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Chapter 10: Intermolecular Forces: the Uniqueness of Water

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Chapter 10: Intermolecular Forces: the Uniqueness of Water Chapter 10: Intermolecular Forces: The Uniqueness of Water Learning Objectives 10.1: Intramolecular Forces versus Intermolecular Forces • Be able to compare and contrast intramolecular forces of attraction with intermolecular forces of attraction for ionic, covalent, molecular, and metallic materials. See the table given below. Comparing Intramolecular and Intermolecular Forces of Attraction Intramolecular Force Intermolecular Force(s) Examples Ionic Bond Electrostatic force NaCl, BaF2 Covalent Bonds forming a Covalent Bond C(diamond), SiO2 Network Solid Covalent Bond Hydrogen bonding See examples in previous table. Dipole-dipole forces London dispersion forces Metallic Bond (“sea of electrons”) Ag, Na • Be able to use the Kinetic Molecular Theory of Matter to compare and contrast the physical properties of gases, liquids, and solids. Focus on volume, shape, compressibility, relative density, and molecular motion. 10.2: Dispersion Forces & 10.3: Interactions among Polar Molecules • Be able to compare and contrast the intermolecular forces of attraction (IFA or IMF; sometimes called “van der Waals forces”) for molecular compounds. See the table given below and Table 10.3 in text. • Understand that as molar mass increases, melting points of similar nonpolar compounds increase due to a greater number of electrons and the resulting ease in polarizability (see Tables 10.1 & 10.2; see Fig. 10.3 in text). • Be able to use intermolecular forces of attraction to explain relative melting points, boiling points, surface tensions, viscosities, specific heats, and vapor pressures for a given series of molecular compounds. Intermolecular Forces of Attraction (“van der Waals forces”) Type Strength Attraction Examples Hydrogen bonding Strongest “H” on one molecule is attracted to “F,” H2O, NH3, HF, base “O,” or “N” on an adjacent molecule. pairs in DNA Dipole-dipole forces Intermediate Polar molecules attract each other like HCl, H2S tiny magnets. (Larger dipole moments result in greater interactions.) London dispersion Weakest Induced dipoles attract each other for a CH4, CCl4 forces fraction of a second; works best with atoms and molecules having lots of electrons, which makes them more Chapter 10: Intermolecular Forces: The Uniqueness of Water, Page 2 polarizable. All covalent compounds have London dispersion forces. 10.4: Polarity and Solubility • Be able to distinguish between solute, solvent, solution, and solubility. • Remember that solubility is defined as the maximum amount of a solute that will dissolve in a given quantity of solvent at a specific temperature. • Be able to describe general solubility trends: • As temperature increases, the solubility of a solid in water increases. • As temperature increases, the solubility of a gas in water decreases. • As pressure increases, the solubility of a gas in water increases. Ex. Soda pop effervescence. • Be able to use both words and graphs to compare and contrast saturated, unsaturated, and supersaturated solutions. • Be able to distinguish between miscible and immiscible liquids. • Be able to use electronegativity values to determine bond polarity. • Be able to combine knowledge of bond polarity and molecular geometry to predict molecular polarity. • Be able to clearly explain and apply the solubility rule “like dissolves like,” which means that the solute and solvent are alike in polarity: • Nonpolar solutes will dissolve in nonpolar solvents. • Polar and ionic solutes will dissolve in polar solvents. • Be able to explain how soap works. Soap molecules have ionic “heads” that dissolve in water and nonpolar “tails” that dissolve in grease. Thus, soap and water can remove grease despite the fact that water and grease, which are polar and nonpolar compounds, respectively, will not dissolve in each other. 10.5: Solubility of Gases in Water • As pressure increases, the solubility of a gas in water increases. Ex. Soda pop effervescence. • Be able to perform calculations with Henry’s Law: � = ��, where � is the molar concentration of the dissolved gas (moles/L), � is a constant that depends only on temperature (moles/L-atm), and � is the pressure of the gas over the solution at equilibrium (atm). • Most gases obey Henry’s Law; exceptions include gases that undergo reactions like: • NH + H O NH + + OH- 3 2 4 • CO + H O H CO 2 2 2 3 • Hb + 4 O Hb(O ) where Hb stands for hemoglobin in blood 2 2 4 • Be able to use Henry’s Law to perform calculations related to deep sea diving and “the bends.” 10.6: Vapor Pressure of Pure Liquids ⎛ P1 ⎞ ΔH vap ⎛ 1 1 ⎞ • Be able to perform calculations with the Clausius-Clapeyron equation: ln⎜ ⎟ = ⎜ − ⎟ ⎝ P2 ⎠ R ⎝ T2 T1 ⎠ 10.7: Phase Diagrams: Intermolecular Forces at Work • Be able to draw and explain phase diagrams. Be able to explain the significance of the triple point and the critical point. 10.8: Some Remarkable Properties of Water • Be able to distinguish between surface tension, capillary action, cohesion, and adhesion. • Be able to explain why water has “surprising” physical properties Chapter 10: Intermolecular Forces: The Uniqueness of Water, Page 3 • Water is polar because it has polar bonds and bent geometry. It is also capable of hydrogen bonding, which resulting in several unusual properties. 1. Water has a higher melting point and boiling point than expected. 2. Water has considerable surface tension. 3. Water has a high capacity to absorb heat. 4. Ice is less dense than liquid water and will therefore float. • In addition to these properties, polar and ionic substances will dissolve in water. .
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