acids and bases

• Bronsted acid

  proton donor
• Bronsted base

  proton acceptor
• conjugate acid-base pair

  two species that are different from each other by an H+ ion
• pH

  pH = -log[H+]
• [H+]

  [H+] = 10^-pH
• strong acids

  -completely ionised in solution

  HA (aq) → H+ (aq) + A- (aq)

  examples: HCl, HBr, HI, HNO3, H2SO4, HClO4
• the ionic product of water

  -In all aqueous solutions, an equilibrium exists in water where a few water molecules dissociate into protons and hydroxide ions

  H2O(l) <---> H+(aq) + OH-(aq)

  -Kw = [H+][OH-]
• strong bases

  -are completely ionised in solution

  BOH(aq) ---> B+(aq) + OH-(aq)
• weak acids

  -partially dissociates in aq solutions eg. carbox. acids

  -constant Ka
• pH of weak acids

  Ka = [H+]^2 / [HA]
• pKa

  -logKa
• thymol blue

  in acid - red

  in alkali - yellow

  pKa - 1.7

  pH range - 1.2-1.8
• methyl orange

  in acid - red

  in alkali - yellow

  pKa - 3.7

  pH range - 3.1-4.4
• bromophenol blue

  in acid - yellow

  in alkali - blue

  pKa - 4.1

  pH range - 3.4-4.6
• methyl red

  in acid - red

  in alkali - yellow

  pKa - 5.1

  pH range - 4.4-6.2
• phenolphthalein

  in acid - colourless

  in alkali - pink

  pKa - 9.3

  pH range 8.3-10.0
• strong acid strong base

  -pH changes from 4-10

  -methyl red and phenolphthalein suitable

  -methyl red not ideal but shows good enough colour change
• weak acid strong base

  -pH changes from 7-10

  -phenolphthalein only suitable one
• strong acid weak base

  -pH change from 4-7

  -methyl red most suitable

  -methyl orange often used as it shows good enough colour change
• weak acid weak base

  -no sudden pH change so no suitable indicator
• buffer solution

  -solution which resists changes in pH when small amounts of acid or alkalis are added

  -used to keep pH almost constant

  -consists of weak acid - conjugate base

  -consists or weak base - conjugate acid
• uses of buffer solutions in controlling pH of blood

  -HCO3- ions act as a buffer to keep blood pH between 7.35 and 7.45

  -body cells produce CO2 during aerobic respiration

  -CO2 will combine with H20 in blood to form a solution containing H+ ions

  CO2 (g) + H2O (l) ⇌ H+ (aq) + HCO3- (aq)

  -if conc of H+ not regulated, pH would drop and cause acidosis - too much acid in blood - cause malfunctioning