Chemistry Exam 3 Preparation Chapter 7 Learning Objectives • Characterize the different states of matter Gas – Adopt volume and shape of their container. Ideal Gas Law Volume of gas varies inversely with pressure – Boyle’s Law Volume of gas is directly proportional to temp. – Charles’ Law Each component of a gaseous mixture exerts it’s own pressure; Partial Pressure: Molar Concentration: Filler Dalton’s Law of Partial Pressures: Liquid – Energy of attraction comparable to thermal energy. Maintain volume but adopt shape of container. Viscosity is the resistance of liquids to flow. Surface Tension is the energy required to increase the surface area of a liquid by a fixed amount. Units are in J/m2. Increases with increasing intermolecular forces. Adhesion and cohesion are present in liquids. Surface tension and viscosity come from cohesion, which is the force that causes beads of water. Shown in Fig 7.12 Solid – Shape, volume independent of container. Strong intermolecular force. • Describe the behavior of gases The behavior of ideal gases can be explained by the kinetic molecular theory, also gases possess weak intermolecular forces • Explain the postulates of Kinetic-Molecular Theory• Define the absolute temperature scale Measure in Kelvin • Discuss the meaning of the term thermal energy Thermal energy: The average kinetic energy of the molecules in a system is often referred to as the thermal energy the system, or the average energy. When the thermal energy exceeds the potential energy that holds the molecules in a crystal, the crystal melts. Thermal energy drives chemical reactions. • Apply some of the general concepts of KMT to phase equilibria Thermal energy of a system depends on it’s temperature • Explain the origin of intermolecular forces Exist because there is an attraction between regions of opposite charge on molecules. Polarity among molecules leads to molecular dipoles. • Show how to determine if a molecule has a permanent dipole If the centers of positive and negative charge do not coincide, two poles are produced and the molecule has a permanent dipole. The magnitude of the dipole depends upon both the size and the separation of the two centers of charge; if the centers of charge coincide, their separation is zero and they have no permanent dipole. Molecules with permanent dipoles are said to be a polar, while those that do not have permanent dipoles are said to be nonpolar. • Describe the unique properties of water that result from hydrogen bonding• Compare thermal energy to the energy of interaction for a solid, liquid & gas -Substances are gases when their thermal energy is much greater than their energy of interaction -They are liquids when their thermal energy and their energy of interaction are comparable -They are solids when their energy of interaction is much greater than their thermal energy • Describe some properties of liquids and relate them to the strengths of the intermolecular interactions If thermal energy is relatively low compared to the energy of interaction, the molecules cannot readily break the interactions, so the flow is retarded and the liquid is said to be viscous. Thus, viscosity decreases as the liquid is heated. For example, oil is quite viscous when cold, but flows easily when hot. The energy required to increase the surface area of a liquid by a fixed amount is called the surface tension of the liquid. The units of surface tension are J/m2. Surface tension increases with increasing intermolecular forces. Molecules on the surface experience attractive forces from the bulk but not from above, so there is a net inward force, which causes the liquid to minimize its surface area. Molecules in the bulk of a liquid experience forces in all directions, so there is no net force on the molecule. Forces between like molecules are called cohesive, while those between unlike molecules are said to be adhesive. • Describe the relative energies of the three states of matter Solids have the lowest potential energy, liquid’s is slightly higher, gas has the highest. As heat is added to a substance, the kinetic energy will increase until it reaches the point of increasing potential energy. When potential energy is increased, a change of matter occurs. • Describe the various changes in state at a molecular level Solid-Liquid Transition Melting/freezing point can reach a dynamic equilibrium called a melt where it stays either frozen/liquid as long as heat stays consistent Heat of Fusion is the heat required to melt a solid.Substances with high melting points interact strongly with one another. The pressure on an equilibrium mixture involving different states always moves toward the more dense state, ex. Ice skating Liquid-Vapor Transition Not all molecules in liquid have the same kinetic energy -When a small fraction of molecules have enough energy to escape their intermolecular interactions, they enter the gas phase and evaporate -When colliding molecules don’t have sufficient energy to escape their interaction, so they stick to each other in condensation -The pressure of the gas where liquid and vapor reach their dynamic equilibrium is called the vapor pressure -Compounds with stronger intermolecular interactions have lower vapor pressures than those that interact only weakly because the stronger interactions bond again to condense more easily -Bubbles form at the temperature where the vapor pressure of the liquid equals the external pressure, or the boiling point. Normal boiling point is at 1 atm -Stronger intermolecular interactions equal higher normal boiling points -Hydrogen bonding affects boiling points -Molar mass affects boiling points -Dispersion forces affect boiling point -Heat of Vaporization is the energy required to convert a liquid to a vapor at a given temperature. -Critical temperature is where liquid can no longer exist, critical pressure is the pressure required to liquefy it at the critical temp. -Critical Point is defined by the critical temperature and pressure. Beyond the critical point, substance behaves like gas but has the density of a liquid, forming a supercritical fluid Solid-Vapor Transition -Sublimation: the solid to vapor transition -Deposition: The vapor to solid transition -Heat of Sublimation: The energy required to convert a solid to the gas phase. • Demonstrate the use of a phase diagram and how it is