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CHM 1020--Chemistry for Liberal Studies--Spring 1999 |
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Chemistry 1020—Lecture 14—Notes Hydrogen bonds The H-O bond has a special property, because as the electron pair is pulled from H, a very tiny positive charge (the proton) is exposed. This exposed proton can snuggle up to another pair of electrons, forming a stronger than expected attraction if the electron pair is on a small atom. Such a bond is called a hydrogen bond. Hydrogen bonds can only form between very small and very electronegative atoms. For most cases, this includes only F, O, and N. covalent bond O-H..........O hydrogen bond In this case OH is called the donor and O (with a lone pair of electrons) is the acceptor). In water, the oxygen of each water molecule can be both a donor and an acceptor. The structure of water is one in which each water molecule is surrounded by four others—sort of a tetrahedral structure. (See figure 5.3). Hydrogen bonds are not as strong as covalent bonds, but are stronger than other dipole-dipole attractions. It is this bonding that is responsible for:
It takes a lot of energy to break up this hydrogen bonding network. Also, the tetrahedral-like structure of water when it is ordered in an ice structure (see figure 5.4) takes up more volume than if a few bonds were broken and the molecules could nestle closer to one another. This arrangement explains why water expands when it freezes (or contracts when it melts). Hydrogen bonds have energies from about 4 kJ/mol to 25 kJ/mol. Compare these values to the values of covalent bonds in table 4.1, page 121, where covalent bonds vary from 150-450 kJ/mol. The normal H-O covalent bond energy is 459 kJ/mol. Hydrogen bonds are very important in the structure of large biological molecules such as proteins and nucleic acids. The energy of one bond is not very great, but in these large molecules there are many such bonds and collectively they contribute to the overall structural stability of these molecules. Polyatomic ions In discussing solution of ionic substances, we note that many of the minerals we find in water consist of ions that contain more than one atom. We call such ions polyatomic ions. So at this point we need to discuss some additional structural information. Ions are electrically charged species. The atoms of a polyatomic ion are being held together by covalent bonds, but the group of atoms has an excess or deficiency of electrons. You might think of the group as a charged molecule. A few polyatomic ions are positively charged. (Positively charged ions are called cations). The only example we will cite for the moment is NH4+, the ammonium ion. Rules for Lewis structures are the same, except that when there is a positive charge, one must remove a valence electron. Draw the Lewis structure of ammonium ion.
Most polyatomic ions are negatively charged, and most of them contain one or more oxygen atoms. (Negatively charged ions are called anions). In this case the normal ide ending for a negative ion is replaced by ate. For example:
Example Lewis structures:
There is no standard pattern here relating number of oxygens or charge. You just need to learn the names of these principle oxy-anions. Once you have learned these names, though, you can relate the names of several others. If there is one less oxygen than normal, the ending is ite.
The oxyanions of the halogens are a bit more complicated because there are four of them. As above, one less oxygen than the ate structure has the ite suffix. Two less oxygens has in addition the hypo prefix, while one more oxygen than the ate structure takes the per prefix. So:
(Give the structures of:
A couple of other important ions are hydroxide (OH-) and acetate (C2H3O2-). Let’s note the Lewis structure of the acetate ion:
Solutions of covalent compounds You noted that sugar dissolves in water readily, but that the solution is not an electrolyte, indicating that ions were not formed. However, there is still the strong possibility of molecular interactions between the water and the sugar, because there are many O-H bonds in sugar. (Note figure 5.10). These OH groups can hydrogen bond with water, serving both as hydrogen bond acceptors and hydrogen bond donors. Hydrocarbons have only C-C and C-H bonds, which are non-polar, and even the small bit of polarity tends to be canceled by the tetrahedral geometry. Hence compounds like hexane and octane cannot interact with water and therefore do not dissolve to any appreciable extent. Thus one has the notion that like dissolves like. Polar molecules tend to dissolve in polar solvents, non-polar molecules in non-polar solvents. Compare the alcohol series as to solubility.
Amphipathic molecules (Detergents) Some molecules have both a very polar and a non-polar end. One example would be the sodium salt of palmitic acid, called sodium palmitate. CH3CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2COO- Na+or
Such amphipathic compounds tend to self associate in water in such a way as to expose the polar COO- end of the molecule to water, and the non-polar hydrocarbon chain away from water, forming aggregate structures called micelles. Salts of fatty acids like this are soaps, and have detergent properties, in that the hydrocarbon center of a micelle can dissolve non-polar substances such as grease, suspending the grease in water. In pioneer days, one made such soaps by reaction of sodium and potassium hydroxides with fats to produce these salts. (I’ll bet no one remembers the old song "Grandma’s Lye Soap"). Modern detergents have better properties than these fatty acid salts, but the principle of the structure is the same, a molecule with a large non-polar end and a very polar end. Water purification There are many different types of contaminants that one must deal with in trying to purify water to the extent that it is potable. First of all, bacterial contamination can spread disease. Something must be done to kill potential microorganisms in the water. Normally in city water supplies, this is done by chlorination, although in some circumstances ozone has been used. But ozone is not as stable or long lasting as chlorine. A drawback of chlorination is that it can react with some organic substituents to produce compounds in the water that might have some degree of toxicity. Sea water is unfit to drink because it contains too high a concentration of mineral ions—higher than our kidneys can handle. (Sea water has about 3.5% salt, while our body tissues contain water with about 0.9% salt. Drinking sea water would cause water to be pulled from our cells, resulting in their dehydration.) Sea water can be purified by distillation, but this process is energetically very expensive. A more common and less expensive procedure is that of reverse osmosis, in which the water is forced under pressure through a membrane that has holes of a sufficient size to pass water, but excludes ionic species. Water from lakes, rivers, and aquifers will be potable as far as mineral content is concerned, but will often contain various mineral species depending on the kinds of minerals the water has been exposed to. Water that is high is Ca2+ and Mg2+ is considered hard water. The problem with hard water is that these divalent cations form insoluble precipates with soaps and detergents, creating a scum when washing things and making it difficult for the detergents to properly remove grease and dirt. Such water can be softened by removal of these ions. One procedure involves adding sodium carbonate, causing calcium carbonate and magnesium carbonate to precipitate from solution. Another is a process known as ion exchange. The water is passed through a substance such as a zeolite or an ion exchange resin, in which the divalent ions are exchanged for Na+. High sodium concentrations can cause problems, though, for some individuals with high blood pressure. Water treatment plants also use procedures such a filtration through beds of sand to remove suspended particulate material. Sometimes it is necessary to add a floculating agent, such as a mixture of aluminum sulfate (alum) and calcium hydroxide (slaked lime) which form a gelatinous precipitate of aluminum hydroxide. This precipitate traps fine particles and helps in their removal by filtration. Often the water is subject to aeration to allow dissolved organic substances to react with the oxygen of the air, thereby removing some things that might have disagreeable odors. |
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