acid base

• dissociation vs ionisation

  dissociation: break apart

  ionisation: form ions
• acid base theories:

  1)arrhenius: acid released H+ base releases OH- (limitation: restricted to acid base reaction in aq)

  2)bronsted-lowry: proton donor/ accepter

  3)lewis: epa/ epd
• conjugate pairs: differ by 1 proton
• H2O is amphiprotic: can accept/donate proton, forming acid or base
• as temperature increases, Kw increases as H>0
• units of Ka/Kb: moldm-3
• the larger the KA, the stronger the acid

  the larger the pKa, the weaker the acid
• is Ka or pH better indicators?

  • Ka, as it is constant with constant temperature & doesnt vary with acid conc, Ka of SA always larger than WA
  • pH varies with acid conc: pH of SA can be larger or smaller than WA depending on conc
• Ka1 > Ka2 > Ka3. successive dissociation involves acidic species that are increasingly more -ve charged, making donation of +ve H+ more difficult
• the larger the Ka of acid, the smaller the Kb of CB. if HA is stronger than HX, A- weaker base than X-, rxn between X- and HA will occur to a greater extent (SA & SB rxn)
• salt will undergo hydrolysis if:

  • cation is CA of WB
  • anion is CB of WA

  eg. NA+ will not hydrolyse due to low CD, Cl- will not hydrolyse as it is an anion of SA
• what if both cation and anion hydrolyses?

  compare Ka/Kb
• buffer: solution that is able to resist pH change when a small amount of acid or base is added
• buffer capacity: ability of buffer to resist pH change (when pH=pKa / pOH=pKb)
• effective buffer range: pH range which buffer acts effectively in maintaining pH approximately constant (ph=pka+-1)
• why does diluting buffer decrease buffer cap but not pH?

  • buffer cap: for the vol of buffer used for dilution, diluted buffer has less HA & A- to cope with addition of H+ & OH-
  • pH: dilution of HA & A- to the same extent
• at 25.C, Kw=1.0 x 10^-14
• pKw = pH + pOH = 14 at 25.C
• Ka= [H+][A-]/[HA]
• Kb= [BH+][OH-]/[B]
• Kw= Ka x Kb
• buffer:

  pH= pKa + lg[A-]/[HA]

  pOH= pKb + lg[BH+]/B
• Kw = [H+][OH-]
• weak acid if [H+] < [HX]
• suitable indicator

  pH range coincides with region of rapid pH change in titration curve
• why is h3po4 tribasic but h3po3 is dibasic

  O more e- neg than H, e- neg diff btwn P & H smaller, O-H more polar than P-H bond, O- more easily formed