Chapter 4: Intermolecular Forces of Liquids and Solids PDF

Chemistry 1 Chapter 4: Intermolecular Forces of Liquids and Solids Part 1: Intermolecular Forces (see chapter 11 in the text-book) 1  Definition : Intermolec...

Chemistry 1 Chapter 4: Intermolecular Forces of Liquids and Solids Part 1: Intermolecular Forces (see chapter 11 in the text-book) 1  Definition : Intermolecular forces are the attraction forces that exist between atoms, ions and molecules which is responsible about a lot of properties Intermolecular forces give properties of.. Solids/ Liquids gases Boiling/ Vapor Surface Capillary Ideal Real Melting point pressure Tension tension 2 I. The Kinetic Molecular Theory : Gas, Liquids and Solids According to the Kinetic Energy theory of matter, gas, liquid and solids consists of molecules in constant motion, these molecules have kinetic energy KE. What’s a Kinetic energy of Molecule The kinetic energy is the type of energy that anything has if it’s moving A phase is a homogeneous part of the system in contact with other parts of the system that can be separated mechanically from a nonhomogeneous mixture. A phase may consist of a single substance or a mixture of substances. The three fundamental phases of matter are: gas, liquid and solid. 3 I.1. In gases phase In gases, the distances between molecules are so great (compared with their diameters) that at ordinary temperatures and pressures (say, 25°C and 1 atm), there is no appreciable interaction between the molecules. There is no force of attraction or repulsion between gas particles. Because there is a great deal of empty space in a (space that is not occupied by molecules) -gases can be readily compressed. Compressed gas The lack of strong forces between molecules also allows a gas to expand to fill the volume of its container. Furthermore, the large amount of empty space explains why gases have very low densities under normal conditions (temperature T=0°C, Pressure P=1 atm). The kinetic energy is very high and gas molecules are in constant movement even at room temperature Heated air expand 4  Liquids and solids are quite a different story. The principal difference between the condensed states (liquids and solids) and the gaseous state is the distance between molecules. I.2. In liquid phase In a liquid, the molecules are so close together that there is very little empty space. Thus, liquids are much more difficult to compress than gases, and they are also much denser under normal conditions. Molecules in a liquid are held together by one or more types of attractive forces, which will be discussed in the next Section. A liquid also has a definite volume, because molecules in a liquid do not break away from the attractive forces. The molecules can, however, move past one another freely, and so a liquid can flow, can be poured, and assumes the shape of its container. Liquid molecules are relatively locked compared gas molecules. So its kinetic energy is lower. 5 I.3. In a solid phase In a solid, molecules are held rigidly in position with virtually no freedom of motion. Many solids are characterized by long-range order; that is, the molecules are arranged in regular configurations in three dimensions. There is even less empty space in a solid than in a liquid. Thus, solids are almost incompressible and possess definite shape and volume. The kinetic energy decreases as matter is in gas than in liquid than in solid phase. Characteristic properties of gases, liquids and solids 6 II. Intermolecular forces Intramolecular forces hold atoms together in a molecule. Intermolecular forces are attractive forces between molecules. Intramolecular forces stabilize individual molecules, whereas intermolecular forces are primarily responsible for the bulk properties of matter (for example, melting point and boiling point). 7 II. Intermolecular forces Physical properties affected by the presence of intermolecular forces : Generally, intermolecular forces are much weaker than intramolecular forces. The boiling points of substances often reflect the strength of the intermolecular forces operating among the molecules. At the boiling point, enough energy must be supplied to overcome the attractive forces among molecules before they can enter the vapor phase. If it takes more energy to separate molecules of substance A than of substance B because A molecules are held together by stronger intermolecular forces, then the boiling point of A is higher than that of B. The same principle applies also to the melting points of the substances. 