acids and bases

• acid-base equilibria involve the transfer of protons between substances. therefore substances can be classified as acids or bases depending on their interaction with protons
• a bronsted-lowry acid is a proton donor (e.g NH4+)
• a bronsted-lowry base is a proton acceptor (e.g OH-)
• a strong acid is an acid that completely dissociates to ions when in soltuion with a pH 3-5
• a weak acid is an acid that only slightly dissociates when in a solution with pH 0-1
• strong bases have pH 12-14
• weak bases have a pH 9-11
• pH = -log 10 [H+]
• [H+] = 10^-pH
• As temperature changes, the value of Kw changes
• The forward reaction in the equilibrium of water


  Is endothermic and is therefore favoured when temperature of the water is increased
• As temperature increases


  More H+ ions are produced meaning the water becomes more acidic
• on a pH titration curve, the equivalence point/ neutralisation point is the large vertical section on a graph
• in strong acid-strong base reactions, the equivalence point is around pH7
• in a strong acid-weak base, the equivalence point is <pH7 so it is more acidic
• in a weak acid-strong base, the equivalence point is >pH7 (more basic)
• Methyl Orange


  Used for reactions with a more acidic neutralisation point, orange in acids and turns yellow at the neutralisation point
• Phenolphthalein


  Used for reactions with a more basic neutralisation point, pink in alkalis and turns colourless at the neutralisation point
• Indicator colours in acidic and basic solutions


  • Methyl Orange: red in acid, yellow in basic
  • Litmus: red in acid, blue in basic
  • Phenolphthalein: colourless in acid, pink in basic
• a buffer solution is


  a solution that is able to resist changes in pH when small volumes of acid or base are added
• calculate buffer in acid+base

  find number of moles in each species

  calculate their concentration when at equilibrium using the total volume

  use Ka to find [H+] and pH
• calculate buffer in acid+salt

  find the moles of the salt

  use Ka to find pH
• adding small amounts of H+ to a buffer solution will increase the concentration of the acid and the overall solution will get more acidic
• adding small amounts of OH- to a buffer solution decreases the concentration of acid in the solution and makes the overall solution more basic
• buffer solutions are common to keep systems regulated
• strong acids and bases completely dissociate in water whereas weak acids and bases don’t; they only partly dissociate into their own ions
• strong acid examples

  HCl, H2SO4, H3PO4
• strong bases examples

  NaOH, CaCO3, Na2CO3
• weak acid examples

  CH3COOH (ethanoic), any organic acid
• weak base examples

  NH3
• Ka expression


  acid HA: HA<->H+ + A-

  Ka = [H+][A-]

  [HA]
• a titration is the addition of an acid/base of known titration to base/acid of unknown titration to determine the concentration. an indicator/pH meter is used to show that neutralisation has occured
• around the equivalence point there is a large and rapid change in pH except in the weak-weak titration
• the end point is the volume of acid or alkali added when the indicator just changes colour. if the correct indicator is used then the equivalence point=end point
• properties of a good indicator for a reaction 


  sharp colour change (not gradual) - no more than 1 drop needed for a colour change

  end point is the same as the equivalence point

  distinct colour change so end point is obvious