PHASE CHANGES. * melting * boiling * sublimation. * freezing * condensation * deposition. vs.

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1 PHASE CHANGES endothermic * melting * boiling * sublimation vs. vs. exothermic * freezing * condensation * deposition

2 H enthalpy: heat content of a system under constant pressure HEATING CURVE: Where is this energy going if the temperature doesn't rise or lower during a phase change?

3 "STICKY" FORCES OF ATTRACTION BETWEEN MOLECULES ARE RESPONSIBLE FOR THEIR MELTING AND BOILING POINTS These are called INTERMOLECULAR FORCES (IMFs) INTERMOLECULAR FORCES: attractive forces between molecules. * weaker than intramolecular forces * determine phase of substance * hold matter together * counteracts KINETIC ENERGY High kinetic energy = IMFs are completely destroyed medium kinetic energy = IMFs are losing the battle lower kinetic energy = IMFs have enough strength to keep molecules locked into position

4 Do not confuse IMFs with intramolecular forces (bonding forces). Bonding Forces There are THREE types of IMFs: 1. London dispersion forces 2. Dipole-dipole 3. H-bonding

5 1. Dispersion forces (aka London forces) weak forces that result from temporary shifts in the density of electrons in electron clouds. *electrons in the cloud are in constant motion * electron density around each nucleus can momentarily be greater in one region of the cloud = a temporary dipole. * if they get closer, dispersion forces take place *exist between all particles *become stronger as the size of the particles increase (more electrons means stronger temporary dipole

6 2. Dipole dipole forces attractions between oppositely charged regions of polar molecules. * polar molecules contain permanent dipoles (parts are always partially positive and partially negative) * small polar molecules with large dipoles have stronger forces than dispersion BUT in many polar molecules dispersion forces dominate dipole dipole forces.

7 3. Hydrogen Bonds a special type of dipole dipole bond that occurs between molecules containing a hydrogen atom bonded to a small, highly electronegative atom with at least one lone pair of electrons. must be bonded with F, O, N dominates the other two forces in strength The presence of hydrogen bonds explain why water is a liquid at room temperature, while compounds of comparable masses are gases.

8 Do not confuse INTERmolecular forces with INTRAmolecular bonds: The attractive forces that hold particles together in ionic, covalent, and metallic bonds are called intramolecular forces. intra means within

9 When energy is added or removed, one phase can change into another Two variables combine to control the phase of a substance: Temperature & Pressure Phase Diagram: a graph of pressure verses temperature that shows in which phase a substance exists under different conditions Point A is the triple point. This is the point on the graph where all lines meet. This represents the temperature and pressure at which all 3 phases of a substance coexist. All 6 phase changes can occur at the triple point. Point B is the critical point. This indicates the critical temperature and pressure above which water can not exist as a liquid. *Phase Diagrams are different for each substance because normal boiling and freezing points of substances are different.

10 How Do We Describe the Behavior of Liquids at the Molecular Level? *Take the shape of their container * definite volume * relatively incompressible * more dense than gas * positions are random and constantly changing = fluid Hydraulic braking system in a car: A. Surface tension * directly related to the strength of the IMFs Molecules at the surface have greater attractions to the inside of the liquid compared to the atmosphere. These molecules are then pulled inward toward the center. This creates an "elastic skin".

11 B. Vapor Pressure * evaporation: when SURFACE molecules of a liquid are moving fast enough to escape into the gaseous space. Saturated vapor pressure

12 c. Adhesion vs. cohesion * capillary rise:

13 D. Viscosity: resistance to flow

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