8 II. 1- Van der Waals forces Van der Waals forces are weak intermolecular forces that are dependent on the distance between atoms or molecules. II.1.1. Dipole-dipole (permanent) Dipole-dipole intermolecular forces are attractive forces between polar molecules, that is, between molecules that possess dipole moments. Molecules with dipole moments can attract each other electrostatically by lining up so that the positive and negative ends are close to each other, as shown in figure below. The electrostatic interaction of two polar molecules 9 II.1.1. Dipole-dipole (permanent) That is, the molecules orient themselves to maximize the positive-negative charge interactions and to minimize positive- positive and negative-negative interactions, as represented below: The interaction of many dipoles in a condensed state Example: Interaction between HCl molecules 10  II.1.2. Induced dipole This dipole and induced dipole type of interaction occurs when we place an ion or a polar molecule (this means a molecule with dipole moment) near an atom (or a nonpolar molecule), the electron distribution of the atom (or molecule) is distorted by the force exerted by the ion or the polar molecule, resulting in a kind of dipole. when we place an ion when we place a polar molecule (dipole) He atom or other nonpolar molecule Induced dipole caused by the appraoch of a cation Induced dipole caused by the appraoch of a dipole ion-induced dipole interaction dipole-induced dipole interaction The dipole in the atom (or nonpolar molecule) is said to be an induced dipole because the separation of positive and negative charges in the atom (or nonpolar molecule) is due to the proximity of an ion or a polar molecule. 11 II.1.2. Induced dipole Examples : δ- δ- δ+ e- e- δ+ e- Ar Cl H e- Ar e- Cl H δ- δ+ e- e- Polar molecule the proximity H-Cl (polar molecule) to argon atom (non-polar) Argon : induced dipole creates an induced dipole δ+ δ- NO3- (nonpolar) polar I2 (nonpolar): induced dipole 12  What causes the appearance of instantaneous induced dipole ? The dipole moment being induced depends on : 1-The charge on the ion (cation or anion) or the strength of the dipole (of polar molecule) or polarity. The higher the polarity of the molecule the easier the induced dipole is formed. 2- The polarizability of the atom or molecule (nonpolar ones) Polarizability ? that is, the ease with which the electron distribution in non-polar entities i.e the atom (or molecule) can be distorted. Generally, the larger the number of electrons and the more diffuse the electron cloud (*) in the atom or molecule, the greater its polarizability. (*) By diffuse cloud we mean an electron cloud that is spread over an appreciable volume, so that the electrons are not held tightly by the nucleus. 13  I.1.3. Dispersion forces The London dispersion forces are the weakest intermolecular forces. This type of interaction happens between non polar atoms or molecule. For example, in a helium atom the electrons are moving at some distance from the nucleus. At any instant it is likely that the atom has a dipole moment created by the specific positions of the electrons. So an instantaneous dipole of one He atom can induce a dipole in each of its nearest neighbors At the next moment, a different instantaneous dipole can create temporary dipoles in the surrounding He atoms. The important point is that this kind of interaction produces dispersion forces, attractive forces that arise as a result of temporary dipoles induced in atoms or molecules. A B A-A B-B 1- Two atoms, no polarization 1- Two nonpolar molecules A-A and B-B : no polarization 2- instantaneous dipole on A atom 2- instantaneous dipole on A-A molecule 3-instantaneous dipole appears 3-instantaneous dipole appears also on B-B also on B atom molecule 14  I.1.3. Dispersion forces This Interaction between two instantaneous dipoles is governed by the polarizability which allows gases containing atoms or nonpolar molecules (for example, He and N2) to condense. In fact, at very low temperatures (and reduced atomic speeds), dispersion forces are strong enough to hold He atoms together, causing the gas to condense (gas to liquid). Note ! London dispersion forces result from the coulombic interactions between instantaneous dipoles. Dispersion forces are present between all molecules (and atoms) because of the constant motion of the electrons, an atom or molecule can develop a temporary (instantaneous) dipole when its