Oceans are filled with??? The Water Molecule. Hydrogen Bonding between Polar Molecules: WATER!!!!! Let s understand water then. 1

Transcription

1 Oceans are filled with??? WATER!!!!! Let s understand water then. 1 The Water Molecule V -shaped molecule: 105º angle between H nuclei Due to covalent bonds involving Oxygen s p-orbitals P-orbitals are 90º apart, but the hydrogen nuclei repel each other (Born repulsion) resulting in 105º between hydrogen nuclei Polar molecule: Electronegativity of Oxygen pulls electrons away from Hydrogen nuclei Electron cloud around Oxygen results in negative charge at the Oxygen side and positive charge at the Hydrogen side of the molecule Polar property leads to hydrogen bonding 2 Hydrogen Bonding between Polar Molecules: Hydrogen bonds positive H ends attract negative O ends into a weak bond Water molecules thus stick together The polar water molecule and resulting hydrogen bonds are responsible for ALL of water s REMARKABLE properties! E.g. Surface Tension Highest among common liquids Water forms drops! Other strange properties resulting from Hydrogen Bonds, large heat capacity, large latent heats of fusion and vaporization, Strange Density variations 3 1

2 Freezing & Density: Ice floats! Freezing = removal of heat, Slows vibrations (lowers Temperature) Structure becomes compact - increase density Read pp Max density 1g/cc at 4ºC Below 4ºC rigid framework develops (0.99g/cc 0ºC) Ice - expanded structure: 105º to 109º 0.92 g/cc at 0ºC 4 Phases of Water: Boiling point and freezing point are in normal range of temperature! Thus, water is present nearly everywhere No other substance is present in all phases on Earth s surface! 5 Phase Changes - change P and T (condensation, sublimation, boiling) Heat & Phase Changes: The Calorie is the unit of measure of Heat Energy Definition of a Calorie: Heat required to raise 1 gram of H 2 O by 1ºC The more water you have, the more heat energy is required to raise T! Temperature is not heat! Heat = Energy. This energy causes molecules to vibrate. The vigor of the vibration is dependent on the number of molecules present. The more heat you add, the faster the molecules will vibrate. The more molecules you have vibrating, the more heat energy in the system. Temp ~ how fast molecules are vibrating Temperature does not depend on or account for the number of molecules. 6 2

3 Which contains more heat? A. Candle flame B. Warm Bath 7 Heat Capacity = amount of heat required to raise Temperature of a substance 1ºC (this is a definition!) Heat Capacity of Water = 1 calorie/mole/ºc (notice this is the definition of calorie) Highest of all common solids & liquids! Hydrogen boding of water causes this LARGE heat capacity need lots of heat energy to vibrate all those hydrogen bonds and raise temperature! Note to science majors: the heat capacity per gram of substance is the Specific Heat Heat Capacity of water > most solids (20 x that of quartz) Few liquids have greater Heat Capacity - e.g. ammonia Examples: water vs. ethyl alcohol Water vs. Ethyl alcohol Ethyl alcohol s Heat Capacity = 1/3 calorie Quartz sand (beach) vs. water Heat Capacity of quartz = 0.2 cal 8 Big Point about Heat Capacity: Water has very high Heat Capacity. Thus it takes a great amount of heat exchange to change water s temperature. So, oceans RESISTS changes in Temperature while absorbing or releasing heat from the Sun. Compared to continents, Oceans require more heat to change temperature! Therefore, little to no wide diurnal or seasonal variations in ocean temperature. THUS vast amounts of Heat are moved around the globe as oceans circulate! Which is to say Oceans store & transport lots of heat! Thus, oceans control Earth s heat budget 9 3

