dissociation constant & salts

• Ka is the acid dissociation constant
• the acid dissociation constant, Ka, is given by

  • Ka = [products]/[reactants]

  or

  • pKa = -log10Ka
• in a solution of weak acid, only a small proportion of the original acid molecules dissociate into ions, therefore the acid concentration is the same as the original concentration and is given by the symbol C
• every weak acid which dissociates into ions, forms one hydrogen conjugate acid ion and one conjugate base ion
• when a weak acid dissociates, ignoring the hydrogen ions from the water molecules, it can be said that

  • [H+] = [A-]

  and

  • Ka = [H+]2/C
• within weak acids, the value of Ka gives a measure of how weak or dissociated the acid is
• the smaller the value of Ka the weaker the acid is
• the larger the value of pKa, the weaker the acid (the less it dissociates into ions)
• the equilibrium constant lies towards the left
• pKa is also used for calculating the approximate pH of a weak acid by using

  pH = 1/2 pKa-1/2 log10C
• the weaker the base, the stronger the conjugate acid that will be formed with a higher Ka value
• the conjugate acid of a weak base is a strong acid
• conductivity

  • strong acid = higher
  • weak acid = lower
• rate of reaction

  • strong acid = faster
  • weak acid = slower
• pH

  • strong acid = lower
  • weak acid = higher
• volume to neutralise acid

  • strong acid = same
  • weak acid = same
• the differences in conductivity, rate of reaction and pH is a result of the fact strong acids have a higher number of hydrogen ions than weak acids of the same concentration
• the volume of alkali required to neutralise the acid remains the same
• then hydroxide ions in the alkali react with all the available hydrogen ions in solution
• in a weak acid, the hydroxide ions remove all the hydrogen ions from the equilibrium and causes the acid molecules to release more hydrogen ions, which continues until all acid molecules have dissociated and the acid is neutralised
• the volume of alkali required depends on concentration of the acid and not the strength (strong/weak)
• salts are defined as one of the products of the neutralisation of an acid by a base
• a salt is formed when the hydrogen ions of an acid are replaced by metal ions
• naming salts

  • 1st part = identifies which base was used
  • 2nd part = identifies which acid was used
• the pH of the salt solution depends on the strength of the parent base and parent acid from which it's formed, whichever is stronger the pH will be pulled to that end of the scale
• all salts are strong electrolytes and dissociate completely in solution
• in soluble salts formed by a strong base and a strong acid, both the parent acid and base will fully dissociate in water and the equilibrium is unaffected and the pH will remain the same as in pure water , 7
• when a weak acid or base is dissolved, the ions will set up and equilibrium with the ions in the water which disrupts the water equilibrium
• strong parent acid + weak parent base

  pH of salt = acidic
• strong parent acid + strong parent base


  pH of salt = neutral
• weak parent acid + strong parent base

  pH of salt = alkali