+ accepts them and is the acid. Four neutral ligands leave the total charge at +2.
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