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.
Principal among these is. For example, this guideline could be used to predict that ethanol, which is composed of polar molecules, would be soluble in water, which is also composed of polar molecules. Likewise, pentane C5H12 , which has nonpolar molecules, is miscible with hexane, which also has nonpolar molecules. We will use the Like Dissolve Like guideline to predict whether a substance is likely to be more soluble in water or in hexane.
It can also be used to predict which of two substances is likely to be more soluble in water and which of two substances is likely to be more soluble in a nonpolar solvent, such as hexane: Polar substances are likely to dissolve in polar solvents. For example, ionic compounds, which are very polar, are often soluble in the polar solvent water.
Nonpolar substances are likely to dissolve in nonpolar solvents. For example, nonpolar molecular substances are likely to dissolve in hexane, a common nonpolar solvent. Nonpolar substances are not likely to dissolve to a significant degree in polar solvents.
For example, nonpolar molecular substances, like hydrocarbons, are likely to be insoluble in water. Polar substances are not likely to dissolve to a significant degree in nonpolar solvents.
For example, ionic compounds are insoluble in hexane. It is more difficult to predict the solubility of polar molecular substances than to predict the solubility of ionic compounds and nonpolar molecular substances. Many polar molecular substances are soluble in both water and hexane. For example, ethanol is miscible with both water and hexane. The following generalization is helpful:. Substances composed of small polar molecules, such as acetone and ethanol, are usually soluble in water.
A second property of any system, its entropy, is also important in helping us determine whether a given process occurs spontaneously. A perfect crystal at 0 K, whose atoms are regularly arranged in a perfect lattice and are motionless, has an entropy of zero.
In contrast, gases have large positive entropies because their molecules are highly disordered and in constant motion at high speeds. The formation of a solution disperses molecules, atoms, or ions of one kind throughout a second substance, which generally increases the disorder and results in an increase in the entropy of the system.
Thus entropic factors almost always favor formation of a solution. In contrast, a change in enthalpy may or may not favor solution formation. The London dispersion forces that hold cyclohexane and n-hexane together in pure liquids, for example, are similar in nature and strength. Mixing equal amounts of the two liquids, however, produces a solution in which the n-hexane and cyclohexane molecules are uniformly distributed over approximately twice the initial volume.
In other cases, such as mixing oil with water, salt with gasoline, or sugar with hexane, the enthalpy of solution is large and positive, and the increase in entropy resulting from solution formation is not enough to overcome it.
Thus in these cases a solution does not form. The column on the far right uses the relative magnitudes of the enthalpic contributions to predict whether a solution will form from each of the four.
Keep in mind that in each case entropy favors solution formation. In two of the cases the enthalpy of solution is expected to be relatively small and can be either positive or negative. Thus the entropic contribution dominates, and we expect a solution to form readily. In the other two cases the enthalpy of solution is expected to be large and positive. The entropic contribution, though favorable, is usually too small to overcome the unfavorable enthalpy term. Hence we expect that a solution will not form readily.
In contrast to liquid solutions, the intermolecular interactions in gases are weak they are considered to be nonexistent in ideal gases. Consequently, all gases dissolve readily in one another in all proportions to form solutions. In contrast, naphthalene is a nonpolar compound, with only London dispersion forces holding the molecules together in the solid state. The slight positive charges on the hydrogen atoms in a water molecule attract the slight negative charges on the oxygen atoms of other water molecules.
This tiny force of attraction is called a hydrogen bond. This bond is very weak. Hydrogen bonds are formed easily when two water molecules come close together, but are easily broken when the water molecules move apart again. They are only a small fraction of the strength of a covalent bond, but, there are a lot of them and they impart some very special properties to the substance we call water.
Water is a Liquid at Room Temperature. Over three-quarters of the planet earth is covered with water. Life probably started in such a liquid environment and water is the major component of living things humans are over 60 percent water. At room temperature anywhere from zero degree centigrade to degrees centigrade , water is found in a liquid state.
This is because of the tiny, weak hydrogen bonds which, in their billions, hold water molecules together for small fractions of a second. Water molecules are constantly on the move. If they are moving fast enough they become a gas.
0コメント