electrons are distributed unsymmetrically around the nucleus. Examples: I2 (non-polar molecule) 15  I.1.3. Dispersion forces What causes the appearance of instantaneous induced dipole ? Molar mass : molecules with larger molar mass tend to have more electrons, and dispersion forces increase in strength with the number of electrons. Furthermore, larger molar mass often means a bigger atom whose electron distribution is more easily disturbed because the outer electrons are less tightly held by the nuclei. The comparison of the melting points of non polar compounds of similar substances shows that the melting point increases as the number of electrons in the molecule increases. Melting point (°C) and molar weight (g/mol) of similar nonpolar compounds Compound Weight mass (g/mol) Melting point (°C) CH4 16.04 -182.5 CF4 88.00 -150.0 CCl4 153.82 -23.0 CBr4 331.63 90.0 CI4 519.63 171.0 16  II. 2- Hydrogen bonds Force interaction by hydrogen bonds is dipole–dipole type interaction in which hydrogen is bound to a highly electronegative atom, such as nitrogen (N) , oxygen (O), or fluorine (F). Because dipole–dipole attractions of this type are so unusually strong, they are given a special name called hydrogen bonding. Figure below shows hydrogen bonding among water molecules, which occurs between the partially positive H atoms and the lone pairs on adjacent water molecules. Hydrogen bonds (in dots) 17 II. 2- Hydrogen bonds Two factors account for the strengths of these interactions: 1- The great polarity of the bond and the close approach of the dipoles (distance), allowed by the very small size of the hydrogen atom. 2- The strength of a hydrogen bond is also determined by the coulombic interaction between the lone-pair electrons of the electronegative atom and the hydrogen nucleus. For example, fluorine is more electronegative than oxygen, and so we would expect a stronger hydrogen bond to exist in liquid HF than in H2O. In the liquid phase, the HF molecules form zigzag chains: HF H2O NH3 Note that the O, N, and F atoms all exhibit at least one lone pair that can interact with the hydrogen atom in hydrogen bonding. 18  II. 2- Hydrogen bonds Hydrogen bonding has a very important effect on physical properties. Boiling point : In general, the boiling points of a series of similar compounds containing elements in the same periodic group increase with increasing molar mass due to the increase in dispersion forces for molecules with more electrons. Hydrogen compounds of Group 4A follow this trend. The lightest compound, CH4, has the lowest boiling point, and the heaviest compound, SnH4, has the highest boiling point. However, the lightest compound (NH3, H2O, and HF) has the highest boiling point, contrary to our expectations based on molar mass. This observation must mean that there are stronger intermolecular attractions in NH3, H2O, and HF, compared to other molecules in the same groups. In fact, this particularly strong type of intermolecular attraction is due to the hydrogen bond. 19 Question : Which of the following can form hydrogen bonds with water? CH3OCH3, CH4, F-, HCOOH, Na+ 20 II.3. Ion-Dipole Forces Ion-dipole forces, is the attraction forces between an ion (either a cation or an anion) and a polar molecule. The strength of this interaction depends on the charge and size of the ion and on the magnitude of the dipole moment and size of the molecule. 21  Exercise : Ion-Dipole Forces Q1: consider two cations Na+ and Mg2+. Which ion interacts more strongly with water molecules? Answer: Because the Mg2+ ion has a higher charge and a smaller ionic radius (78 pm) than that of the Na+ ion (98 pm), it interacts more strongly with water molecules. Q2: Which of these two cations has higher heat hydration? Hint : Note that the heat of hydration of a salt is defined as the heat change when it combines with a specific amount of water to form a stable hydrated salt. It is the result of the favorable interaction between the cations and anions of an ionic compound with water. Answer:neach ion is surrounded by a number of water molecules in solution. Consequently, the heats of hydration for the Na+ and Mg2+ ions are -405 kJ/mol and -1926 kJ/mol, respectively. Similar differences exist for anions of different charges and sizes. 