PH Of Weak Acids & Plyprotic Acids - Chapter 7 Notes

02/18/2022 02/21/2022 •pH of weak acid -[H]=A[Acid] •when [HA] ka the simple approximation Works •doesn't work for stronger weak acids at low concentrations - Ex. CH3COOH CH3COOH 10.10 1x107 C -x E 0.10-x KA= 1.8x105 0.30- (assuming x20.10) \[x=[H^{+}]=\sqrt{(1.1\times10^{-7}(.10)}=1.34\times10^{-3}M\] \[[H+]=\frac{-L_{m}+\sqrt{L_{m}}^{2}+4L_{m}DdL_{m}}{2}\] • use for shonger weak acids at low concentrations •pla = plt of the Ka Mixtures of Weak Acids -should be treated based on relative Ka values of acids in mixture - strongest acid determines [H] - •To check approximation, put x over initial concentration of multiply by 100 -if is less than 3% it is accurate approximation Mixture of weak acids -use concentration of H* from the stronger weak acid as initial concentration for second weak acid -Weak Bases Base 5 Base Conjugate Acid Strength Acid Strength (CH, NH \[4.4\times10^{4}\] (CH, NH NH \[1.8\times10^{-5}\] NH CN \[1.6\times10^{15}\] HCN \[1.5\times10^{-41}\] HF -usually have Nitrogen -solve for OH concentration instead of H •can use equations for pH of KA, just remember to subtract from 14 02/21/2022 02/23/2022 Polyprotic Acids -monoprotic acid - one H+ dissociates -diprotic acid - 2 H+ dissociates •Step Equilibria - H₂A+H₂O H₃O+HA- : Ka₁ - HA-+H₂O H₃O+A²⁻ : Ka₂ - Ka₁Ka₂ •Coulomb's Law - explains stepwise dissociation - force between charged particles is proportional to the product of their charges - H₂A+H₂O H₃O+HA- : q₁=+1, q₂=-1 - HA-+H₂O H₃O+A²⁻ : q₁=+1, q₂=-2 (twice as strong attraction) Distribution functions for weak acids & bases - monoprotic acids [H₃O+] = Ka₁[HA] / [A-] [A-] = Ka₁[HA] / [H₃O+] αHA = [HA] / Ctotal αA- = [A-] / Ctotal mole fraction of HA = αHA mole fraction of A- = αA- mole fraction of HA + mole fraction of A- = 1 Ctotal = total concentration of HA-A Distribution functions for polyprotic acids 02/21/2022 02/25/2022 amphoteric ion - either acts as an acid or a base - HCO₃⁻ - hydrolysis of salt solutions - a cation or anion reacts with H₂O to form H₃O⁺ & OH⁻ - Salt derived from a strong acid & a strong base - neutral solution (pH=7) - NaCl from NaOH + HCl - Salt derived from weak acid of strong base - basic solution (pH>7) - NaCH₃O (from NaOH & CH₃COOH) - Salt derived from strong acid & weak base - acidic solution (pH<7) - NH₄Cl (from NH₃ + HCl) - Salt derived from weak acid of weak base - pH depends on acid/base involved - no spectator ions when amphoteric salt is created Amphoteric - to find which are amphoteric - find conjugate acid - find conjugate base - find the ones that have both weak conjugate acids & conjugate bases - pH is the average of the two pKs Lewis Acids & Bases - Lewis acid - any substance that can accept a pair of electrons - small cations - molecules with unfilled octets - ex. - highly charged transition metal cations: Fe²⁺, Fe³⁺, Co²⁺ - group 3 cations: Al³⁺, Ga³⁺, compounds: AlCl₃ - smaller group 2 cations: Be²⁺, Mg²⁺