General practicals

• how do you purify a solid

  Recrystallisation:

  1. Dissolve product in (hot) solvent to remove impure solid
  2. filter out insoluble impurities using gravity filtration
  3. leave solution to cool and form crystals
  4. remove soluble impurities by filtering under reduced pressure
• what things to consider about recrystallisation

  solvent - choose solvent that doesn’t dissolve the product we want at low temperature, only dissolves our product at high temperatures

  dissolve impure solid in a minimum volume of solvent

  keep the solution hot when we filter soluble impurities.

  leave solution to recrystallise to form as many crystals as possible

  wash crystals during filtration to remove any remaining soluble impurities
• How to we make sure the product formed from recrystallisation is pure ? 


  To determine the melting point of the recrystallised solid and compare to data book value to pure solid.

  The melting point of an impure solid is lower than a pure solid. impure solids will melt over a range of temperatures whereas pure solids will melt at an exact temperature.
• What apparatus can you use to measure the volume ?

  beaker

  measuring cylinder

  burette

  glass pipette
• How do you calculate percentage uncertainty ?

  uncertainty of measurement / measurement
• A gas syringe measures to the nearest 0.05cm^3?

  what is the smallest volume of gas that can be measured of the percentage uncertainty is less than 0.16%?

  The uncertainty is half the resolution of the apparatus = 0.025cm^3

  when using a gas syringe we need to measure the initial and final volume. As we are making two measurements, the uncertainty in the volume change is double the uncertainty in each indiquas reading.

  uncertainty = 0.16 = 0.05 / y x 100

  measurement = 0.05 / 0.16 x 100 = 31 cm^3
• Why do we use a thermometer in a distillation ?

  To make sure the boiling point remains below that of the alcohol
• What are anti-bumping granules

  and what do they do ?

  anti-bumping granules are:

  • made up of silica
  • Large surface area that allow small bubbles to form on them

  anti-bumping granules prevent:

  • the solution from splashing
  • the flask from shattering
  • an impure product from being formed
• What filtering technique do we use if we want to keep the solid ?

  filter under reduced pressure
• What are the advantages of filtering under reduced pressure ?

  filtering under reduced pressure:

  • removes liquid at a much faster rate
  • removes more liquid
• Label this diagram 


  Label from top to bottom :

  A - Büchner flask or side arm flask

  B - filter paper

  C - Buckner funnel

  D - vacuum

  
• Explain how you separate solvents 


  Add our impure solvent to a separating funnel

  add water to our impure solvent and shake

  this forms two layers (aqueous layer + organic layer)

  open stopcock to retombe bottom layer
• practical considerations during solvent extraction

  add sodium chloride. (Forms strong ion-dipole forces with water, this was the water molecules to abandon product leaving it in the organic layer)

  add sodium carbonate solution if there are acidic impurities (H2SO4 catalyst) neutralises sulfuric acid

  if solvent reacts to form a gas - invert and open separating funnel to prevent gas building up
• How do you remove water from a liquid

  Use a drying agent
• How to reduce percentage uncertainty ?

  change instrument you measure with to one with a smaller resolution

  increases the quantity of whatever you’re measuring