Phase Change (State Change): A change in physical form but not the chemical identity of a substance.

Transcription

1 CHM 123 Chapter Phase change, evaporation, vapor pressure, and boiling point Phase Change (State Change): A change in physical form but not the chemical identity of a substance. Heat (Enthalpy) of Fusion (ΔH fusion ): The amount of energy required to overcome enough intermolecular forces to convert a solid to a liquid. Heat (Enthalpy) of Vaporization (ΔH vap): The amount of energy required to overcome enough intermolecular forces to convert a liquid to a gas. Depends on the amount of attractive (intermolecular )forces between molecules Stronger the force the higher ΔH vap (or boiling point) For the solid liquid phase change in water ΔG = ΔH TΔS = 0 at equilibrium Or T = ΔH/ΔS Dang1

2 11.3 Evaporation, Vapor Pressure and Boiling point: Vapor Pressure: The partial pressure of a gas in equilibrium with a liquid at a constant temperature At equilibrium: rate (vap) = Rate (cond.) The conversion of a liquid to a vapor is visible when the liquid boils, but it occurs under other conditions as well. Molecules that enter the vapor phase in an open container can escape from the liquid and drift away until the liquid evaporates entirely, but molecules in close container are trapped. As more and more molecules pass from the liquid to the vapor, the chances increase that random motion will some of them to return occasionally to liquid. Ultimately, the number of molecules returning to liquid and the number escaping become equal, at which point a dynamic equilibrium exists. The higher the temperature and the lower the boiling point of the substance, the greater the fraction of molecules in the sample that have sufficient kinetic energy to break free from the surface of the liquid and escape into the vapor Intermolecular forces Enthalpy of Boiling point Vapor Pressure vaporization Low Low Low High High High High Low Clausius-Clapeyron Equation - this gives us a way of finding the heat of vaporization, the energy that must be supplied to vaporize a mole of molecules in the liquid state. - Alternatively, once the heat of vaporization and the vapor pressure at one temperature are known, the vapor pressure at one temperature are known, the vapor pressure of the liquid at any other temperature can be calculated. Dang2

3 Clausius-Clapeyron equation leads to a convenient way to measure the heat of vaporization in the laboratory. C is a constant, its value is the same at any two pressures and temperature. Therefore the Clausius-Clapeyron equation can be re-written as R = J/mol K For water, 25.0 C? H vap=44.0 kj/mol and its vapor pressure is 1.0 atm at C. What is vapor pressure of water at Ether has P vap = mmhg at 17.9 o C and a normal boiling point of 34.6 o C. What is the heat of vaporization, H vap for ether in kj/mol? Dang3

4 11.9 Phase diagram - summarizes the effect of temperature and pressure on a substance in a closed container. Every point in this diagram represents a possible combination of temperature and pressure for the system. The diagram is divided into three areas, which represent the solid, liquid, and gaseous states of the substance Normal Boiling Point: The temperature at which boiling occurs when there is exactly 1 atm of external pressure Normal Melting Point: The temperature at which melting occurs when there is exactly 1 atm of external pressure Critical Point: A combination of temperature and pressure beyond which a gas cannot be liquefied. Critical Temperature: The temperature beyond which a gas cannot be liquefied regardless of the pressure. Critical Pressure: The pressure beyond which a liquid cannot be vaporized regardless of the temperature. Supercritical Fluid: A state of matter beyond the critical point that is neither liquid nor gas. Triple Point: A point at which three phases coexist in equilibrium. Water Carbondioxide Dang4

5 Refer to the phase diagram below when answering the questions on this worksheet: 1) What is the normal freezing point of this substance? 2) What is the normal boiling point of this substance? 3) If I had a quantity of this substance at a pressure of 1.25 atm and a temperature of C and lowered the pressure to 0.25 atm, what phase transition(s) would occur? 4) At what temperature do the gas and liquid phases become indistinguishable from each other? 6) If I had a quantity of this substance at a pressure of 0.75 atm and a temperature of C, what phase change(s) would occur if I increased the temperature to C? At what temperature(s) would they occur? Dang5