What is it A. Polarity, Solubility, and Miscibility Molecules do not have the same properties, which contributes to the wonderful diversity of things around us and the different ways such things react with the world. One of the major factor that determines what properties molecules have is their polarity and the two important practical manifestations of polarity are solubility and miscibility. Solubility refers to the ability of a solute to dissolve in a certain amount of solvent. Miscibility is the ability of two liquids to mix in all proportions. You may have notice common solubility problems in your everyday life. If you get bike chain grease on your pants, sap from a pine tree on your shirt, or wax from a candle on the table top, these substances are hard to remove with just water. Why do you think this demonstrates a solubility problem? What is the polarity of these substances, and how do you eventually get the stain out of your clothes? It is possible to determine the polar nature of various substances knowing that ‘like dissolves like.’ Polar substances WILL DISSOLVE in WATER (a polar solvent), while non-polar solutes DO NOT. Non-polar substances dissolve only in other non-polar materials. Now I want you to try the exercises below in order to assess how much you have understood the solubility (and miscibility) rule of substances in relation to their polarity. Which of the following substances below will most likely mix with each other? a. water (H2O) and chloroform (CHCl3) b. benzene (C6H6) and chloroform (CHCl3) c. water (H2O) and vinegar (CH3COOH) d. acetone (C3H6O) and toluene (C6H5CH3) e. carbon tetrachloride (CCl4) and water (H2O) In this lesson, we will not discuss Intermolecular Forces of Attraction (IMFA) in so much detail because it will be tackled to you in the next lesson. We will just focus on the following salient points of IMFA and its effect on the physical properties of covalent compounds: 1. There are several types of IMFA and below they are arranged from STRONGEST to WEAKEST. Ion-dipole → H-bonding → Dipole-dipole → Dipole-induced dipole → London forces of attraction 2. The strength of IMFA greatly affects the physical properties of substances such as boiling point, melting point, vapor pressure, surface tension, etc. Table 1 shows the comparison of the various types of IMFA while table 2 shows the physical properties of polar and nonpolar molecules as affected by the type of IMFA present. Table 1. Summary of Types of Intermolecular Forces of Attraction (IMFA) Types of IMFA Ion-dipole Hydrogen bonding Dipole –dipole Dipole-induced dipole London forces Interacting Substances Ion (cation or anion) and a polar molecule Polar molecules containing H chemically bonded to a small and highly electronegative nonmetal atom such as N, O, and F Polar molecules Polar and nonpolar molecules All substances and solely for nonpolar molecules and noble gases Examples NaCl dissolved in H2O; Ca2+ and PCl3 H2O, NH3, CH3OH CH3F and H2S; HCl; HI and CH4 O2, N2, He gas, Br2 Table 2. General Properties of polar and nonpolar molecules Polar molecules • IMFA type: H-bonding and dipole-dipole • exist as solids or liquids at room temperature • High boiling point • High melting point • High surface tension • Low vapor pressure • Low volatility • Soluble in water Nonpolar molecules • IMFA type: London dispersion • exist as gases at room temperature • Low boiling point • Low melting point • Low surface tension • High vapor pressure • High volatility Insoluble in water Let us define the physical properties of substances: A. Boiling point: the temperature at which a liquid boils and turns to vapor B. Melting point: the temperature at which a given solid will melt. C. Surface tension: is the tendency of liquid surfaces to shrink into the minimum surface area D. Viscosity: resistance of a fluid to flow or a change in shape. E. Vapor pressure: the pressure of a vapor in contact with its liquid or solid form F. Volatility: tendency of a substance to evaporate at normal temperatures In the simplest sense, boiling point, melting point, viscosity and surface tension increase as the strength of intermolecular forces increases. On the other hand, vapor pressure and volatility decrease with increasing strength of IMFA. London dispersion forces increase as the molecular mass of a substance increases. Unlike in H-bonding, as the molar mass increases, the boiling point, melting point, viscosity and surface tension decrease. Lesson 5 Types of Intermolecular Forces What is It Intermolecular forces or IMFs are physical forces between molecules while intra molecular forces are forces between atoms within a single molecule. As far as the strength is concerned, intermolecular forces are weaker than intra molecular forces. The intermolecular forces explain why substance exists as solid, liquid or gas at room temperature. The four main types of intermolecular forces: 1. Ion-dipole interaction 2. Dipole-dipole interaction 3. Hydrogen bonding 4. Dispersion forces also known as London forces in honor of Fritz London. Ion-dipole interaction Ion-dipole interaction happens when an ion meets a polar molecule. In this case, the ionic charge determines which part of the molecule attracts and which part repels. A cation or positive ion would be attracted to the negative part of a molecule and repelled by the positive part. An anion or negative ion would be attracted to the positive part of a molecule and repelled by the negative part. Example: An example of the ion-dipole interaction is the interaction between a Na+ ion and water (H2O) where the sodium ion and oxygen atom are attracted to each other, while the sodium and hydrogen are repelled by each other. Dipole-Dipole Interaction Dipole-dipole interaction occurs whenever two polar molecules get near each other. The positively charged portion of one molecule is attracted to the negatively charged portion of another molecule. Since many molecules are polar, this is a common intermolecular force. Example: An example of dipole-dipole interaction is the interaction between two sulfur dioxide (SO2) molecules, in which the sulfur atom of one molecule is attracted to the oxygen atoms of the other molecule. Example: Hydrogen bonding is considered a specific example of a dipole-dipole interaction always involving hydrogen. A hydrogen atom of one molecule is attracted to an electronegative atom of another molecule, such as an oxygen atom in water. Hydrogen Bond The bond that exists between water molecules is Hydrogen bond (Fig.3). It is a special kind of dipole-dipole interaction between Hydrogen which is a polar molecule and a highly electronegative elements Fluorine, Oxygen and Nitrogen. In Hydrogen bond, the highly electronegative element F, O, N causes the hydrogen to become strongly positive. The ability of water to form H-bond relates to its ability as a universal solvent. Hbond prevents the water from evaporating quickly into the atmosphere. It also causes ice to float in water since at freezing temperature, water molecules tend to form a crystal lattice as it expands. London Dispersion Forces It is present in all molecules. It is the weakest intermolecular force which is formed due to temporary dipoles of a non-polar molecule. The strength of the dispersion forces increases as the molecular weight of the substance increases. Think about an atom like argon. It’s an inert gas, right? But if you cool it to –186 °C, you can actually condense it into liquid argon. The fact that it forms a liquid it means that something is holding it together. That “something” is dispersion forces.
