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THE INTERMOLECULAR FORCES Intermolecular forces include dipole-dipole forces, THE PHASES OF MATTER, KINETIC ion-dipole forces, hydrogen bon...

THE INTERMOLECULAR FORCES Intermolecular forces include dipole-dipole forces, THE PHASES OF MATTER, KINETIC ion-dipole forces, hydrogen bonding, and London MOLECULAR THEORY, AND dispersion forces. Collectively, these INTERMOLECULAR FORCES intermolecular forces are also known as Van Der The Kinetic Molecular Theory (KMT) is based Waals Forces, named after a Dutch chemist on the assumption that all states of matter have Johannes van der Waals (1837-1923). Van der component molecules that possess kinetic energy. Waals forces are fundamentally electrical in It postulates that the amount of kinetic energy, the nature; they result from the attraction between motion, and the arrangement of the molecules centers of opposite charge in two molecules close differ for each of the three states of matter. Hence, to each other. the KMT gives explanations for the properties of When two nonpolar molecules approach each matter as they exist as gases, liquids, or solids. other, the electron clouds in which the positive Assumptions of the KMT explain the shape and nuclei are embedded repel each other. This distorts volume of matter in the three states as follows: or polarizes each molecule that gives rise to 1. Matter in the gas state does not have definite induced dipoles and weak van der Waals forces shape and volume. It can fill a container of any size which momentarily exist between them. The and shape. The constant, free, random motion, the greater the number of atoms in a molecule, the relatively very large distance, and the very weak or more readily it is polarized. Hence, in general, van negligible attraction between the molecules bring der Waals forces increase with increasing about these properties of gaseous matter. molecular weight. In other words, van der Waals attractive forces are formed when the molecules 2. Matter in the liquid state does not have definite are close enough such that one molecule produces shape but has definite volume. The shape is not varying electrical fields within itself as it is affected definite because the molecules have enough energy by the electrical fields of the adjacent molecules. to slide over one another and slip out of the Thus, the induced polarization of molecules ordered arrangement to conform to or follow the produced by the fluctuations of charges causes shape of their container. Although particles in a those close enough and properly oriented liquid are in constant motion, the attractive forces molecules to attract each other. The van der Waals between them are more effective or stronger than type of attraction between no polar groups is a those in a gas. nonspecific attractive force between two molecules This keeps the molecules close enough to touch that are about 3 to 4 Angstrom apart. It is a weak one another and keep them together so that their interaction with a bond energy of about 1 kcal/mol total volume is definite. for a pair of atoms. It is considered weaker than electrostatic and hydrogen bonds, but nonetheless, 3. Matter in the solid state has definite shape and important. definite volume because of the very strong attractive forces between the component particles LONDON DISPERSION FORCES (LDF) OR and their low kinetic energies. The component LONDON FORCES, OR SIMPLY particles are tightly packed together and organized DISPERSION FORCES in a fixed position. They have vibrational motion, London dispersion forces are weak attractions that at the atomic level, but don't move around. You are used to explain the attraction between must have noted that the differences in properties nonpolar molecules, as it is apparent that even of matter in the different phases can be attributed nonpolar molecules can have dipoles for short to liquid and solid molecules having attractive periods of time. Dispersion forces increase with forces between them. increasing molecular mass and decrease with Remarkably, the presence of attractive forces, even increasing distances between the molecules. This if they appear negligible in gases, accounts for the explains why the boiling and melting points of processes of phase transitions in matter - gaseous homologous nonpolar molecules (e.g., straight- substances condense to form liquids and liquids, in chain hydrocarbons) increase regularly with turn, form solids. Now, let us look into increasing mass. Although dispersion forces are intermolecular forces, also called noncovalent only one type of van der Waals force, some books bonds. use the term van der Waals forces exclusively for dispersion forces.  