4 Heat flow and how oceans regulate climate: Clausius taught us that heat flows down a temperature gradient Analogy: balls roll down hill under force of gravity Heat flows from hot to cold under the force of entropy (dq = T/dS) Oceans have high heat capacity compared to continents Thus, no great change in temperature of oceans relative to the continents Continental temperature difference ~ 140ºC N.Africa ~50ºC, Antarctic ~-90ºC Oceanic temperature difference ~ 34ºC Tropics ~ 34ºC, Arctic ~-2ºC The lake Effect or the Thermostatic Effect of Oceans During winter, oceans are warmer than continents and heat is transferred to the continents During summer, oceans are cooler than continents and heat is transferred to the oceans The Sun s heat warms oceans unevenly - warm tropics, cool high latitudes This causes convection currents How much heat is transferred (by convection!)? How does water transfer heat? See discussion of Latent Heat 10 Review: What is Heat Capacity A. A Measure of Energy B. The capacity of a substance to radiate heat C. The amount of heat needed to change the temperature of a substance 11 Review: The high heat capacity of water allows. A. Water to transport great quantities of heat B. Water to store great quantities of heat C. Oceans to regulate climate D. All of the above 12 4

5 Which way does heat flow? A. The baby will boil B. The candle will cool C. Cannot know 13 What happens in a phase change? Melting ice: Ice crystals have long-range order to the arrangement of molecules Adding Heat vibrates the structure and disrupts this order = melting to water Liquid water has v. short-range order Boil water: Adding Heat vibrates hydrogen bonds and disrupt order, super energetic molecules escape to vapor Vapor has no order - individual molecules rocketing through the atmosphere 14 Latent Heat & Phase Changes: Cartoon What is Latent Heat? E required to melt or boil Consider icy cool lemonade Initially 0ºC, as ice melts what is the Temp? Latent heat fusion = Heat consumed during melting of ice The latent heat of fusion of water = 80 cal/g ice Add latent heat to melt ice, remove latent heat to freeze water. 15 5

6 Water s Latent Heat of Fusion = 80 cal/g Water s Latent Heat of Fusion is quite large Heat required to melt ice Also, Heat liberated during freezing! Highest among all common solids/liquids! 16 Water s Latent Heat of Vaporization = 540 cal/g Water s Latent Heat Vaporization is HUGE Heat require to turn liquid at 100ºC into vapor Water s Latent Heat Evaporation = 585 cal/g Heat require to turn liquid < 100ºC into vapor Liberated molecule must take heat energy from surrounding water molecules - thus the cooling effect of evaporation. This is also the Heat liberated during condensation Highest of all common substances - VERY IMPORTANT to ocean/atmosphere Zeer Pot Nigerian invention uses LHE to refrigerate 17 food When water condenses in the atmosphere to form rain or clouds.. A. Heat is transferred to the atmosphere, supplying energy for circulation of the atmosphere. B. Heat is transferred to the water, which falls back to the oceans, supplying energy for currents to circulate. C. Heat is transferred to the continents, warming them and making high latitudes habitable. 18 6

7 Thermostatic effects of water s latent heat Evaporation and condensation on a global scale cools the oceans and warms the atmosphere at low latitude evaporation removes heat from ocean (cools tropical seas) At high latitude condensation releases heat to to the atmosphere (warming)! 19 Heat stored/released by sea ice: Annual ice volume change: ~18,000 km 3 How much heat is exchanged? ~1.44 x10 21 calories absorbed during melting! ~1.44 x10 21 calories liberated during freezing! 20 Heat transported by Gulf Stream Gulf Stream outflow is 10ºC warmer than return flow. Flow is 55 x 10 6 m 3 per second Calculate 550 trillion calories per second! Ocean to Atmospheric transfer of heat is even greater! Recall Latent heat of evaporation is very large. 21 7

8 Convection in the Oceans-! Cooling of surface water at high latitude! Warming of surface water at low latitude North Atlantic Cooling Equator Heating We will return to this latter in the semester 22 Review Questions What single feature of water is responsible for some of its important properties (e.g., high heat capacity)? Which direction does heat flow during melting of ice, crystallization of ice, evaporation of water and condensation of water? How do the oceans influence climate? 23 8