Chapter 11 Intermolecular Forces and the Liquid State Lecture Notes PDF

Chapter 11. Intermolecular Forces and the Liquid State © 2018 Cengage Learning. All Rights Reserved.  Intermolecular Forces (IMFs) Forces between particles that hold one molecule near another molecule – Relative strengths of IMFs closely mirror that of the density rank...

Chapter 11. Intermolecular Forces and the Liquid State © 2018 Cengage Learning. All Rights Reserved.  Intermolecular Forces (IMFs) Forces between particles that hold one molecule near another molecule – Relative strengths of IMFs closely mirror that of the density ranking (greatest in solids and weakest in gases) 2 © 2018 Cengage Learning. All Rights Reserved.  Intermolecular Forces (IMFs) (continued 1) Attractions or repulsions between molecules Differ from chemical bonds – Chemical bonds explain why atoms in a molecule stay near one another in a solid or liquid – IMFs explain why molecules stay near one another in a solid or liquid Function of: – Charge (ions) – Polarity (molecular shape and dipoles) – Molar mass 3 © 2018 Cengage Learning. All Rights Reserved.  Intermolecular Forces (continued 2) (IMFs) Influence chemistry and are directly related to the following properties: – Melting point – Boiling point – Energy to convert a solid to liquid – Energy to convert a liquid to vapor Vary in strength 4 © 2018 Cengage Learning. All Rights Reserved.  Intermolecular Forces (continued 3) (IMFs) Important in determining: – Solubility of gases, liquids, and solids in various solvents – Structures of biologically important molecules, such as DNA and proteins Much weaker than covalent bonds 5 © 2018 Cengage Learning. All Rights Reserved.  Properties of Solids, Liquids, and Gases Of the three states of matter: – Liquids are the most difficult to describe precisely Particles in a liquid interact with their neighbors but have a long-range order in their arrangement – Structures of solids can be described easily Particles that make up solids are usually in an orderly arrangement Under ideal conditions, the molecules in a gas are far apart – Considered to be independent of one another 6 © 2018 Cengage Learning. All Rights Reserved.  Ranking IMFs Solids – Strongest IMF – Molecules are held firmly near one another – Molecules cannot swap positions Liquids – Relatively strong IMF – Molecules are held near one another but less strongly than in solids – Molecules can swap positions Gases – Ideal gases experience little or no IMF 7 © 2018 Cengage Learning. All Rights Reserved.  Types of IMFs Dipole–dipole Ion–dipole Dipole–induced dipole Induced dipole–induced dipole 8 © 2018 Cengage Learning. All Rights Reserved.  Dipole–Dipole IMFs Attractive forces that occur between two polar molecules (have a permanent dipole) – Positive end of one molecule lines up with the negative end of another molecule Influence the evaporation of a liquid and the condensation of a gas 9 © 2018 Cengage Learning. All Rights Reserved.  Dipole–Dipole IMFs - Example HCl, a polar molecule – Dipole–dipole IMFs between HCl molecules allow gaseous HCl to condense to form a liquid at low temperatures 10 © 2018 Cengage Learning. All Rights Reserved.  Hydrogen Bonding Unusually strong dipole–dipole IMF that occurs between molecules with H—N, H—O, or H—F bonds 11 © 2018 Cengage Learning. All Rights Reserved.  Reasons for Strength of Hydrogen Bonding Elements bonded to H are very electronegative, and H has a low electronegativity – Low electronegativity results in highly polar bonds, which result in large partial charges and therefore strong IMFs H has a very small size, allowing hydrogen- bonding molecules to approach closely – Decrease in distance results in a strong force of attraction 12 © 2018 Cengage Learning. All Rights Reserved.  Ion–Dipole IMFs Occur when an ionic compound and a polar covalent compound are mixed – Polar molecule can be attracted to an ion of the opposite charge Stronger than dipole–dipole attractive forces 13 © 2018 Cengage Learning. All Rights Reserved.  Ion–Dipole IMFs - Example Consider a solution of ionic salt in water Polar nature of water provides for attractive forces between ions and water – Cations are attracted to the negative end – Anions are attracted to the positive end Na+ Cl  14 © 2018 Cengage Learning. All Rights Reserved. Example Problem 11.1.1 - Identify Species That Can Form Hydrogen Bonds 15 © 2018 Cengage Learning. All Rights Reserved.  Dipole–Induced Dipole IMFs Attractive forces that occur between polar and nonpolar molecules Nonpolar molecules do not have permanent dipoles, but an induced dipole can be created – Induced dipole: Temporary dipole created in a nonpolar molecule 16 © 2018 Cengage Learning. All Rights Reserved.  Dipole–Induced Dipole IMFs - Example Consider the interaction between polar H 2O and nonpolar O2 molecules When the negative end of an H2O molecule approaches an O2 molecule, the negative partial charge repels the O2 electron cloud – This distorts the cloud and creates a temporary dipole on the O2 molecule 17 © 2018 Cengage Learning. All Rights Reserved.  Polarizability Ease with which an electron cloud can be distorted and a dipole can be induced Increases with increasing number of electrons in a molecule – Therefore, it increases with increasing molar mass and molecular size 18 © 2018 Cengage Learning. All Rights Reserved.  