L2 CHEM NOTES

Writing word equations

• In a compound the first ion is always cation and second is always anion
• The cation in the first compound will react with the anion in the second compound
• So the calcium will react with the sulphate and the chloride will react with the sodium
  E.g. calcium chloride + sodium sulphate ---> calcium sulphate + sodium chloride
• Whatever is insoluble is a solid as it is a precipitate and whatever is soluble it is aqueous
• Identifying precipitation reactions
• Make sure to only look at the product of the word equation
• If even one of the compounds of the product is insoluble the reaction is a precipitation
• This is called a precipitation reaction
• If both of the products are soluble the reaction is a double displacement reaction
• Writing ionic equations
• To write ionic equations you must have the ionic charge of each ion and then swap and drop to find the ionic compound
• Ca2+ Cl 1- Na1+ SO42- -----> Ca2- SO42- Na1- Cl1-
• CaCl2 + Na2SO4 -----> CaSO4 + NaCl
  (aq) (aq) (s) (aq)
• Whatever compound is in an aqueous state is broken up into their ions
• If the compound is a solid it is left as it is
• CaCl + NaSO4 ---> CaSO4 + NaCl
  (aq) (aq) (s) (aq)
• Ca2+ Cl 1- + Na1+ SO42- -----> CaSO4 + Na1- Cl1-
• Then cancel the spectator ions (ions that are the same on both the reactant and the product side) excluding the precipitates, then add together what is left
• Ca2+ Cl 1- + Na1+ SO42- -----> CaSO4 + Na1- Cl1-
• Ca2+ + SO42- ---> CaSO4
• If there is more than one ion then put the number of ions in front of the ion e.g.
  CaCl2
  Ca2+ 2Cl-
• Complex ions
• To identify a complex you add excess of a compound and if the solution turns clear then the solution is a complex ion
• All complex ions are soluble
• There is always a metal cation and an attached anion (ligand)
• If the ligands are ammonia hydroxide or water then they are always in 4s apart from when silver is the cation it is 2
• Transition metals in water
  • Cu dissolves in water
• Cu2+ + 4H2O ---> [Cu(H2O)4]2+
  (aq)
• Complex ions
• To identify a complex you add excess of a compound and if the solution turns clear then the solution is a complex ion
• All complex ions are soluble
• There is always a metal cation and an attached anion (ligand)
• If the ligands are ammonia hydroxide or water then they are always in 4s apart from when silver is the cation it is 2 • Transition metals in water
  • Cu dissolves in water
• Cu2+ + 4H2O ---> [Cu(H2O)4]2+
  (aq)
  odium hydroxide (NaOH)
  Is a useful reagent because many cations form precipitates with the hydroxide ion.
  Some of these precipitates are coloured, and the colour allows us to easily determine the cation that is present.
  E.g. iron(II) hydroxide is a green precipitate.
  Fe2+(aq) + 2OH− (aq) → Fe(OH)2 (s)
  (green precipitate)

1. Ammonia solution (NH3)
   Is another reagent that forms hydroxide precipitates with some cations, since ammonia is a weak base that produces hydroxide ions in solution.
   Some hydroxide precipitates will dissolve in excess sodium hydroxide, or excess ammonia, due to the formation of complex ions.
   E.g. copper hydroxide precipitate dissolves in excess ammonia to form a dark blue complex ion.
   Cu(OH)2 (s) + 4NH3(l) → [Cu(NH3)4]2+(aq) + 2OH− (aq)
   (dark blue)
2. Silver nitrate (AgNO3)
   Is a reagent used in the identification of iodide ions and chloride ions.
   The silver ion will form a yellow precipitate with iodide ions (silver iodide), and a white precipitate with chloride ions (silver chloride).
   Ag+(𝑎𝑞) + I−(𝑎𝑞) → AgI(𝑠)
   (yellow precipitate)
   Ag+(𝑎𝑞) + Cl−(𝑎𝑞) → AgCl(𝑠)
   (white precipitate)
   If excess ammonia is added to these precipitates:
   · AgI will be unaffected.
   · AgCl will disappear due to the formation of a complex ion:
   AgCl(𝑠) + 2NH3(𝑎𝑞) → [Ag(NH3)2]+(𝑎𝑞) + Cl−(𝑎𝑞)
3. Potassium thiocyanate (KSCN)]
   Is a reagent used in one scenario - to confirm that a solution contains iron(III) ions.
   A few drops of KSCN added to a solution containing Fe3+ ions turn the solution dark red due to the formation of a complex ion with the thiocyanate ion.
   Fe3+(𝑎𝑞) + SCN−(𝑎𝑞) → [Fe(SCN)]2+(𝑎𝑞)
   (dark red)
4. Barium nitrate (Ba(NO3)2) and barium chloride (BaCl2)
   Used in the identification of sulfate ions, as the barium ion forms a white precipitate with the sulfate ion.
   Ba2+(𝑎𝑞) + SO42−(𝑎𝑞) → BaSO4(𝑠)
   (white precipitate)
5. Dilute acids
   · Hydrochloric acid (HCl) and Sulfuric acid (H2SO4)
   Can be used as reagents to identify various cations and anions in solutions.
   Dilute acids will react with and dissolve precipitates that contain carbonate ions or hydroxide ions
   Dilute acids will react with carbonate ions and hydroxide ions in precipitates and solution as well.
   If you're unsure whether a solution or precipitate contains carbonate ions or hydroxide ions, the addition of a dilute acid will give you the answer.
   CaCO3(𝑠) + 2H+(𝑎𝑞) → Ca2+(𝑎𝑞) + H2O(𝑙) + CO2(𝑔)
   (bubbles)
   Ca(OH)2(𝑠) + 2H+(𝑎𝑞) → Ca2+(𝑎𝑞) + 2H2O(𝑙)

• Carbonates produce bubbles of carbon dioxide gas when they react with acids
• Hydroxides do not produce bubbles.
  · Sulfuric acid (H2SO4)
  Dilute sulfuric acid can also be used in the identification of barium ions and lead ions, as they will form a white precipitate with the sulfate ion.
  Ba2+(𝑎𝑞) + SO42−(𝑎𝑞) → BaSO4(𝑠)
  (white precipitate)
  Pb2+(𝑎𝑞) + SO42−(𝑎𝑞) → PbSO4(𝑠)
  (white precipitate)

Reagents

how to test with reagents
Reagent:
1.
NaOH and NH3
2.
KSCN
3.
AgNO3
4.
Ba(NO3)2 and BaCl2
What it's used for:
1.
Identifying various cations.
2.
Confirming the presence of Fe3+ ions.
3.
Distinguishing between I− ions and Cl− ions.
4.
Identifying SO