Liquids. properties & structure

Transcription

1 Liquids properties & structure

2 Energetics of Vaporization when the high energy molecules are lost from the liquid, it lowers the average kinetic energy if energy is not drawn back into the liquid, its temperature will decrease therefore, vaporization is an endothermic process and condensation is an exothermic process vaporization requires input of energy to overcome the attractions between molecules 2

3 Vaporization molecules in the liquid are constantly in motion the average kinetic energy is proportional to the temperature however, some molecules have more kinetic energy than the average if these molecules are at the surface, they may have enough energy to overcome the attractive forces therefore the larger the surface area, the faster the rate of evaporation this will allow them to escape the liquid and become a vapor 3

4 Vaporization 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.

5 Distribution of Thermal Energy only a small fraction of the molecules in a liquid have enough energy to escape but, as the temperature increases, the fraction of the molecules with escape energy increases the higher the temperature, the faster the rate of evaporation 5

6 Condensation some molecules of the vapor will lose energy through molecular collisions the result will be that some of the molecules will get captured back into the liquid when they collide with it also some may stick and gather together to form droplets of liquid particularly on surrounding surfaces we call this process condensation 6

7 Evaporation vs. Condensation vaporization and condensation are opposite processes in an open container, the vapor molecules generally spread out faster than they can condense the net result is that the rate of vaporization is greater than the rate of condensation, and there is a net loss of liquid however, in a closed container, the vapor is not allowed to spread out indefinitely the net result in a closed container is that at some time the rates of vaporization and condensation will be equal 7

8 Effect of Intermolecular Attraction on Evaporation and Condensation the weaker the attractive forces between molecules, the less energy they will need to vaporize also, weaker attractive forces means that more energy will need to be removed from the vapor molecules before they can condense the net result will be more molecules in the vapor phase, and a liquid that evaporates faster the weaker the attractive forces, the faster the rate of evaporation liquids that evaporate easily are said to be volatile e.g., gasoline, fingernail polish remover liquids that do not evaporate easily are called nonvolatile e.g., motor oil 8

9 Heat of Vaporization the amount of heat energy required to vaporize one mole of the liquid is called the Heat of Vaporization, DH vap sometimes called the enthalpy of vaporization always endothermic, therefore DH vap is + somewhat temperature dependent DH condensation = -DH vaporization 9

10 Phase change Consider the heating curve below to answer the following questions: What is the melting point of this substance?.. What is the boling point of this substance?.. How do calculate the amount of heat required during phase change?

11 Vapor Pressure the pressure exerted by the vapor when it is in dynamic equilibrium with its liquid is called the vapor pressure remember using Dalton s Law of Partial Pressures to account for the pressure of the water vapor when collecting gases by water displacement? the weaker the attractive forces between the molecules, the more molecules will be in the vapor therefore, the weaker the attractive forces, the higher the vapor pressure the higher the vapor pressure, the more volatile the liquid Intermolecular forces Enthalpy of vaporization Boiling point Vapor Pressure Low Low Low High High High High Low 11

12 Dynamic Equilibrium 12

13 Vapor-Liquid Dynamic Equilibrium if the volume of the chamber is increased, that will decrease the pressure of the vapor inside at that point, there are fewer vapor molecules in a given volume, causing the rate of condensation to slow eventually enough liquid evaporates so that the rates of the condensation increases to the point where it is once again as fast as evaporation equilibrium is reestablished at this point, the vapor pressure will be the same as it was before Tro, Chemistry: A Molecular Approach 13

14 Example What happens to the vapor pressure of substance when its surface area is increased at constant pressure a. The vapor pressure increases a. The vapor pressure remains the same a. The vapor pressure decreases

15 Dynamic Equilibrium a system in dynamic equilibrium can respond to changes in the conditions when conditions change, the system shifts its position to relieve or reduce the effects of the change 15

16 Vapor Pressure vs. Temperature increasing the temperature increases the number of molecules able to escape the liquid the net result is that as the temperature increases, the vapor pressure increases small changes in temperature can make big changes in vapor pressure the rate of growth depends on strength of the intermolecular forces 16

17 Boiling Point when the temperature of a liquid reaches a point where its vapor pressure is the same as the external pressure, vapor bubbles can form anywhere in the liquid not just on the surface this phenomenon is what is called boiling and the temperature required to have the vapor pressure = external pressure is the boiling point the normal boiling point is the temperature at which the vapor pressure of the liquid = 1 atm the lower the external pressure, the lower the boiling point of the liquid 17

18 Heating Curve of a Liquid as you heat a liquid, its temperature increases linearly until it reaches the boiling point q = mass x C s x DT once the temperature reaches the boiling point, all the added heat goes into boiling the liquid the temperature stays constant once all the liquid has been turned into gas, the temperature can again start to rise 18

19 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.

20 Clausius-Clapeyron Equation 2-Point Form the equation below can be used with just two measurements of vapor pressure and temperature however, it generally gives less accurate results fewer data points will not give as accurate an average because there is less averaging out of the errors as with any other sets of measurements can also be used to predict the vapor pressure if you know the heat of vaporization and the normal boiling point remember: the vapor pressure at the normal boiling point is 760 torr ln P P 2 1 DH R vap 1 T 1 1 T 2 20

21 Example For water, H vap =44.0 kj/mol and its vapor pressure is 1.0 atm at C. What is vapor pressure of water at 25.0 C and at C? 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?

22 Phase Diagram Normal: Occurs at 1 atm. 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.

23 Example What is the normal freezing point of this substance? 2) What is the normal boiling point of this substance? 4) 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? 5) 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?