Induced Dipole–Induced Dipole IMFs Attractive forces that occur between nonpolar molecules Fluctuations in electron distribution in the nonpolar molecule induce a temporary dipole – Region with excess electron density has partial negative charge – Region with depleted electron density has partial positive charge 19 © 2018 Cengage Learning. All Rights Reserved.  Induced Dipole–Induced Dipole Forces - Example Iodine consists of nonpolar I2 molecules – When two I2 molecules approach each other, the electron clouds repel and temporary dipoles form in each I2 molecule 20 © 2018 Cengage Learning. All Rights Reserved.  Induced Dipole–Induced Dipole Forces London dispersion forces (LDF): Attractive forces between temporary dipoles – Generally weak – Stronger than dipole–dipole forces when they occur between highly polarized molecules 21 © 2018 Cengage Learning. All Rights Reserved.  Effect of Polarizability on Physical Properties Consider a series of atoms and molecules that have only dispersion(London dispersion) forces – As molar mass increases, number of electrons increases More electrons = larger electron cloud = increased polarizability = stronger attractions – As molecular size increases, strength of IMFs increases More surface-to-surface contact = larger induced dipole = stronger attraction 22 © 2018 Cengage Learning. All Rights Reserved.  Example Problem 11.1.2 - Identify Intermolecular Forces in a Substance 23 © 2018 Cengage Learning. All Rights Reserved. Intermolecular Forces and the Liquid State 11.2 Intermolecular Forces and the Properties of Liquids 24 © 2018 Cengage Learning. All Rights Reserved.  Phase Changes IMFs play an important role in influencing the physical state of substances Fusion and vaporization require energy to break the intermolecular bonds in liquids and solids Energy is released when a less ordered phase is converted to a more ordered phase as IMFs are formed – Condensation results in the release of energy 25 © 2018 Cengage Learning. All Rights Reserved. Table 11.1.2 - Phase Changes 26 © 2018 Cengage Learning. All Rights Reserved. Example Problem 11.1.1 - Investigate Phase Changes 27 © 2018 Cengage Learning. All Rights Reserved.  Enthalpy of Vaporization (ΔHvap) Energy required to convert one mole (1 g) of a liquid to vapor at a given temperature Also called heat of vaporization As the strength of IMFs in a series of liquids increases, ΔHvap values for the liquids increase – Values are much smaller than the energies of chemical bonds in molecules 28 © 2018 Cengage Learning. All Rights Reserved. Table 11.1.3 - Enthalpy of Vaporization for Some Common Substances 29 © 2018 Cengage Learning. All Rights Reserved. Table 11.1.1 - Properties of Solids, Liquids, and Gases 30 © 2018 Cengage Learning. All Rights Reserved. Example Problem 11.1.2 - Relate Enthalpy of Vaporization to IMFs 31 © 2018 Cengage Learning. All Rights Reserved. Intermolecular Forces and the Liquid State 11.2 Vapor Pressure 32 © 2018 Cengage Learning. All Rights Reserved.  Properties of Liquids Particles are in constant motion and are in close contact Almost incompressible IMFs are relevant – Liquids: Flow when poured out of a container Take on the shape of the container Evaporate to form vapor 33 © 2018 Cengage Learning. All Rights Reserved.  Vapor Pressure In order for a liquid to vaporize, sufficient energy must be available to overcome intermolecular forces – Process of vaporization is therefore endothermic When molecules of liquid are in the vapor state, they exert a vapor pressure 34 © 2018 Cengage Learning. All Rights Reserved.  Dynamic Equilibrium State in which rates of vaporization and condensation become equal Equilibrium vapor pressure – Pressure exerted by a vapor over a liquid in a closed container when a dynamic equilibrium is reached at a given temperature 35 © 2018 Cengage Learning. All Rights Reserved. Table 12.2.1 - Vapor Pressures (mm Hg) of Some Common Liquids 36 © 2018 Cengage Learning. All Rights Reserved.  Vapor Pressure and Temperature As temperature of a liquid increases: – More molecules have minimum KE needed to enter the gas phase – Number of molecules in the gas phase increases – Vapor pressure increases Comparing higher T to lower T, the area under the curve for the higher T is greater, so more molecules can escape to the vapor phase 37 © 2018 Cengage Learning. All Rights Reserved.  Vapor Pressure and IMFs Vapor pressure is: – Affected by the nature of a liquid – Dependent on the IMFs of a liquid Note that the two plots have identical shapes – Both liquids are at the same T – KE is dependent only on temperature 38 © 2018 Cengage Learning. All Rights Reserved.  Vapor Pressure1)and IMFs (continued More molecules of methanol are in the vapor state (greater shaded area) at this temperature IMFs of methanol must be less than that of water – This observation is in agreement with the enthalpy of vaporization values 39 © 2018 Cengage Learning. All Rights Reserved.  Vapor Pressure2)and IMFs (continued Inverse relationship – As the strength of the IMFs in a series of liquids increases, the vapor pressures of the liquids decrease 40 © 2018 Cengage Learning. All Rights Reserved.