Intermolecular Forces and Properties of Matter PDF

Chapter 5 Intermolecular Forces and Properties of Matter  Molecular Forces Intramolecular forces : Forces are within a molecule (polar and nonpolar covalent bond, ionic bond and metallic bond). Intermolecular forces occur between molecules and include: üVan Der Wa...

Chapter 5 Intermolecular Forces and Properties of Matter  Molecular Forces Intramolecular forces : Forces are within a molecule (polar and nonpolar covalent bond, ionic bond and metallic bond). Intermolecular forces occur between molecules and include: üVan Der Waals interactions: o Keesom forces (between polar molecules) o Debye forces (between a nonpolar molecule and a polar molecule) o London dispersion forces (between nonpolar molecules) üHydrogen bond Intramolecular forces (bonds) are stronger than intermolecular forces.  Polar and nonpolar molecules Reminder: Electronegativity is a measure of the tendency (ability) of an atom to attract a bonding pair of electrons towards itself. Nonpolar molecules occur when there is an equal sharing of electrons between their atoms. Examples of nonpolar molecules: § O2, H2, Cl2 , I2, Br2 Cl : Cl § CO2 There is an equal sharing of electrons. § Hydrocarbons: CH4 , C2H6 , C3H8 ,…. Cl2 is a nonpolar molecule. Polar molecules occur when atoms do not share electrons equally. A dipole forms, with part of the molecule carrying a slight positive charge and the other part carrying a slight negative charge. This happens when there is a difference between the electronegativities of the atoms or because of the molecular geometry. Examples of polar molecules: § HBr, HCl, HI, H2O, HF, NH3 § SO2 , O3 H :Cl Cl is more electronegative than H. The two atoms did not share electrons equally. HCl is a polar molecule. HCl molecule forms a dipole.  Keesom Forces (Dipole-dipole interactions) q Act between polar molecules. q Oppositely charged ends attract and like ends repel. q Moderate strength Interaction between permanent dipoles. Examples: HCl, HBr, HI Keesom forces increase when q the difference of electronegativity between the atoms forming the molecule increases. q the temperature decreases. q the distance between molecules decreases.  Debye Forces q Occur between a nonpolar molecule and a polar molecule. q The permanent dipole in the polar molecule induces an electric dipole in the nonpolar molecule when it comes extremely close. Examples: Debye forces between O2 and H2O Debye forces When O2 comes extremely close to H2O  London Dispersion Forces q Polarizability is the ease with which the electron distribution around an atom or molecule can be distorted ('How easy is it to form an induced dipole'). q Instantaneous dipole, that occurs accidentally in an atom or a molecule due to electrons movement, induces a similar dipole in a neighboring atom or molecule. q London forces occur in nonpolar molecules. q Weak forces Examples: Homonuclear diatomic molecules: H2, Cl2, I2, O2, F2, Br2, … Hydrocarbons: CH4, C3H8, C6H14 , …. Carbon dioxide CO2 London dispersion forces become stronger when q the polarizability increases. Polarizability increases when Ø The size of atoms or molecules increases. Ø The number of electrons increases. q the distance between molecules decreases. q the chain of carbon atoms in hydrocarbons becomes longer.  PRACTICE Q1: Which of the followings shows the highest London dispersion forces? a) C3H8 b) C4H10 c) C5H12 d) C6H14 Answer : C6H14 shows the highest London dispersion forces C3H8 < C4H10< C5H12 < C6H14 Increasing London forces Q2: According to the following order from the lowest to the highest atomic size: F < Cl < Br < I explain why F2 and Cl2 are gases, Br2 is a liquid and I2 is a solid at room temperature. Answer : F2, Cl2, Br2 and I2 are nonpolar molecules, and they show London dispersion forces. As the size increased, London forces increased: Increased London dispersion forces  Hydrogen Bonding q Hydrogen on one molecule attached to a highly electronegative atom (N, O, or F) and either (N, O, or F) on another molecule. q Strong dipole-dipole force. q Occurs between Polar molecules. Examples: H2O, HF, NH3, CH3COOH Ø Order from the strongest to the weakest intermolecular forces: Hydrogen bond > Keesom force >Debye force > London force  Properties of liquids Vapor pressure q The condition in which two opposing processes are occurring simultaneously at equal rates in a closed system is called dynamic equilibrium. qThe pressure exerted by the vapor on the surface of the liquid at equilibrium is called the vapor pressure. In a closed system, when the rate at which the liquid is entering the gas phase equals the rate at which the vapor is returning to the liquid phase, the system is at equilibrium. After this time, the liquid level will remain constant. The pressure exerted by the vapor on the surface of the liquid at equilibrium is called the vapor pressure. The vapor pressure of a liquid increases when q the intermolecluar forces decreases. q the temperature increases.  Boiling Point The temperature at which the vapor pressure equals the external pressure (atmospheric pressure) is called the boiling point. This is the point where bubbles of vapor form within the liquid. q The boiling point of a liquid at 1 atm pressure is the normal boiling point. qThe boiling point increases with increasing external pressure. qThe molecules that have strong intermolecular forces have high boiling point.  PRACTICE Which of the compounds hexane (C6H14) and propane (C3H8) has the highest boiling point ? Which of these two compounds has the lowest vapor pressure ? Explain your answer. Answer: Hexane C6H14 has a longer chain than propane C3H8. Therefore, C6H14 is held by higher attractive molecular forces via London forces than C3H8. Hexane C6H14 has lower vapor pressure than propane C3H8. Hexane C6H14 has higher boiling point than propane C3H8. The substance having the highest boiling point has the lowest vapor pressure and conversely, the substance having the lowest boiling point has the highest vapor pressure.  Viscosity of a liquid q Viscosity is the resistance to flow of a liquid. When the viscosity of a liquid increases, the liquid flows more slowly. The viscosity of honey is higher than the viscosity of water. Honey flows more slowly than water. Viscosity of a liquid increases when üthe intermolecular forces increases üthe temperature decreases  Phase Changes Phase changes are transformations from one phase to another. The energy required to go from one state to another state is called enthalpy change (DH). melting, vaporization and sublimation are endothermic processes (need energy, DH>0). freezing, condensation and deposition are exothermic processes (release energy, DH