22 Resume : different sorts of intermolecular forces Intermolecular forces Van der Waals forces London Hydrogen bonding Dipole-dipole Induced dipole Ion-dipole forces dispersion forces H…..O Two polar H…..N non-polar ion + polar molecules molecules molecule H…..F Strong dipole-dipole attraction or atoms ion-induced dipole interaction dipole-induced dipole interaction ion + non-polar polar molecule + non-polar molecule molecule 23 Question : what types of intermolecular forces between the following pairs: (a) HBr and H2S (b) Cl2 and CBr4 (c) I2 and NO3- (d) NH3 and C6H6 Classify the species into three categories: ionic, polar (possessing a dipole moment), and nonpolar. Keep in mind that dispersion forces exist between all species. Both molecules are polar: dipole-dipole forces and dispersion forces Both molecules are nonpolar: there is only dispersion forces between molecules c) I2 is homonuclear (same atom) diatomic molecule and therefore nonpolar, so the forces between NO3- and I2 are ion-induced dipole forces and dispersion forces. d) NH3 is polar C6H6 nonpolar so the forces between NH3 and C6H6 are dipole-induced dipole forces and dispersion forces 24 III. Properties of Liquids Intermolecular forces give rise to a number of structural features and properties of liquids. III.1. Surface Tension What are some real-life examples of surface tension? Surface tension enables the water Rain water droplets on leaf strider to “walk” on water. Floating a needle Surface tension is the ability of liquid surfaces to shrink into the minimum surface area possible. “Surface tension is the tension of the surface film of a liquid caused by the attraction of the particles in the surface layer by the bulk of the liquid, which tends to minimize surface area”. 25 III.1. Surface Tension How does surface tension causes a needle to stay at the surface of water? Molecules within a liquid are pulled in all directions by intermolecular forces. However, molecules at the surface are pulled downward and sideways by other molecules, but not upward away from the surface. These intermolecular attractions thus tend to pull the molecules into the liquid and cause the surface to tighten like an elastic film. The resistance of a liquid to an increase in its surface area is called the surface tension of the liquid. Liquids that have strong intermolecular forces also have high surface tensions. Thus, because of hydrogen bonding, water has a considerably greater surface tension than most other liquids. 26  How does surface tension causes a droplet to take the shape that it does? Because there is little or no attraction between polar water molecules and, say, the nonpolar surface of leaf, a drop of water assumes the shape of a small round bead, because a sphere minimizes the surface area of a liquid. Surface tension is capillary action Polar liquids typically exhibit capillary action, which is the spontaneous rising of a liquid in a narrow tube. Two different types of forces are responsible for this property: cohesive forces, the intermolecular forces among the molecules of water mercury the liquid adhesive forces, the forces between the liquid molecules and their container. ✓ When adhesion is greater than cohesion, the liquid (a water) rises in the capillary tube. ✓ When cohesion is greater than adhesion, as it is for mercury, a depression of the liquid in the capillary tube results. Note that glass is polar 27 III.2. Viscosity Viscosity is a measure of a fluid’s resistance to flow. The greater the viscosity, the more slowly the liquid flows. The viscosity of a liquid usually decreases as temperature increases. Liquids that have strong intermolecular forces have higher viscosities than those that have weak intermolecular forces Hydrogen bonds Ex: glycerol, whose structure is has an unusually high viscosity due mainly to its high capacity to form hydrogen bonds using its O-H groups. 28  Why does the viscosity of a liquid decrease with increasing temperature? When a liquid is heated, the kinetic energy of its molecules increases, and the intermolecular attraction becomes weaker. Hence, the viscosity of a liquid decreases with increase in its temperature. Why is ice less dense than water? When ice is formed it forms more hydrogen bonds with other water molecules to form hexagonal lattice structures. In the middle of the hexagons there is a lot of empty space. Ice takes up more space than water. Lower density When ice melts, a number of water molecules have enough kinetic energy to break free of the intermolecular hydrogen bonds. These molecules become trapped in the cavities of the three-dimensional structure. As a result, there are more molecules per unit volume in liquid water than in ice. Higher density 29