Acids and Bases Topic 8

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' Interesting Background (not required)

Two Acid-Base Definitions

1.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

HCl (g) + H 2 O (l) ⇌ H 3 O + (aq) + Cl - (aq) – in the forward reaction, HCl is the acid (will donate H + ) H 2 O is the base (will accept H + ) – in the reverse reaction, H 3 O + is the acid (will donate H + ) Cl - is the base (will accept H + ) acidbaseacidbase

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

Brønsted-Lowry conjugate pairs acidbase acid

Strong acid  Weak conjugate base Weak acid  Strong conjugate base Strong base  Weak conjugate acid Weak base  Strong conjugate acid AcidStrengthBaseStrength H 2 SO 4 Very Strong Fairly Strong Weak Very Weak HSO 4 - Very Weak Weak Less Weak Fairly Strong HClCl - HNO 3 NO 3 - H3O+H3O+ H2OH2O HSO 4 - SO 4 2- CH 3 COOHCH 3 COO - H 2 CO 3 HCO 3 - NH 4 + NH 3 HCO 3 - CO 3 2- H2OH2OOH -

14 Water is amphiprotic/amphoteric (can act as acid or base) AcidBase Conjugate Acid Conjugate Base HCl+ H 2 O   H 3 O + +Cl - H 2 PO H 2 O   H 3 O + + HPO 4 2- NH H 2 O   H 3 O + +:NH 3 Base Acid Conjugate Acid Conjugate Base :NH 3 + H 2 O   NH 4 + +OH - PO H 2 O   HPO OH -

Donating protons… monoprotic acids contain a single proton that can be donated (HCl, HNO 3, HNO 2, CH 3 COOH) diprotic acids contain two protons that can be donated (H 2 CO 3, H 2 SO 4, H 2 SO 3 ) triprotic acids contain three protons that can be donated (H 3 PO 4 ) for a substance to be an acid, the hydrogen usually has to be attached to oxygen or a halogen – for example, in CH 3 COOH, only the H on “OH” is able to be donated, the three hydrogens on carbon are non-acidic (do not write this C 2 H 4 O 2 )

2.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 base acid

when ammonia donates a pair of electrons to hydrogen, its a Lewis base when boron trifluoride accepts a pair of electrons from nitrogen (in NH 3 ) its a Lewis acid + +