Chapter 7 Liquids, Solids, and Phase Changes

Chapter 7: Liquids, Solids, and Phase Changes LIQUIDS - A form of matter that has a definite volume and takes the shape of its container. - A liquid expands and contracts only very slightly w/ a change in temperature and lacks the compressibility typical of gases. Surface Tension - Molecules at the surface of a liquid experience attractive forces downward, toward the inside of the liquid, and sideways, along the surface of the liquid. On the other hand, molecules in the center of the liquid experience uniformly distributed attractive forces. This imbalance of forces at the surface of a liquid results in a property called surface tension. PHASE EQUILIBRIUM Observation of this closed system would show an initial small drop in the water level, but after some time the level would be constant. At first, more energetic molecules near the surface are escaping into the gaseous phase faster than some of the gaseous water molecules are returning to the surface and possibly being caught by the attractive forces that will retain them in the liquid phase. After some time the rates of evaporation and condensation equalize, known as phase - equilibrium. In a closed system like this, when opposing changes are taking place at equal rates, the system is said to have dynamic equilibrium. At higher temperatures, since the number of molecules at higher energies increases, the number of molecules in the liquid phase will be reduced and the number of molecules in the gaseous phase will be increased. The rates of evaporation and condensation will again become equal. Le Chatelier's Principle: When a system at equilibrium s disturbed by the application of a stress (a change in temperature, pressure, or concentration), it reacts so as to minimize the stress and attain a new equilibrium position. Heat + 𝐻2 𝑂(l) ⇆ 𝐻2 𝑂(g) - The equation shifts to the right (any similar system that is endothermic shifts to the right when temperature is increased) until equilibrium is reestablished at the new temperature. The molecules in the vapor that are in equilibrium w/ the liquid at a given temperature exert a constant pressure, known as equilibrium vapor pressure. BOILING POINT - When a liquid is heated in an open container, the liquid and vapor are not in equilibrium and the vapor pressure increases until it becomes equal to the pressure above the liquid. - At this point the average kinetic energy of the molecules is such that they are rapidly converted from the liquid to the vapor phase within the liquid as well as the surface, the temperature at which this occurs is known as the boiling point. SOLIDS - Particles are fixed in rather definite positions and maintain definite shapes. - When heated at certain pressures, some solids vaporize directly w/o passing through the liquid phase, called sublimation. - The temperature at which atomic or molecular vibrations of a solid become so great that the particles break free from fixed positions and begin to slide freely over each other in a liquid state called the melting point. - The amount of energy required at the melting point temperature to cause the change of phase to occur is called the heat of fusion. Water Calorimetry Problems - A calorimeter is a container well insulated from outside sources of heat or cold so that most of its heat is contained in the vessel. From this graph, heat is being used at 0∘ 𝐶and 100∘ 𝐶to change the state of water, but not its temperature. One gram of ice at 0∘ 𝐶needs 80 calories or 3.34 × 102 joules to change the water at 0∘ 𝐶,called heat of fusion. Energy is being used at 100∘ 𝐶to change water to steam, not to change the temperature. One gram of water at 100∘ 𝐶absorbs 540 calories or 2.26 × 103 joules of heat to change 1 gram of steam at 100∘ 𝐶, called heat of vaporization. SOLUBILITY - Water is referred to as “the universal solvent” b/c of the number of common substances that dissolve in water. When substances are dissolved in water to the extent that no more will dissolve at that temperature, the solution is said to be saturated. The substance dissolved is called the solute and the dissolving medium is called a solvent. As a soluble solute is added to water at a given temperature, the solute will continue to go into solution until the water cannot dissolve anymore (sloped portion of graph). From that point on, the solute will fall to the bottom and does not dissolve, b/c an equilibrium has established b/w molecules leaving and entering the solid phase (line on the graph becomes horizontal). This solution is holding the maximum amount of dissolved solute in this amount of water and at 20∘ 𝐶, this is a saturated solution. A solution that contains less solute than a saturated solution under the existing conditions is described as an unsaturated solution.