Preparation of a sample of Soap

Key steps + observations in the procedure for the preparation of a sample of soap:

• set up a reflux apparatus

  • oil containing glycerol tristearate, sodium hydroxide, ethanol and antibumping granules are added to a pear shaped flask

  • gyceryl tristearate is the key component for making soap

  • sodium hydroxide is the base present for base hydrolysis/saponification to occur

  • antibumping granules ensures a smooth, contolled boil

  • ethanol acts as a solvent for the oil

• reflux for 30 minutes to ensure completetion and no loss of ethanol solvent

  • the walls of the coated become coated with the sodium salts of the fatty acids that have not been hydrolysed

• set up distillation apparatus to collect ethanol, which has a lower boiling point

• the contents of the pear shaped flask is poured into a beaker containing brine, which will precipitate the soap and dissolve excess sodium hydroxide

• use a small amount of water to ensure all washings go into the brine solution, only a small amount to ensure the solution is still saturated

• soap (sodium stearate) is filtered off and washed with ice cold water, to remove excess sodium hydroxide; the water being ice cold prevents the soaps from dissolving too quickly

• soap is allowed to air dry overnight on filter paper

What is refluxing and why is is used

• a lab technique in which a liquid is boiled in a container that is attached to a vertical condenser

• vapour from the boiling liquid condenses and flows back into the flask which keeps the liquid boiling without the loss of vapour 

• this prevents the flask running dry whilst allowing sufficient time for the reaction to occur, maximising the yield

What is brine?

a concentrated solution of sodium chloride (NaCl or salt) in water

What is the test to confirm that soap has been produced?

• put in a test tube with water, stopper and shake

• the soap forms a lather in the presence of water

Write a word + chemical equation for the preparation of soap:

Glycerol tristearate + Sodium hydroxide → Sodium Stearate (Soap) + Glycerol

What is the IUPAC name for glycerol?

Propan-1,2,3-triol

Draw the structure of sodium stearate:

sodium octadecanoate