7.7
From Fats to Soaps
1. Glycerides and Their Fatty Acid
Content
This activity considers gylcerides and the effect of their parent fatty
acids on their properties.
A glyceride is an ester of glycerol with fatty acids.
In a glyceride, three fatty acids form ester links with
glycerol.
The fatty acids in a glyceride can be the same or different.
The fatty acids in fats and oils usually contain an even
number of carbon atoms from C2 to C24.
Physical properties:
The presence or absence of double bonds in a fatty acid
affects the melting point of the glyceride.
Saturated fatty acids are linear and the molecules can pack
closely together.
This means that the intermolecular forces are stronger
resulting in a higher melting point.
Unsaturated fatty acids are bent due to the double bonds.
These molecules can’t pack closely together.
This means that the intermolecular forces are weaker
resulting in a lower melting point.
2.
Soaps and Cleansing Action
This activity considers how soaps are made and how the structure of
the soap molecule allows it to act as a cleansing agent.
A typical soap is the sodium salt of a fatty acid.
Soaps are made by alkaline hydrolysis of a glyceride using
sodium hydroxide:
glyceryl
+
tristearate
sodium
hydroxide
sodium
stearate
+
glycerol
A soap molecule has an ionic head at one end and a long
covalent tail at the other end.
The Action of Soaps
3.
Percentage Yield
This activity is about calculating the percentage yield of a product in
a chemical reaction.
The percentage yield of a product in a chemical reaction is
the actual mass produced compared with the theoretical
mass expressed as a percentage.
Yield =
actual yield
x 100 %
theoretical yield
Example:
500g of sodium stearate soap (RFM = 306) was formed by the
reaction of 610g of glyceryl tristearate fat (RFM = 890) with sodium
hydroxide. Calculate the percentage yield.
C3H5(C17H35COO)3 + 3NaOH
1 mole
3 moles
C3H5(OH)3 + 3C17H35COONa
1 mole
3 moles
Calculate the theoretical mass:
glyceryl tristearate
1 mole
890g
1g
610g
sodium stearate
3 moles
306g x 3
918
890
918 x 610
890
= 629.2g
So the theoretical yield = 629.2g
actual yield
x 100 %
theoretical yield
= 500 x 100 = 79.5 %
629.2
Yield =