Grade 9
Lipids and fats
Fats and oils are esters of glycerol with fatty acids; they store energy, do not mix with water, and can be turned into soap.
IntuitionIntuition: why oil and water do not mix
Shake cooking oil with water and the oil soon gathers in a separate layer. Fat molecules are mostly long chains of carbon and hydrogen, which do not attract water molecules. The same chains make fats a compact energy store: a gram of fat gives more than twice the energy of a gram of starch or sugar.
SchoolSchool level: structure, saturated and unsaturated fats, soap
Definition:
An ester formed from one glycerol molecule and three fatty acids (long-chain carboxylic acids ). Solid ones are called fats, liquid ones oils.
| Fatty acid | Formula | Type |
|---|---|---|
| Palmitic acid | C₁₅H₃₁COOH | Saturated |
| Stearic acid | C₁₇H₃₅COOH | Saturated |
| Oleic acid | C₁₇H₃₃COOH (one cis C=C) | Unsaturated |
Saturated chains are straight and pack closely, so animal fats such as lard are solid at room temperature. A cis C=C double bond bends the chain, so molecules pack poorly and plant oils such as olive oil stay liquid. Heating a fat with aqueous NaOH breaks the ester bonds (saponification) and gives glycerol and soap, the sodium salt of fatty acids.
Example: Soap from tristearin
Saponify 89 g of tristearin, (M = 890 g/mol), with excess NaOH. What mass of sodium stearate (M = 306 g/mol) forms?
Solution
n(tristearin) = 89/890 = 0.100 mol. One mole gives 3 mol of soap, so n(soap) = 0.300 mol and m = 0.300 × 306 = 91.8 g.
References
- Lehninger Principles of Biochemistry (8th ed.) · David L. Nelson, Michael M. Cox, Aaron A. Hoskins, 2021