EXPERIMENT 17

[soap.lab]

OBJECTIVES:

1. To obtain some historical perspective on the history of soap and its production.

2. To be able to make soap.

3. To gain knowledge of the structure and properties of soap molecules.

DISCUSSION:

Soap is made by reacting an alkali (a base) such as sodium hydroxide or potassium hydroxide with fats or oils. Most of the fats come from tallow and most of the oils come from coconut oil. Spring was a traditional time for farmers to make soap. All during the winter fats would be saved from slaughtering animals and from cooking. These could be kept cold outside. At the same time, ashes from the fireplace were saved. When the time came to make soap, the ashes were put into a large barrel and water was trickled slowly through the barrel. This leached out the alkali in the ashes. The fats were boiled in large pots kept for this purpose and then the alkali and the salt were added. Usually a very strong soap was obtained. Refinements to this process led to milder soaps and the use of different kinds of fats and oils. Now the word "soap" can mean many different kinds of soap. Some are very mild and can be used for a baby's skin. Others are very strong and should not be touched with the hands. Perfume can be added for a nice scent.

The overall reaction leading to the formation of soap is known as saponification. It produces a soap molecule (a soluble sodium or potassium salt of long-chained fatty acid) and glycerol (obtained as a by-product). See equation below.

O "R" can be any organic radical,

" but usually has 10 - 22 C atoms

CH₂ -- O - C - "R" in its chain (ie: -C₁₀H₂₁).

|

| O CH₂-OH O

| " | "

CH -- O - C - "R" + 3 K-OH ----> CH-OH + 3 K - O - C - R

| |

| CH₂-OH

| O a soap molecule

| "

CH₂ -- O - C - "R" glycerol

a fat molecule

When "R" is -C₁₇H₃₅, the fat molecule is named tristearin (or glycerol stearate) and the soap molecule is potassium stearate. This soap has the formula K-OOCC₁₇H₃₅ , or C₁₇H₃₅COO-K

EXPERIMENTAL PROCEDURE: Work in pairs.

A. Preparation of Soap

1. Measure out about 10 mL of a vegetable oil (ie: corn oil, coconut oil, olive oil) and put it into a clean evaporating dish.

2. To the dish add 20 mL of ethyl alcohol [C₂H₅OH], 10 mL of distilled water and then 10 mL of sodium hydroxide (50% solution).

3. Stir the mixture in the dish with a glass rod. Heat it for about 15 minutes. Keep it near boiling, but DO NOT BOIL the mixture. The alcohol is flammable, so use a low flame. Stir the mixture constantly. The mixture should thicken as the soap forms. Have a beaker cover ready to put over the dish if the alcohol catches on fire.

4. While one person is heating the oil/NaOH, the other partner should prepare the salt solution. Weigh a small beaker. Add about 50 g of NaCl. (Instead of weighing, you can fill a beaker to the 40-mL mark with NaCl.) Put 150 mL of water in a 400 or 600 mL beaker and dump in the NaCl. Stir it well. It will be a saturated solution, so all of it may not dissolve.

5. Quickly pour the soap mixture into the salt solution. Stir the mixture for several minutes and then cool it in an ice bath. Soap will form as a solid on top of the water. The glycerol formed as a by-product will dissolve in the water.

6. Set up a Buchner funnel with a filter paper. DECANT the soap mixture through the filter. Keep most of the soap in the beaker, pouring out the liquid.

7. Add about 50 mL of ice water to the beaker containing the soap. Stir, then decant through the filter paper. Repeat this washing several more times with 50 mL of ice water each time. This should remove most of any NaOH that might remain as well as salt water. If you wish, you can put any soap in the filter back into the beaker. Discard the filtrate. It contains the glycerol and excess NaOH.

8. Test the soap as described on the Report Sheet.

9. Give the Instructor a small sample of your soap. He will test it to see if you washed it well and removed the excess NaOH.

Exp 17 - LAB REPORT and QUESTIONS Name ______________ Sec ____

B. Testing Soap Oil used ___________________

1. Put a small piece of the soap with a little distilled water in a test tube. Shake well.

Does it foam?

2. Put a small piece of the soap with a little tap water in another test tube. Shake well. Does it foam?

3. Rub a small piece of the soap on the palm of your hand with a small amount of distilled water. Does it lather?

4. Rub a small piece of the soap on your hand with a small amount of tap water.

Does it lather?

5. Instructor's tests: foaming __________ ; pH __________

QUESTIONS

1. Write the full equation for the saponification reaction of lard with sodium hydroxide. Suppose the formula for lard is:

O

"

CH₂-O-C-(CH₂)₁₆-CH₃

|

| O

| "

CH-O-C-(CH₂)₁₆-CH₃ + NaOH ==>

|

| O

| "

CH₂-O-C-(CH₂)₁₆-CH₃

2. C₁₁H₂₃COOH is lauric acid. What is the formula of its sodium salt?

Its chemical name is ____________________________.

3. What is "tallow" ?

4. Explain any differences in foaming is Tests B-1 and B-2.