Change of Phase Chapter 23

Evaporation Evaporation – a change of phase from liquid to gas that takes place at the surface of a liquid The molecules at the surface of a liquid may gain enough kinetic energy by being bumped by other molecules to break free of the liquid, comprising a vapor The average kinetic energy of the molecules left behind is lowered, therefore evaporation is a cooling process

Evaporation

Condensation Condensation – the changing of phase from a gas to a liquid Vapor molecules can run into slower-moving particles of a cool surface and slow themselves down enough to reform as a liquid The air always contains some water vapor Saturated – at any given temperature, there is a limit to the amount of water vapor that can be in the air Relative Humidity – indicates how much water vapor is in the air, compared with the limit for that temperature At a relative humidity of 100%, the air is saturated Fog is formed when water vapors stick together as they cool, moist air moves in close to the ground and water vapor condenses out to form a cloud

Condensation

Evaporation and Condensation Rates Evaporation and Condensation are occurring continuously at equal rates The molecules and energy leaving the liquid’s surface by evaporation are counteracted by as many molecules and as much energy returning by condensation The liquid is in equilibrium – in a state of balance – since evaporation and condensation have canceling effects If evaporation exceeds condensation, a liquid is cooled If condensation exceeds evaporation, a liquid is warmed

Boiling Boiling – change of phase which occurs beneath the surface from liquid to gas The pressure of the vapor within the bubbles of a boiling liquid must be great enough to resist the pressure of the surrounding water As atmospheric pressure is increased, the molecules in a vapor are required to move faster to exert increased pressure within the bubble, thereby increasing the boiling point of a liquid Boiling, like evaporation, is a cooling process

Freezing Freezing – the change in phase from a liquid to a solid When energy is extracted from a liquid, it freezes If sugar or salt is dissolved in water, the freezing temperature is lowered; these molecules get in the way of the formation of ice crystals The ocean, with its high salinity, rarely freezes, even on the top

Regelation Regelation – the phenomenon of melting under pressure and freezing again when the pressure is reduced (specific to water) This is do to the very open structure of ice crystals, the application of pressure lowers the melting point

Regelation

Energy and Changes of Phase Energy must be put into a substance to change from a solid all the way to a gas, and energy must be extracted to change a substances phase from a gas to a solid The phase change sequence is reversible! You can boil an ice cube, and then turn it back into an ice cube While a substance changes phase, its temperature does not change Much more energy is given off when water vapor condenses than when an equal mass of water freezes

Energy and Changes of Phase

Thermodynamics Chapter 24

Absolute Zero As the thermal motion approaches zero, the kinetic energy of the atoms approaches zero, and the temperature of the substance approaches a lower limit Absolute Zero – no more energy can be extracted from a substance and no further lowering of its temperature is possible Absolute zero corresponds with zero degrees on the Kelvin scale (0 K) Unlike the Celsius scale, there are no negative numbers on the thermodynamic scale (Kelvin)

Absolute Zero

First Law of Thermodynamics When the law of energy conservation is applied to thermal systems, we call it the first law of thermodynamics: Whenever heat is added to a system, it transforms to an equal amount of some other form of energy More specifically the first law states: Heat added = (increase in internal energy) + (external work done by the system) Adding heat is not the only way to increase the internal energy of a system Changes in internal energy are equal to the work done on or by the system

Change in air temperature ~ pressure change Adiabatic Processes Adiabatic – the process of compression or expansion of a gas so that no heat enters or leaves a system Adiabatic changes of volume can be achieved by performing the process rapidly so that heat has little time to enter or leave, or by thermally insulating a system from its surroundings Adiabatic form of the first law: Change in air temperature ~ pressure change Adiabatic processes are happening all the time in the atmosphere and in your car

Adiabatic Cooling

Second Law of Thermodynamics The second law of thermodynamics tells us the direction of heat flow in natural processes: Heat will never of itself flow from a cold object to a hot object. Heat only flows one way, from hot to cold

Heat Engines and the 2nd Law Heat Engine – any device that changes internal energy into mechanical work Mechanical work can be obtained only when heat flows from a high temperature to a low temperature In every heat engine only some of the heat is transformed into work Heat flows out of a high-temperature reservoir into a low-temperature reservoir Every heat engine will (1) absorb heat from a reservoir of higher temperature, increasing its internal energy, (2) convert some of its energy into mechanical work, and (3) expel the remaining energy as heat into some lower-temperature reservoir (sink)

Heat Engines and the 2nd Law When work is done by a heat engine running between two temperatures, T hot and T cold, only some of the input heat at T hot can be converted to work, and the rest is expelled as heat at T cold. Carnot efficiency – the ideal efficiency of a heat engine: Ideal efficiency = (T hot - T cold )/ T hot The greater the temperature difference between the hot and cold reservoirs, the greater the efficiency In practice , friction is always present in all heat engines, and efficiency is always less than ideal

Heat Engines

Order Tends to Disorder Organized energy in the form of electricity that goes into electric lights degenerates to heat energy and has no further use. The quality of energy is lowered with each transformation Energy of an organized form tends to disorganized forms Natural systems tend to proceed toward a state of greater disorder. Disordered energy can only be transformed to ordered energy only at the expense of some organizational effort or work input

Entropy Entropy – the measure of the amount of disorder in a system Whenever a physical system is allowed to distribute its energy freely, it always does so in a manner such that entropy increases while the available energy of the system for doing work decreases Entropy will normally increase for physical systems The first law of thermodynamics is a universal law of nature for which no exceptions have been observed, while the second law is a probability statement

Entropy

Assignment Read Chapters 23 & 24 (pg. 339-367) Do Chapter 23 #26-47 (pg. 352-353); Appendix F #1-9 (pg. 681-682) Do Chapter 24 #30-43 (pg. 369-370); Appendix F #1-3 (pg. 682)