lon-dipole interactions are involved in solution processes like in the case of sodium chloride DIPOLE-DIPOLE FORCES dissolving in water. For example, when sodium Dipole-dipole interaction is the attraction of bond chloride crystal is placed in dipoles in different molecules. Bond dipoles arise water, the polar ends of the water molecules exert from the unequal sharing of electrons by covalently attractive forces on the surface ions of the crystal bonded atoms. The tendency of dipoles to be (Figure 1.7) and pull them away from each other, perfectly oriented with respect to one another is preventing them from rejoining as each ion is observed to be effective at reasonably larger surrounded by water molecules. distances than van der Waals forces. Figures 1.1 and 1.2 are illustrations of how dipole- dipole forces act Hydrogen Bonding The hydrogen bond is a weak bond formed when a hydrogen with partial positive charge (a hydrogen bonded to a small, highly electronegative atom) is Another representation of dipole-dipole close to an atom in a molecule with lone pairs of interaction between HCI molecules is Figure 1.3: electrons or with excess electronic charge (negative). It is formed as a result of weak electrostatic interaction between the partially positive hydrogen and the negative group or lone pair of electrons. It is a weak type of electrostatic attraction (2 to 10 kcal/mol). The strength of the H where the solid lines represent covalent bonds; the bond is influenced by the electronegativity of the dotted lines are dipole-dipole forces. For water atom to which it is bonded. The H bond can be molecules, the dipole- dipole interaction is looked upon as a bridge between two highly represented as: electronegative atoms, either F, O, or N with the latter being covalently bonded to other hydrogens. Molecules that contain hydrogen covalently Figure 1.4. Dipole-Dipole bonded to F, O, or N have significant hydrogen Interaction in Water bonding ability. Examples of H bonds are shown in Molecules. Figure 1.8. The solid lines represent covalent bonds; the dotted lines are H-bonds. These intermolecular forces are weaker than. where the hydrogen forms the + end and the either ionic or covalent bonds, but their oxygen forms the end of the water dipole. importance cannot be underestimated. They are ION-DIPOLE FORCES responsible for determining whether a molecular compound is a gas, a liquid, or a solid at a given These forces exist when polar molecules are temperature. They also account for the differences attracted to ions. Figures 1.5 and 1.6 show that the in some physical properties of matter in the positive pole is attracted to a negative ion (anion), different phases. while the negative pole is attracted to a positive ion (cation). Figure 1.8. (A) bondo between the same molecules 4. Evaporation - It is an indication of the escape and bonds between those of different molecules of molecules from the surface of the liquid. It is evidence of molecular motion. A liquid in an open MATTER IN A LIQUID PHASE container eventually evaporates completely Another property of a liquid which is determined 5. Cooling Effect of Evaporation - The by the attractive forces between the molecules and molecules that escape are the ones with greatest the molecular geometry is viscosity. The viscosity velocity. Therefore, the average velocity and the of a liquid is a measure of its tendency to resist average kinetic energy (KE) of the molecules left in flowing motion. Polar molecules and molecules the liquid are reduced as evaporation proceeds. with complex structures (with "branches") tend to The amount of heat energy for the given number of have higher viscosity, being less able to slip and molecules is reduced as well as their temperature; slide over another than those with simple thus, evaporation always results in a cooling effect. structures and less polarity. For example, cooking oil (with chains of more than 12 carbons) is more 6. Vapor Pressure - When a liquid vaporizes in viscous than gasoline (with 7-8 carbons) due to a closed container, the space above the liquid stronger London dispersion forces; and glycerol, becomes saturated with vapor and an equilibrium with three OH groups, is more viscous than state exists between the liquid and the vapor. rubbing alcohol because of more H-bonding. A liquid with high viscosity is said to be viscous or The equilibrium equation is: simply "thick." When viscosity is so high that it cannot flow anymore, the matter is said to be glassy or vitreous. The opposite of viscosity is fluidity. Highly fluid liquid is said to be free-flowing, mobile, or "thin." Other properties observed when matter is in the At equilibrium, the molecules in the vapor exert a liquid phase include the following: pressure. 1. Capillary Action - This refers to the The pressure exerted by a vapor in equilibrium spontaneous rising of a liquid in a narrow tube. with its liquid is known as the vapor pressure of the This action results from the cohesive forces liquid. This may be considered as a measure of the (intermolecular forces) within the liquid and the "escaping" adhesive forces between the liquid and the walls of the container. When the attraction between the tendency of molecules to go from the liquid to the liquid and the walls of the container is greater than vapor state. those within the liquid itself, the liquid will rise 7. Boiling Point - This is the temperature at within the container. This property of matter in the which the vapor pressure of a liquid is equal to the liquid state explains how plants get nourishment external pressure atmospheric pressure above the (water and dissolved minerals) from the soil liquid). Thus, when the boiling point is given, the through their roots and to all parts of the plants. pressure should also be stated. When we express 2. Incompressibility - In ordinary conditions, the boiling point without the pressure, it is this is another property of liquids. Since the interpreted to be the normal boiling point at the molecules in a liquid are already close and standard atmospheric pressure of 760 mm Hg at touching one another, they cannot be crowded sea level. The boiling point is one of the most together anymore unless they are squeezed and commonly used