34 Pre-Lab Questions 1. Below is a general outline of a soap molecule. Indicate

Question

34 Pre-Lab Questions 1. Below is a general outline of a soap molecule. Indicate on the outline the area EXPERIMENT 34 to be hydrophobic and the area that is expected to be hydrophilic. 2. With respect to the above molecular outline, what chemical properties would you expect for the (a) hydrophobic area and for the (b) hydrophilic area? 3. How should a good emulsifying agent behave? ehs be havier of rms adarer containing Ca and Mg" salts. How do you explain 5. Soaps work best when water is neutral or alkaline, but not very well when water is acidic. Explain this behavior of soap 6. How are soaps prepared? custom page 81

Explanation / Answer

The long tail like end of soap molecule is the Hydrophobic (lipophilic)

2. The organic part of a natural soap is a negatively-charged, polar molecule. Its hydrophilic (water-loving) carboxylate group (-CO2) interacts with water molecules via ion-dipole interactions and hydrogen bonding.

The long hydrocarbon part is hydrophobic, the hydrophobic (water-fearing) part of a soap molecule, its long, nonpolar hydrocarbon chain, does not interact with water molecules. The hydrocarbon chains are attracted to each other by dispersion forces and cluster together, forming structures called micelles. In these micelles, the carboxylate groups form a negatively-charged spherical surface, with the hydrocarbon chains inside the sphere. Because they are negatively charged, soap micelles repel each other and remain dispersed in water.

3.      The good emulsifier is capable of dispersing one liquid into another immiscible liquid. This means that while oil (which attracts dirt) doesn't naturally mix with water, soap can suspend oil/dirt in such a way that it can be removed.

4.      Although soap is a good cleaning agent, its cleaning capacity is reduced when used in hard water. Hardness of water is due to the presence of sulphates, chlorides or bicarbonate salts of Ca2+ or Mg2+ ions. Soaps are sodium or potassium salts of long chain fatty acids. When soap is added to hard water, the Ca2+ and Mg2+ ions present in hard water react with soap. The sodium salts present in soaps are converted to their corresponding calcium and magnesium salts which are precipitated as scum. The insoluble scum sticks on the clothes and so the cleaning capacity of soap is reduced. This process removes soap ions from solution, and decreases the cleaning effectiveness of soaps.

5.      Soaps, the sodium salts of fatty acids, are water soluble, but the fatty acids themselves are not. A soap can be converted into the fatty acid by means of a reaction with a strong mineral (non-organic) acid. Thus, the acidification of a soap solution causes the fatty acid to precipitate. Acidification of detergents, on the other hand, produces acids which are often water soluble

6. Soaps are carboxylate salts with very long hydrocarbon chains. Soap can be made from the base hydrolysis of a fat or an oil. This hydrolysis is called saponification, and the reaction has been known for centuries. Traditionally, soaps are made from animal fat and lye (NaOH). (Lye was traditionally made by pouring water through wood ashes.) fats and oils are triesters of glycerol and three fatty acids. Esters can be hydrolyzed to their alcohol and carboxylic acid components in the presence of acid or base. Fats, oils, and fatty acids are insoluble in water because their hydrophobic tails are so long. If a base is used for hydrolysis, the fatty acids produced are deprotonated and are present as the corresponding carboxylate salts. Because these product carboxylate salts are charged, they are much more soluble in water than the corresponding uncharged fatty acids. Since the carboxylate salts also each have a long nonpolar tail, they are also compatible with nonpolar greases and oils.

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