Theories of Acids and Bases - PowerPoint PPT Presentation

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Theories of Acids and Bases

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  1. Theories of Acids and Bases Topic 8.1

  2. Interesting Background (not required) • concepts acids and bases were loosely defined as substances that change some properties of water • criteria that was often used was taste • substances were classified • salty-tasting • sour-tasting • sweet-tasting • bitter-tasting • sour-tasting substances would give rise to the word 'acid', which is derived from the Greek word oxein, which mutated into the Latin verb acere, which means 'to make sour'

  3. Three Acid-Base Definitions

  4. Arrhenius definition • form hydrogen ions (H+) in aqueous solution • with Arrhenius bases forming hydroxide ions (OH-) in solution

  5. Brønsted – Lowry definition • involves the transfer of a proton (H+) • a “proton” is really just a hydrogen atom that has lost its electron • acids: molecule or ion that acts as proton (H+) donor • bases: molecule or ion that acts as proton (H+) acceptor

  6. acid base acid base • HCl(g) + H2O(l) ⇌ H3O+(aq) + Cl-(aq) • in the forward reaction, • HCl is the acid (will donate H+) • H2O is the base (will accept H+) • in the reverse reaction, • H3O+is the acid (will donate H+) • Cl- is the base (will accept H+)

  7. Conjugate Pairs • an acid-base reaction always involves (at least) two conjugate pairs that differ by an H+ • conjugate acids and conjugate bases are compounds formed when a H+ ion is gained and a H+ is lost • a conjugate pair is: • an acid and its conjugate base • a base an its conjugate acid

  8. Brønsted-Lowry conjugate pairs base acid acid base

  9. Strong acid  Weak conjugate base • Weak acid  Strong conjugate base • Strong base  Weak conjugate acid • Weak base  Strong conjugate acid

  10. Water is amphiprotic/amphoteric(can act as acid or base) • Acid Base Conjugate AcidConjugate Base • HCl + H2O  H3O+ + Cl- • H2PO4- + H2O  H3O++HPO42- • NH4+ + H2O  H3O++ :NH3 • Base Acid Conjugate AcidConjugate Base • :NH3 + H2O  NH4++ OH- • PO43- + H2O  HPO42- + OH-

  11. Donating protons… • monoprotic acids contain a single proton that can be donated (HCl, HNO3, HNO2, CH3COOH) • diprotic acids contain two protons that can be donated (H2CO3, H2SO4, H2SO3) • triprotic acids contain three protons that can be donated (H3PO4) • for a substance to be an acid, the hydrogen usually has to be attached to oxygen or a halogen • for example, in CH3COOH, only the H on “OH” is able to be donated, the three hydrogens on carbon are non-acidic (do not write this C2H4O2)

  12. Lewis Acid and Base Definitions • most general/encompassing definition • must have lone pairs (ligands) available to donate • Lewis acids accept a pair of electrons to form a dative covalent bond • Lewis bases donate a pair of electrons to form a dative covalent bond acid base

  13. LONE PAIR ACCEPTOR = Acid LONE PAIR DONOR = Base

  14. LONE PAIR DONOR = Base LONE PAIR ACCEPTOR = Acid

  15. when ammonia donates a pair of electrons to hydrogen, its a Lewis base • when boron trifluoride accepts a pair of electrons from nitrogen (in NH3) its a Lewis acid + +