Precipitation Reactions + Solubility Rules

• When does a precipitation reaction occur?

  when cations and anions in an aqueous solution combine to form an insoluble ionic solid called a precipitate.
• what is a salt

  a compound that is formed when the hydrogen atom in an acid is replaced by a metal
• when naming salts what are the two parts?


  first part: metal, metal oxide, or metal carbonate used in the reaction

  second part = acid
• method A: adding acid to a solid metal, insoluble base or insoluble carbonate (preparing soluble salts)


  heat acid until warm, then add metal/base/carbonate, stirring constantly until it stops disappearing

  filter mixture to remove excess base, transfer solution to evaporating basin

  Evaporate water from solution using a bunsen burner until crystals appear

  Remove evaporating basin from heat and allow filtrate to dry and crystalise
• Example of method A


  preparation of pure, hydrated copper (II) sulfate crystals; acid = dilute sulfuric acid; insoluble base = copper (II) oxideEquation = copper(II) oxide + sulfuric acid → copper(II) sulphate + waterCuO (s) + H2SO4(aq) → CuSO4(aq) + H2O (l)
• Method B: reacting a dilute acid and alkali (soluble base)


  Use a pipette to measure the alkali into a conical flask and add a few drops of indicator (thymolphthalein or methyl orange)

  Add acid to the burette, noting the starting volume

  Add acid to alkali slowly until the indicator changes color. Calculate the volume of acid added (final volume of acid-initial volume). Repeat steps 1-3 without indicator

  Transfer solution to an evaporating basin, heat to partially evaporate water

  Remove evaporating basin from heat and allow filtrate to dry and cystalise
• how can insoluble salts be prepared?


  insoluble salts can be prepared using a precipitation reaction. The solid salt obtained is the precipitate; thus, in order to successfully use this method, the solid salt being formed must be insoluble in water and the reactants must be soluble.
• Using two soluble reactants

  Add soluble salts to water and mix

  Filter to remove precipitate from the mixture

  Wash the precipitate with distilled water to remove traces of the solution

  Dry the precipitate insoluble salt in an oven

  Ex: Preparation of pure, dry lead(II) sulfate crystals using a precipitation reaction

  Soluble salt 1 = lead (II) nitrate

  Soluble salt 2 = potassium sulfate

  Equation: lead(II) nitrate + potassium sulfate → lead(II) sulfate + potassium nitrate

  Pb(NO3)2(aq) + K2SO4(aq) → PbSO4(s) + 2KNO3(aq)
• Sodium, potassium, ammonium and nitrates,


  soluble = all

  insoluble = none
• Chlorides


  Most are soluble but silver and lead (II) are insoluble
• Sulfates


  most are soluble but barium, calcium and lead (II) is insoluble
• Carbonates


  carbonates of sodium, potassium and ammonium are soluble but most are insoluble
• Hydroxides


  hydroxides of sodium, potassium, ammonium, and calcium (calcium hydroxide is partially soluble) most are insoluble
• when does a double replacement reaction occur?


  when two ionic reactants dissociate and bond with the respective anion or cation from the other reactant. The ions replace each other based on their charges as either a cation or an anion. This can be thought of as "switching partners"; that is the two reactants each "lose" their partner and form a bond with a different partner. A double replacement reaction is specifically classified as a precipitation reaction
• if the rules state that an ion is soluble...


  then it remains in its aqueous ion form
• If an ion is insoluble based on the solubility rules...


  then it forms a solid with an ion from the other reactant
• if all the ions in a reaction are shown to be soluble...


  then no precipitation reaction occurs.
• precipitation of an insoluble salt from a precipitate reaction.


  Barium nitrate + Sodium sulfate ---> Barium sulfate + Sodium nitrate

  Ba(NO3)2(aq) + Na2SO4(aq)---> BaSO4 (s) + 2NaNO3(aq)