Presentation on theme: "February 20, 2008 Objective: To understand heat transfer and the causes of phase changes Check-in Frayer Models CW - Chapter 10 Notes HW - Study for a."— Presentation transcript:
February 20, 2008 Objective: To understand heat transfer and the causes of phase changes Check-in Frayer Models CW - Chapter 10 Notes HW - Study for a super short Conduction vs. Convection vs. Radiation Quiz Monday! 1
Ch 10 Heat Transfer & Change of Phase These notes are available on the Moodle!
10.1 Conduction Conduction- transfer of heat E by the collisions between particles in a substance. ( especially solids!) Conductor- solids whose electrons can move freely. Insulator- molecules that do not have mobile electrons. POOR CONDUCTOR.
EXAMPLES GOOD CONDUCTORS (Poor insulator) Silver Copper Aluminum Iron INSULATORS (Poor conductor) Wood Wool Paper Cork Plastic Air
10.2 Convection Convection- transfer of heat E by the motion of fluids. Fluid – any substance with molecules/atoms that are free to flow past each other. Both gases and liquids are fluids!
Convection Liquid- molecules at bottom move faster, spread apart & less dense. Buoyed up, denser liquid moves to bottom. Gases- Warm air expands, rises, and then cools. Stirs the atmosphere = wind AIR heats up as compressed
10.3 Radiation Transfer of Heat by means of electromagnetic radiation. *** DO NOT CONFUSE W/ RADIATION INVOLVING ATOMIC NUCLEI*** Similar to when you drop a stone in a pond, or shake the end of a rope Electromagnetic radiation is caused by a vibrating charge (electron(s) in atoms on the sun). Ex: radio waves Infrared waves light waves (light bulb)
Wavelenth (λ) of radiation related to frequency & Temperature LONG WAVE = low frequency = cool SHORT WAVE = high frequency = hot
Emission of Radiant E All objects greater than 0 K emit some radiant energy. Frequency relative to temperature f ~ T Ex: sun,fireplace, lamp frequency, λ, temp you feel high freq infrared waves as heat
Absorption of radiant energy Objects emit & absorb radiant energy Good emitters are good absorbers Poor emitters are poor absorbers Dark objects absorb & emit more radiant Energy Ex: black vs white container of water in sun Black warms faster & cools faster.
Temperature vs. Intermolecular Forces Intermolecular Forces – attractive forces that exist between atoms/molecules of a substance. Different strengths for different substances. Determines what phase the substance will be at certain temperatures. Example: solids at 25°C have stronger IMF that liquids, than gases do.
10.5 Evaporation A change of phase from liquid to gas. Liquid – molecules constantly in random motion & gain KE. as high E molecules break free of surface, lower KE left in liquid (TEMP ). EVAPORATION IS A COOLING PROCESS. Ex: sweat. evaporation cools skin.
Boiling Point: Part 1 Boiling is evaporation within a liquid Evaporation occurs beneath surface of liquid when boiling. Bubbles (vapor) forms in liquid closest to the heat source Bubbles only form when pressure equal to the pressure of the surroundings (atmosphere).
Boiling Point: Part 2 Boiling is evaporation at the surface of a liquid Evaporation is always occurring at the surface of a liquid, regardless of temperature. The vapor phase that exists above a liquid exerts a pressure called Vapor Pressure. When vapor molecules have enough KE to exert a pressure equal to that of the atmosphere then the substance is said to be boiling.
BP vs. Atmospheric Pressure atmospheric pressure requires a temp to boil. Vapor molecules require more energy so that they can exert a pressure = to that of the atmosphere altitudes = pressure So in Denver, CO WATER BOILS AT LOWER TEMP. (95 C) … takes longer to cook.
10.6 Condensation A change of phase from a gas to a liquid. Molecules at liquids surface do not have enough KE to overcome IMF and are attracted to liquid Collisions with the surface transfer KE. KE is absorbed by the liquid. CONDENSATION IS A WARMING PROCESS
9.8 Thermal Expansion Molecules jiggle and move farther apart when heated. Most substances expand when heated & contract when cooled. i.e. solids are more dense then liquids, then gases for most substances. Exception: Water contracts until 4 C, then forms crystalline structure w/ open spaces and expands.
Expansion of H 2 O Ice :Hexagonal, open structure Ex: Melt ice cube, H 2 O less volume
10.8 Melting & freezing- opposite directions in phases changes. Melting – substance changes from solid liquid. When heated, molecules gain KE, vibrate more and attractive force between molecules is overcome. SOLID MELTS
Freezing - liquid changes to solid As E is removed from liquid, molecular motion slows. IMF take over & molecules bind together and vibrate in fixed positions. H 2 O w/o impurities - freezes at 0 C. w/ impurities - < 0 C foreign molecules change the vapor pressure to less than that of pure water.
SUBLIMATION Molecules of a solid are transformed directly into gas phase. BP for a substance describes when the vapor molecules exert a pressure equal to that of the atmosphere. Sublimation occurs when vapor molecules at the surface of a solid exert a pressure greater than that of the atmosphere at room temperature.
Phase Diagrams Under different conditions of temperature and pressure substances will exist at either solid, liquid or a gas.
Phase changes TRANSFER OF HEAT occurs whenever matter changes phases. solidliquidgasplasma Gaining thermal energy Plasmagasliquidsolid Losing thermal energy
10.9 Energy & Phase Change 1 st Law defines parameters for heat transfer under conditions of constant pressure (our atmosphere). – Specifically, the gain or loss of thermal energy equals the amount of heat transferred. Enthalpy (H) – the term used to describe heat absorbed or released under constant pressure.
Energy is needed for a change of phase ΔH system = ΔH surrounding or… Energy lost by the system must equal energy gained by the surroundings. Energy may cause an increase in Temp OR a phase change!
Changing Phase Melting Point Boiling Point Changes in state require energy! Add KE and Temp rises At BP/MP temp stops rising and E used to break IMF
Heat of fusion Amount of E needed to change Solid liquid. Heat of fusion of H 2 O = 335 J/ g H 2 O
Heat of vaporization Amount of E required to change a substance from liquid gas heat of vaporization of H 2 O = 2255 J/g H 2 O