Water's hydrogen bonds affect the properties of nonpolar molecules, those with carbon and hydrogen, in water. Because nonpolar molecules do not easily dissolve in water and are hydrophobic, they become squeezed together. This is how cell membranes are formed -- the water-fearing parts of the molecules all face the same direction and squeeze together to prevent water from touching them.
The water cannot get through the membrane. Examples of nonpolar molecules being put in water are easily found, especially in the kitchen. Mix vegetable oil with food coloring and pour it on top of water in a clear cup.
The oil and water do not mix because water is polar and oil is nonpolar. The nonpolar molecules form a membrane between the water and oil. Notice how oil drops in the water half form drops, blocking their insides from the water. However, the food coloring slowly makes its way out of the oil into the water, demonstrating the fluidity in the membrane if the molecules are polar, like food coloring.
Molecular Activity of Water Vs. 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.
For this reason, little energy is absorbed or evolved when a small amount of a hydrocarbon is dissolved in water. To explain why only very small amounts of hydrocarbons such as hexane dissolve in water, therefore, we must look at the change in the entropy of the system. It is not obvious, but when hexane molecules move into the water layer, the particles in the new arrangement created are actually less dispersed lower entropy than the separate liquids.
The natural tendency toward greater dispersal favors the separate hexane and water and keeps them from mixing. This helps explain why gasoline and water do not mix.
Gasoline is a mixture of hydrocarbons, including hexane. Gasoline and water do not mix because the nonpolar hydrocarbon molecules would disrupt the water in such a way as to produce a structure that was actually lower entropy ; therefore, the mixture is less likely to exist than the separate liquids. We can apply what we know about the mixing of ethanol and water to the mixing of two hydrocarbons, such as hexane, C 6 H 14 , and pentane, C 5 H When the nonpolar pentane molecules move into the nonpolar hexane, London forces are disrupted between the hexane molecules, but new London forces are formed between hexane and pentane molecules.
Because the molecules are so similar, the structure of the solution and the strengths of the attractions between the particles are very similar to the structure and attractions found in the separate liquids. When these properties are not significantly different in the solution than in the separate liquids, we can assume that the solution has higher entropy than the separate liquids.
Therefore, when very similar liquids, like pentane and hexane, are mixed, the natural tendency toward increasing entropy drives them into solution. Exothermic changes lead to an increase in the energy of the surroundings, which leads to an increase in the number of ways that that energy can be arranged in the surroundings, and therefore, leads to an increase in the entropy of the surroundings.
Endothermic changes lead to a decrease in the energy of the surroundings, which leads to a decrease in the number of ways that that energy can be arranged in the surroundings, and therefore, leads to a decrease in the entropy of the surroundings. Therefore, exothermic changes are more likely to occur than endothermic changes.
We can use this generalization to help us explain why ionic compounds are insoluble in hexane. For an ionic compound to dissolve in hexane, ionic bonds and attractions between hexane molecules would need to be broken, and ion-hexane attractions would form. The new attractions formed between the ions and hexane would be considerably weaker than the attractions broken, making the solution process significantly endothermic. The tendency to shift to the higher entropy solution cannot overcome the decrease in the entropy of the surroundings that accompanies the endothermic change, so ionic compounds are insoluble in hexane.
Ionic compounds are often soluble in water, because the attractions formed between ions and water are frequently strong enough to make their solution either exothermic or only slightly endothermic. For example, the solution of sodium hydroxide is exothermic, and the solution of sodium chloride is somewhat endothermic. Even if the solution is slightly endothermic, the tendency to shift to the higher entropy solution often makes ionic compounds soluble in water.
The dividing line between what we call soluble and what we call insoluble is arbitrary, but the following are common criteria for describing substances as insoluble, soluble, or moderately soluble. If less than 1 gram of the substance will dissolve in milliliters or g of solvent, the substance is considered insoluble.
If more than 10 grams of substance will dissolve in milliliters or g of solvent, the substance is considered soluble. If between 1 and 10 grams of a substance will dissolve in milliliters or g of solvent, the substance is considered moderately soluble. Although it is difficult to determine specific solubilities without either finding them by experiment or referring to a table of solubilities, we do have guidelines that allow us to predict relative solubilities.
The tiny electrons with negative charges circle rapidly in orbits around the nucleus, forming electron shells at different distances, much like the planets and other objects that circle the sun.
Atoms of each element have varying numbers of electrons in their outermost shells. Atoms become more stable when their outermost electron shells are emptied out or filled up. One way they can achieve this goal is for two atoms to share one or more electrons between them so that each of them can fill or empty that outermost shell. But they can only share the electron s if they stay close to each other, and this is called a covalent bond. In other situations, one atom can become more stable by losing electrons and the other can become more stable by gaining them.
Here's a little joke to help you remember The formation of an ionic bond is a redox reaction. One atom loses electrons oxidation while the other one gains electrons reduction. Atoms that carry a charge, either positive or negative, are called ions and, because opposites attract, they can form an ionic bond.
Ionic and covalent bonds are the most important in all of chemistry. With ionic bonds, atoms give or take electrons. With covalent bonds, they have to share them. Now think about a magnet. So do batteries. So does the Earth.
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