acids and bases

Cards (35)

  • 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