physical properties for deformed, which would require a great amount of characterizing and identifying substances. The energy. heat of vaporization, expressed in cal/g or in kcal/g-atom or in kJ/kg, is the energy required to 3. Diffusibility - This is much less in a liquid than change exactly 1 gram of liquid to vapor at its in a gas, but it takes place at an easily measured normal boiling point. The attractive forces rate. One liquid may diffuse through another, or a between the liquid molecules are overcome during solid may dissolve and diffuse through a liquid. vaporization. Only when all the liquid has become The particles in a liquid are attracted to one a gas will the temperature of the substance again another, but they. are not rigidly held together so increase as more heat energy is added. they can always still move. Thus, they can slide over one another to effect diffusion. Water and Its Properties The liquid most familiar If this happens, we would not have the much- to all of us is water. Every one of us sees and uses needed water in the liquid phase for drinking. water in our everyday life. Water is so common that we often take it for granted. We assume that it SPECIFIC HEAT is a typical liquid, but chemical studies show that The specific heat of water is higher than any nearly all of its chemical and physical properties commonly known liquid, except ammonia, as seen are unusual when compared to other liquids. in Table 1.1. This means that water takes a much Water is a very remarkable substance with its longer time to heat up and a longer time to cool simple composition and structure. It has unique down than most substances on Earth. It can absorb properties. As evidence shows, it is the only natural a large amount of heat with only a slight change in substance that is found in all three phases: temperature. Water temperature fluctuates less than land temperature; in such a way, large bodies liquid, solid, and gas at temperatures normally existing on our planet. Pure water is an odorless of water, like the oceans, serve as moderating influences on the Earth's climate. Water serves as and tasteless liquid. It has a bluish tint. At standard atmospheric pressure (760 mm of Hg or the Earth's thermal regulator, considering that more than 70% of the Earth's surface is covered 760 torr), its freezing point is 0°C (32°F) and boiling point is 100°C (212°F). with water. DENSITY - Water is at its maximum density at a SOLVENT ACTION temperature of 4°C (39°F). It has the unusual Water dissolves more substances any other than property of contracting in volume as it is cooled to common liquid that is why it is the most used polar 4°C, and then expanding when cooled from 4°C to solvent. This property makes water the most 0°C. It is the only liquid that expands when it effective liquid for transporting dissolved freezes. This makes the density of ice lower than nutrients into the bloodstream and eliminating that of liquid water and that is why ice floats on wastes from living tissues in our bodies. water. This dissolving ability also explains why water is Ice crystals are formed from intermolecularly H- the most important agent in bonded water molecules as freezing occurs. As illustrated in Figure 1.12, the H-bonds keep the the erosion of weathered materials on the Earth's molecules in fixed positions but with spaces surface and why tropical areas are much more between (open lattice structure), which results in eroded than deserts. Water that runs over and lower density for solid water (ice) than liquid through the surface of the land dissolves many water. minerals of rocks and soil. Unfortunately, this dissolving ability is also the reason why water is easily polluted, often stays polluted, and remains stagnant for a long time. This simply shows that wherever water flows, either through the ground or through our bodies, it takes along valuable chemicals, minerals, and nutrients. SURFACE TENSION Water has a high surface tension because of strong Figure 1.12. (A) H-bonded water molecules in ice intermolecular H-bonding. and (B) ice crystal sample Boiling Point - Water has a relatively high boiling point (100°C at 1 atm of pressure). Once it Figure 1.13. Model of reaches this, its temperature remains constant and water molecules on undergoes a phase transition into water vapor. the surface held Substances of together by H- bonding comparable molar mass like ammonia and methane are gases at a temperature that water is a liquid. If the properties of water were to be similar with other liquids on Earth, it would boil at normal surface temperatures and thus, exist only as a gas. COMPOSITION AND STRUCTURE OF CHEMICAL PROPERTIES OF WATER WATER Water is a very stable substance. It remains in its By this time, we already know that a single water liquid state at temperatures found in most places molecule consists of two hydrogen atoms and one on the Earth's surface and it has a relatively high oxygen atom. Each hydrogen atom is attached to boiling point of 100°C compared with compounds the oxygen atom by a single covalent bond (Figure of similar molecular weights. Water is directly 1.14). This bond is formed by the overlap of the 1s involved in many chemical reactions. orbital of hydrogen with asp hybrid orbital of oxygen re and that contains the unpaired electron. 