Chemistry 101: Intermolecular Forces of Attraction PDF

C H E M I S T R Y 1 0 1 Have you ever wondered why there are droplets of water on the outer side of a cold glass? GENERAL CHEMISTRY CLASS 2 CHAPTER 1 Intermolecular Forces in Liquids and Solids 1. Kinetic molecular model of liquids and solids...

C H E M I S T R Y 1 0 1 Have you ever wondered why there are droplets of water on the outer side of a cold glass? GENERAL CHEMISTRY CLASS 2 CHAPTER 1 Intermolecular Forces in Liquids and Solids 1. Kinetic molecular model of liquids and solids 2. Intermolecular Forces Dipole-dipole forces Ion-dipole forces Dispersion forces Hydrogen bonds 3. Properties of liquids and IMF Surface Tension Viscosity Vapor pressure boiling point Molar heat of vaporization 4. Structure and Properties of Water Objectives At the end of the lesson students are able to; 1. explain the kinetic molecular model of solids and liquids, 2. differentiate the characteristic of solids and liquids, 3. analyse the intermolecular forces of attraction exist between particles of substances, and 4. appreciate the chemistry of things around us. Concepts in a Box Kinetic Molecular Model Explain the properties of Liquids Solids can be Have particles held classified as together by Surface Tension Viscosity Vapor pressure Intermolecular Forces Amorphous Crystalline Boiling point Dipole-dipole forces Ionic Heat of Ion-dipole forces Molecular vaporization Dispersion forces Metallic Hydrogen bonds Network Group 8A LESSON 1: The Kinetic Molecular Models of liquids and solids Intermolecular Forces of Attraction in Matter  Less closely Arrangement Closely packed packed than in Very far apart of Particles orderly disorderly Kinetic Molecular Theory solid disorderly Particles Kinetic Energy Particles move vibrate and Particles slide of Particles about at great rotate about over each other speed fixed position Particle Motion Very low low High Attractive forces between Very strong strong Very weak particles Dipole-dipole forces Ion-dipole forces Intermolecular Dispersion forces forces Hydrogen bonds Kinetic Molecular Theory Kinetic- Kinetos (Greek word) which means “Movable” It explains the properties of solids, liquids, and gas in terms of intermolecular forces of attraction and the kinetic energy of the individual particles. What do you observe?  Intermolecular forces < kinetic energy of molecules Intermolecular forces > kinetic energy of molecules  Properties Have definite Shape Have no definite Shape Do not flow Flow and take the shape of Virtually incompressible their container Expand when heated, but Very difficult to compress to a lesser extent than Slightly expand when liquids and gases. heated.  Particles Strongly attracted to each Weakly attracted to each other other; break their Vibrate a little, but not interaction easily much compared to liquids Vibrate and move more and gases than those in a solid Vibrate faster when Move faster when heated heated Intermolecular Forces of Attraction Strong intramolecular attraction (Covalent bond) This force holds the molecules together which can be attractive or repulsive. Weak intermolecular attraction The attractions between molecules are not nearly as strong as the intramolecular attractions (bonds) that hold compounds together. Many physical properties reflect intermolecular forces, like boiling points, melting points, viscosity, surface tension, and capillary action. Types of Intermolecular Force Weakest to strongest forces: dispersion forces (or London dispersion forces) dipole–dipole forces hydrogen bonding (a special dipole–dipole force) ion–dipole forces Note: The first two types are also referred to collectively as van der Waals forces. Dipole–Dipole Interactions Polar molecules have a more positive and a more negative end–a dipole (two poles, δ+ and δ−). The oppositely charged ends attract each other. Strong intramolecular attraction (Covalent bond) Weak intermolecular attraction For molecules of approximately equal mass and size, the more polar the molecule, the higher its boiling point. Hydrogen Bonding The dipole–dipole interactions experienced when H is bonded to N, O, or F are unusually strong. A hydrogen bond is an attraction between a hydrogen atom attached to a highly electronegative atom and a nearby small electronegative atom in another molecule or chemical group. What Forms Hydrogen Bonds? Hydrogen bonding arises in part from the high electronegativity of nitrogen, oxygen, and fluorine. These atoms interact with a nearly bare nucleus (which contains one proton).  