9. Salts

• What is a salt?

  It is an ionic compound which contains a cation and an anion. It is also a compound formed when the hydrogen ion in an acid is completely or partially replaced by a metallic ion or ammonium ion
• How are salts formed?


  When an acid reacts with a metal, carbonate, bases or alkalis
• What is water of crystallisation?


  When a salt combines with water molecules forming crystals
• What are hydrated salts?

  Salts that contain water of crystallisation
• What are anhydrous salts?

  Salts that do no contain water of crystallisation
• How to make a hydrated salt anhydrous?

  Heat the hydrated salt
• What are the colour of the 4 common hydrated salts?

  MEMORISE!

  NOTE: The "." represents the amount of water crystallised in the formula

  
• What are soluble salts?


  Salts that can dissolve in water to form an aqueous solution
• What types of salts are soluble?

  MEMORISE!

  • All SPA salts are soluble (Sodium chloride, potassium iodide iodide, ammonium nitrate)
  • C LS [Chloride Lao Sai]
  • S LCB

  
• What are the 3 ways to prepare a salt?

  1. Reaction of acid with excess INSOLUBLE metal/carbonate/base
  2. Titration
  3. Ionic precipitation
• How to know which salt preparation method to choose?

  MEMORISE!

  
• Why must the metal/carbonate/base be added in excess?


  Ensure that all the acid is reacted. Otherwise, the salt produced will be contaminated with acid
• Why must the metal/carbonate/base be insoluble in water?


  So that the excess metal/carbonate/base can be removed from the solution by filtration
• What are the conditions of the metal to react it with an acid to obtain a salt? Why?

  • Must be moderately reactive, so it can react without being explosive
  • Must not be a unreactive metals (copper, gold, silver, platinum) do not react with acids
  • Must not be a very reactive metals (Sodium, Potassium) will react explosively
• Steps to prepare a salt (ie. zinc sulfate) [EXCESS METAL METHOD] ?

  1. Add excess [Zinc] powder into dilute [sulfuric acid]
  2. Stir until effervescence stops
  3. Filter to remove excess unreacted [Zinc] powder as residue
  4. Collect the filtrate, [ZnSO4] solution
  5. Heat filtrate to obtain concentrated [H2SO4] solution
  6. Test for saturation by seeing if crystals form on a glass rod
  7. Leave the solution to cool and crystallise
  8. Filter to obtain crystals as residue
  9. Wash crystals with cold, distilled water
  10. Pat dry between pieces of filter paper

  PRACTICE: How to obtain copper (II) chloride?
• How to determine to use excess metal, carbonate, or base?

  • If the cation in the salt is unreactive, must use excess base or carbonate
  • If the cation in the salt is reactive, you can choose to use excess metal, carbonate or base (typically pick metal, since its easier)

  EXAMPLE:

  Copper (II) sulfate

  • Cu2+ (cation of the salt) is unreactive

  Thus,

  React copper (II) hydroxide or copper (II) carbonate with dilute sulfuric acid.

  
  

  
  

  
  

  Zinc sulfate

  • Zn2+ (cation of the salt) is reactive

  Thus,

  React zinc power with dilute sulfuric acid
• Steps to prepare a salt (ie. Copper (II) sulfate) [EXCESS CARBONATE/BASE METHOD] ?


  1. React excess copper(II) hydroxide/carbonate with dilute [H2SO4]. Some heating is required.
  2. Stir until no more copper(II) hydroxide/carbonate dissolves.
  3. Filter the mixture to remove excess unreacted copper(II) hydroxide/carbonate as residue.
  4. Collect the filtrate
  5. Heat the filtrate to obtain a saturated solution.
  6. Allow filtrate to cool and crystallise.
  7. Filter to obtain crystals as the residue, wash with a little cold distilled water
  8. Dry crystals between pieces of filter paper
• Steps to prepare a salt (ie. sodium nitrate) by titration (1)?


  1. Fill a burette with dilute [nitric acid]
  2. note the initial reading.
  3. Pipette 25.0 cm³ of aqueous [sodium hydroxide/sodium carbonate] solution into a conical flask.
  4. Add 1-2 drops of [methyl orange] to the solution, which will turn [yellow].
  5. Slowly add dilute [nitric acid] from the burette until the solution just turns [orange] (end point).
  6. Record the final burette reading (V2 cm³) and calculate the volume of dilute [nitric acid] used for neutralisation.
• Steps to prepare a salt (ie. sodium nitrate) by titration (2)?


  1. Repeat steps 1-5 without the indicator to obtain a pure [sodium nitrate] solution.
  2. Heat to saturate the solution when titration is complet
  3. Cool to crystallise to obtain [sodium nitrate] crystals
  4. Filter to obtain crystals as residue
  5. Wash with cold distilled water dry between sheets of filter paper.
  6. Dry between sheets of filter paper.
• How to prepare a salt (ie. Barium sulfate) by ionic precipitation?

  1. Add aqueous sodium [sulfate] to a beaker of aqueous [Barium] nitrate
  2. A [white] precipitate of [Barium sulfate] forms
  3. Filter to obtain the precipitate [barium sulfate] as residue
  4. Wash the precipitate with cold, distilled water to remove impurities
  5. Leave the precipitate of [barium sulfate] to dry

  TIP:

  Always use [cation] nitrate + sodium [anion] if they ask for reagents for a ppt reaction!

  
  

  eg. How to produce a salt of Lead (II) chloride?

  use Lead (II) nitrate + Sodium chloride