Acid Base

• Bronsted-Lowry acid is a proton donor - donates a H+ ion
• Bronsted-Lowry base is a proton acceptor - accepts a H+ ion
• when a Bronsted-Lowry acid donates a H+ it becomes a conjugate base
• when a Bronsted-Lowry base accepts a H+ it becomes a conjugate acid
• monoprotic acid - releases one H+ ion per molecule e.g. HCl, HNO3
• Diprotic acid - releases two H+ ions per molecule e.g. H2SO4, H2C2O4
• pH of strong acid = -log[H+]
• [H+] = 10^-pH
• for diprotic acids, multiply the concentration of H+ by 2
• [H+] in a diluted solution = original concentration x original volume/new volume
• Kw = [H+][OH-]. at room temperature the value of Kw is 1 x 10^-14
• In pure water [H+]=[OH-] so Kw = [H+]^2
• pH of mixtures of strong acids and strong bases

  1. calculate moles of H+ (concentration x volume)
  2. calculate moles of OH-
  3. calculate moles of excess acid or base (the one that is bigger)
  4. calculate concentration of excess acid or base by dividing by total volume
  5. calculate pH
• to find concentration of H+ from concentration of OH-, divide Kw by concentration of OH-
• strong acid - completely ionises in water. molecule fully breaks down into its ions
• weak acid - partially ionises in water. small fraction of molecules break down into their ions
• ka is the acid dissociation constant where ka = [H+]^2 / [HA] in water. HA is a weak acid.
• pka = -log(ka)

  higher ka = stronger acid

  lower pka = stronger acid
• methyl orange is an indicator used for more acidic neutralisation points. colour change from red in acid to yellow in alkali between pH 3 and 5
• phenolphthalein is an indicator used for more basic neutralisation points. colour change from colourless in acid to pink in alkali between pH 8 and 10
• a pH curve shows how pH changes as an alkali is added is added to an acid or vice versa
• the equivalence point is when the moles of alkali added equals the moles of acid present. the pH is not always 7 at the equivalence point
• in most acid base reactions, the pH curve shows rapid change in pH around the equivalence point
• buffer solutions are solutions that resist changes in pH when small amounts of acid or alkali are added - the pH does change, but not by much
• acidic buffer solutions are below pH 7 while basic buffer solutions are above pH 7
• acidic buffer solutions are made from a mixture of weak acid and one of its salts. can also be made from an excess of weak acid with strong alkali, resulting in a mixture of HA and A-
• in an acidic buffer, the concentration of the acid and salt is much higher than the concentration of H+
• basic buffer solutions are made from a mixture of weak alkali and one of its salts. can also be made from an excess of weak alkali with strong acid
• in a basic buffer, the concentration of the base and salt is much higher than concentration of OH-