1. Water reacts with metals- For example, potassium, sodium, and calcium react similarly with water, producing metal hydroxides and liberating hydrogen gas, but they differ in the intensity or vigorousness of their reactions. In addition, aluminum, zinc, and iron react with steam at high temperature, forming hydrogen and metallic oxides. 2. Water reacts with nonmetals - The element fluorine reacts violently with cold water, producing As illustrated by Figure 1.15, although the H20 hydrogen fluoride and free oxygen. molecule as a whole is electrically neutral, its O-H bonds are polar, and because of its bent structure, 3. Water reacts with metallic oxides - Soluble water is a polar molecule. It acts as a dipole, with a oxides like CaO and Na2, 0 react with water to partial positive charge on one end (on the H atoms form hydroxides. side) and a partial negative charge on the other end 4. Water reacts with nonmetallic oxides - (on the oxygen atom). CO2, SO2, and N2,05 react with water to form acids. 5. Water can combine with some salts - This combination results in the formation of hydrates which are solids that contain water molecules as part of their crystalline structure. The water in the hydrate is called water of crystallization or water of hydration. Hydrates follow the law of definite composition, having a fixed number of water molecules in the crystalline unit. This number of The intermolecular forces acting between water water molecules is included in the formula of the molecules are the hydrogen bonds. An H-bond is compound using a dot ( ) followed by the number the dipole-dipole attraction between polar of water molecules present. For example, the molecules containing F-H, O-H, or N-H bonds. compound named copper (II) sulfate pentahydrate Thus, water has two types of bonds: (1) covalent contains five molecules of water, so the formula is bonds between H and O atoms within the CuSO4, 5H20. molecule; and (2) H-bonds between H and O atoms in different water molecules (Figure 1.16). Uses of Water Industrial Uses Water is fundamental to industries. It is used for nearly every step of the manufacturing and production processes, as such there is a great demand for it. This demand progressively increases due to the rapidly growing population. The intermolecular H-bonding effectively gives Below is a list of how water is used within water the properties of a much larger, heavier industries: molecule due too three-dimensional aggregation. 1. Water is part of the product - For example, land. Geological studies revealed that as part of the large amounts of water are used in manufacturing cycle, about 425,000 km of ocean water evaporates soft drinks, beer, pastry, and canned foods as well per year. Most of it precipitates back into the as in the formulation of drugs, lotions, cleaning oceans, but about 40,000 km' falls on land, agents, etc. providing most of our water supply. Unfortunately, because of intensifying human activities, global 2. Water is used in the processing of a warming leading to the Earth's rising surface product - For example, huge quantities of water temperature has considerable impacts on the are used in making paper, but the greater part of it hydrologic cycle, such that it alters the amount, does not stay with the product. Water helps in timing of distribution, and quality of available grinding wood chips and mixing the pulp over a water. In addition, there is also evidence of silted wide screen, but the water is drained off the screen rivers and streams, polluted surface and as the paper moves ahead. Other cases are in the groundwater supplies, and lakes that are acidified cleaning of containers like bottles and cans where and biologically dead or prematurely filled by silt the manufactured products are packed. or algal growth. 3. Water is used for cooling the product - For If this environmental issue is not addressed, it will instance, the water may be piped through a steel result in shrinking water supplies that will not mill to absorb heat from the furnace and molten meet the continuing increase in water demand in metal. many areas around the world. AGRICULTURAL USES "The beautiful thing about learning is Lands are irrigated by water from rivers, lakes, and that no one can take it away from you." artificial reservoirs, but there is unavoidable need for man-made irrigation and dams or reservoirs. - Ms. D Dams are used by man to slow down or speed up the flow of water or to stop its flow altogether. When dams stop the flow, water is usually stored in a lake or reservoir so that people can make use of it when needed. DOMESTIC USES An adult human being has a minimum daily need of 2 liters of water for drinking. This is an obvious requirement, but an equally vital one is the large volume of water needed to sustain his or her other domestic needs like for bathing, household cleaning, watering the plants, and laundry purposes. WATER SUPPLY AND WATER QUALITY There is so much amount of water on Earth. However, of all that water, only less than 1 percent is available for consumption of humans and many other living things. Only about 0.3 percent of fresh water, which is our source for drinking water, is found in our surface waters like rivers, lakes, and swamps, and from groundwater sources (wells). Water is considered a renewable resource since it can be replenished naturally after consumption through the complex process called hydrologic cycle. This assures that it would not be in danger of being used up or being depleted. Water continuously circulates from the oceans to the atmosphere, to the land and back to the oceans, providing a renewable supply of purified water on