Exp2423_Lab_updated 3-09-10

Exp2423_Lab_updated 3-09-10 - Techniques Heating...

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Techniques Heating, Precipitation, Gravity Filtration Discussion The process of making soap has remained basically unchanged for hundreds of years. The procedure involves the basic hydrolysis (saponification) of a fat. Chemically, fats are usually referred to as triglycerides, and they contain ester functional groups. Saponification involves heating fat with an alkaline solution. Lye (sodium hydroxide) is used as the source of the alkali. The alkaline solution hydrolyzes the fat to its component parts, the salt of a long-chain carboxylic acid (soap) and an alcohol (glycerol). When salt is added, the soap precipitates. The soap is washed free of unreacted sodium hydroxide and molded into bars. The carboxylic acid salts of soap usually contain 12-18 carbons, arranged in a straight chain. The carboxylic acids containing even numbers of carbon atoms predominate, and the chains may contain unsaturation (i.e. double bonds). H 2 C HC H 2 C COOR COOR COOR a triglyceride (tri-ester) from a fat or oil C 6 H 5 O 6 R 3 + 3 NaOH 3 RCOO - Na + H 2 C HC H 2 C OH OH OH + carboxylic acid salts (soap) RCO 2 Na glycerol (a tri-alcohol) C 3 H 8 O 3 Note: COOR = COR O an ester group (Molar mass = 295 gm/ mol) (Molar mass = 304 gm/ mol) This is an idealized reaction, in which the R groups are the same. Usually, the R groups are not the same, and soap is actually a mixture of salts of carboxylic acids. A disadvantage of soap is that it is an ineffective cleanser in hard water, which contains salts of magnesium, calcium, and iron in solution. The insoluble 'calcium soap' that forms is what causes bathtub ring. However, soap is an excellent cleaner in soft water and is biodegradable --it is removed from the environment by the actions of microorganisms. Safety Sodium hydroxide is extremely corrosive and caustic . Avoid all contact and clean spills immediately. Ethanol is highly flammable . Do not use near open flames. Procedure 1. Begin by setting up a boiling water bath. Place 5 g of cooking oil in a 250-mL Erlenmeyer flask. Record exact mass of the oil. Add 10 mL 95% ethanol and 10 mL of a 6 M S odium hydroxide solution. Organic Chemistry 2423 Lab Pg. 1
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2. Heat the mixture in the water bath for a 30-minute period, keeping it on a low boil. Cover the mouth of the flask with a small watch glass to reduce evaporation of the alcohol from the solution. 3. Prepare a solution of 10 mL of 95% ethanol and 10 mL water, and add it in small portions to the reaction over a second 30-minute period. Stir the solution occasionally during heating. 4. In a 400-mL beaker, place about 75 mL of ice and 15 g of sodium chloride. Pour the hot saponification mixture onto the salt-ice mixture. a. Gently stir the mixture for several minutes until the ice is nearly all melted, and the soap has completely precipitated. Pour the solid soap and its solution through a folded piece of
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Exp2423_Lab_updated 3-09-10 - Techniques Heating...

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