How is vsepr used to classify molecules? What are the units used for the ideal gas law? How does Charle's law relate to breathing? What is the ideal gas law constant?
How do you calculate the ideal gas law constant? Much of what we now know about the tendency of particles to become more dispersed can be used to understand this kind of change as well. Picture a layer of ethanol being carefully added to the top of some water Figure below. Because the particles of a liquid are moving constantly, some of the ethanol particles at the boundary between the two liquids will immediately move into the water, and some of the water molecules will move into the ethanol.
In this process, water-water and ethanol-ethanol attractions are broken and ethanol-water attractions are formed. The attractions that form between the ethanol and water molecules are also hydrogen bonds Figure below.
Because the attractions between the particles are so similar, the freedom of movement of the ethanol molecules in the water solution is about the same as their freedom of movement in the pure ethanol.
The same can be said for the water. Because of this freedom of movement, both liquids will spread out to fill the total volume of the combined liquids. In this way, they will shift to the most probable, most dispersed state available, the state of being completely mixed.
There are many more possible arrangements for this system when the ethanol and water molecules are dispersed throughout a solution than when they are restricted to separate layers. Figure below.
We can now explain why automobile radiator coolants dissolve in water. These substances mix easily with water for the same reason that ethanol mixes easily with water. The attractions broken on mixing are hydrogen bonds, and the attractions formed are also hydrogen bonds. There is no reason why the particles of each liquid cannot move somewhat freely from one liquid to another, and so they shift toward the most probable most dispersed , mixed state. We have a different situation when we try to mix hexane, C 6 H 14 , and water.
If we add hexane to water, the hexane will float on the top of the water with no apparent mixing. The reasons why hexane and water do not mix are complex, but the following gives you a glimpse at why hexane is insoluble in water.
There actually is a very slight mixing of hexane and water molecules. The natural tendency toward dispersal does lead some hexane molecules to move into the water and some water molecules to move into the hexane. When a hexane molecule moves into the water, London forces between hexane molecules and hydrogen bonds between water molecules are broken. New attractions between hexane and water molecules do form, but because the new attractions are very different from the attractions that are broken, they introduce significant changes in the structure of the water.
It is believed that the water molecules adjust to compensate for the loss of some hydrogen bonds and the formation of the weaker hexane-water attractions by forming new hydrogen bonds and acquiring a new arrangement.
Overall, the attractions in the system after hexane and other hydrocarbon molecules move into the water are approximately equivalent in strength to the attractions in the separate substances. Jump to navigation. In case of alcohols, just as it happens in case of many other biological molecules, the basic solubility rule that like dissolves like is a bit more complexed.
Each alcohol consists of a carbon chain always nonpolar and a OH group which is polar. The ionic and very hydrophilic sodium chloride, for example, is not at all soluble in hexane solvent, while the hydrophobic biphenyl is very soluble in hexane. Exercise 2. Decide on a classification for each of the vitamins shown below.
Hint — in this context, aniline is basic, phenol is not! Because water is the biological solvent, most biological organic molecules, in order to maintain water-solubility, contain one or more charged functional groups.
These are most often phosphate, ammonium or carboxylate, all of which are charged when dissolved in an aqueous solution buffered to pH 7. Some biomolecules, in contrast, contain distinctly hydrophobic components. In a biological membrane structure, lipid molecules are arranged in a spherical bilayer: hydrophobic tails point inward and bind together by van der Waals forces, while the hydrophilic head groups form the inner and outer surfaces in contact with water.
Interactive 3D Image of a lipid bilayer BioTopics. Because the interior of the bilayer is extremely hydrophobic, biomolecules which as we know are generally charged species are not able to diffuse through the membrane— they are simply not soluble in the hydrophobic interior. The transport of molecules across the membrane of a cell or organelle can therefore be accomplished in a controlled and specific manner by special transmembrane transport proteins, a fascinating topic that you will learn more about if you take a class in biochemistry.
A similar principle is the basis for the action of soaps and detergents. Soaps are composed of fatty acids, which are long typically carbon , hydrophobic hydrocarbon chains with a charged carboxylate group on one end,. Fatty acids are derived from animal and vegetable fats and oils.
In aqueous solution, the fatty acid molecules in soaps will spontaneously form micelles , a spherical structure that allows the hydrophobic tails to avoid contact with water and simultaneously form favorable van der Waals contacts. Interactive 3D images of a fatty acid soap molecule and a soap micelle Edutopics. Because the outside of the micelle is charged and hydrophilic, the structure as a whole is soluble in water. Micelles will form spontaneously around small particles of oil that normally would not dissolve in water like that greasy spot on your shirt from the pepperoni slice that fell off your pizza , and will carry the particle away with it into solution.
We will learn more about the chemistry of soap-making in a later chapter section Synthetic detergents are non-natural amphipathic molecules that work by the same principle as that described for soaps.
The physical properties of alcohols are influenced by the hydrogen bonding ability of the -OH group. The -OH groups can hydrogen bond with one another and with other molecules. Hydrogen bonding raises the boiling point of alcohols.
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