Hydrogen bonding in ammonia contains highly electronegative atom nitrogen linked to hydrogen atoms Hydrogen bonding in Fluoride Fluorine, having the highest value of electronegativity forms the strongest hydrogen bond. Hydrogen Bonding in Water A water molecule contains a highly electronegative oxygen atom linked to the hydrogen atom. Oxygen atom attracts the shared pair of electrons more and this end of the molecule becomes negative whereas the hydrogen atoms become positive. Ion–Dipole Interactions Ion–dipole interactions are found in solutions of ions. The strength of these forces is what makes it possible for ionic substances to dissolve in polar solvents. Mg⁺ ---- K⁺ Dispersion Forces The figure below shows how a nonpolar particle (in this case a helium atom) can be temporarily polarized to allow dispersion force to form. The tendency of an electron cloud to distort is 0 – 0.5 EN called its polarizability. Summarizing Intermolecular Forces Factors Which Affect Amount of Dispersion Force in a Molecule number of electrons in an atom (more electrons, more dispersion force) size of atom or molecule/molecular weight shape of molecules with similar masses (more compact, less dispersion force) Polarizability & Boiling Point If something is easier to polarize, it has a lower boiling point. Remember: This means less intermolecular force (smaller molecule: lower molecular weight, fewer electrons). LESSON 2: Intermolecular Forces and Properties of Liquid Liquid Properties Affected by Intermolecular Forces boiling point viscosity surface tension Heat of vaporization capillary action Cohesion and Adhesion Intermolecular forces that bind similar molecules to one another are called cohesive forces. Intermolecular forces that bind a substance to a surface are called adhesive forces. These forces are important in capillary action.  Capillary Action The rise of liquids up narrow tubes is called capillary action. Cohesive forces attract the liquid to itself. Adhesive forces attract the liquid to the wall of the tube. Water has stronger adhesive forces with glass; mercury has stronger cohesive forces with itself. Surface Tension Water acts as if it has a “skin” on it due to extra inward forces on its surface. Those forces are called the surface tension. Arrange the following from quickly flowing liquid to liquid that moves more slowly:  Viscosity Resistance of a liquid to flow is called viscosity. It is related to the ease with which molecules can move past each other. Viscosity increases with stronger intermolecular forces and decreases with higher temperature.  Boiling When a liquid is heated to a temperature when all molecules have enough kinetic energy to vaporize, the liquid begins to boil. Boiling happens at a boiling point, Vapor forms within the liquid. What is in the bubble? Boiling At the boiling point, bubbles of vapor form within the liquid. The pressure within the bubble is due to the vapor pressure. When vapor pressure in the bubble equals external pressure, the bubble rises to the surface of the liquid and bursts.  Vapor Pressure Vapor pressure is a pressure of vapor particles exerted on the walls of a container. An increase in the temperature of a liquid increases the vapor pressure. Analyze the diagram Boiling Point (temp. of boiling) The boiling point is the temperature at which the vapor pressure of a liquid is equal to the external pressure.  Normal Boiling Point Normal boiling point is a boiling point when the external/atmospheric pressure is 1 atm. Vapor Pressure vs. Temperature Boiling Point Summary Boiling Point depends on Strength of intermolecular forces in a substance External/atmos pheric pressure Boiling: analyze the process from the point of IMF Liquids have quite strong IMFs Gasses have no IMFs Particles of liquid should break IMFs to move from the liquid phase to the gas phase. Energy is required to break IMFs. Boiling Boiling is a process that takes time and energy. Why do we need to supply energy all the time to support boiling? Try to think about the process considering kinetic energies of particles in a liquid. When particles with higher kinetic energy leave the liquid the temperature of the liquid decreases, constant supply of energy is needed to break IMFs and support boiling temperature. Boiling is an endothermic process. Vaporization  Vapor Pressure At any temperature, some liquid molecules have enough energy to escape the surface and become a gas. As the temperature rises, the fraction of molecules that have enough energy to break free increases. Vapor Pressure As more molecules escape the liquid, the pressure they exert increases. The liquid and vapor reach a state of dynamic equilibrium: liquid molecules evaporate and vapor molecules condense at the same rate. Vapor Pressure The boiling point of a liquid is the temperature at which its vapor pressure equals atmospheric pressure. The normal boiling point is the temperature at which its vapor pressure is 760 torr. Vapor Pressure The natural log of the vapor pressure of a liquid is inversely proportional to its temperature. This relationship is quantified in the Clausius–Clapeyron equation.