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Spark 09/22/2011

Spark 09/22/2011. On a very warm day at the beach, which heats up faster, the water or the sand? Once night rolls in, which cools faster? If some materials heat up faster than others, although exposed to the same energy, then where does the energy go if something is not getting hotter?

elijah-mills
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Spark 09/22/2011

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  1. Spark 09/22/2011 • On a very warm day at the beach, which heats up faster, the water or the sand? • Once night rolls in, which cools faster? • If some materials heat up faster than others, although exposed to the same energy, then where does the energy go if something is not getting hotter? • Why might some materials heat up faster than others?

  2. Reading Questions http://mutuslab.cs.uwindsor.ca/schurko/animations/waterphases/status_water.htm Read the Matter document up to “Temperatures in Kelvin” What is the KMT? What is the difference between heat and temperature? Why is the Kelvin temperature scale used? How does the K scale convert to Celcius?

  3. Enthalpy and Heat The study of energy and its transformations is known as thermodynamics. Energy is commonly defined as the capacity to do work or to transfer heat. Work is energy used to cause an object with mass to move. Heat is energy used to cause the temperature of an object to increase.

  4. Enthalpy and Heat Kinetic energy is the energy of motion, represented by the equation: Ek = 1/2mv2 Atoms and molecules have mass and are in motion, therefore they have kinetic energy.

  5. Enthalpy and Heat • The SI unit for energy is the joule:   J = 1 kg-m2/s2 • A 2 kg mass moving at 1 m/s possesses a kinetic energy of 1 J: • Ek = 1/2mv2   = 1/2 (2 kg) (1 m/s)2 = 1 kg-m2/s2   = 1 J • Since a joule is a small amount of energy, the unit kilojoules, kJ, is often used when describing chemical reactions.

  6. Enthalpy and Heat • Traditionally, energy changes in chemical reactions have been expressed in calories, a non-SI unit. • A calorie was originally defined as the amount of heat energy required to raise the temperature of 1 g of water from 14.5 oC to 15.5 oC. It is now defined in terms of the joule: • 1 cal = 4.184 J (exactly)

  7. Enthalpy and Heat • A related energy unit used in nutrition is the nutritional Calorie (note the capital letter). • 1 Cal = 1000 cal = 1 kcal

  8. Heat Changes

  9. Enthalpy

  10. E N T H A L P Y - H heat content in matter:

  11. E N T H A L P Y - H heat content in matter: natural systems tend to go from a state of high energy to a state of low energy.

  12. E N T H A L P Y The phases of matter represent "classes" of molecular motion found at different temperatures. Solid Gas Liquid

  13. E N T H A L P Y Kinetic Energy X The phases of matter represent "classes" of molecular motion found at different temperatures.

  14. E N T H A L P Y HEAT X The phases of matter represent "classes" of molecular motion found at different temperatures.

  15. Hot Coffee Thermal E = 4 Internal E = 2 Solids = 1 Liquids = 2 Gases = 3 Chemical = 0

  16. Coffee Before Coffee After Missing 2 units of E: Where did they go? Total Energy = 5 units Total Energy = 7 units

  17. How can we diagram energy flow when a cup of coffee cools? • Energy Flow Diagram • Is energy flowing in or out of the system? • 2 units of energy leave causing the coffee to cool. E E Cup of Hot Coffee 7 units - 2 units = 5 units

  18. Spark Does hot water or cold water freeze faster? Explain using the KMT. Are you aware of the Mpemba Effect? What are the four conditions that make the Mpemba Effect possible?(p2-3) Do you have an explanation for the Mpemba Effect?

  19. HMMMM… Can water boil if it’s cold? Explain using the KMT. http://youtu.be/8oCjj8iDB9I http://youtu.be/9q5gEZGoBnk http://youtu.be/QGSo7zcg_qA http://youtu.be/OHY9fFQhX68 http://youtu.be/RPlCO3AITV4

  20. Heat Matter does not contain heat, it contains energy (kinetic, thermal, internal energy, chemical energy). Heat is energy that is transferred from hot to cold Heat is measured in calories or Joules 1 calorie = 4.185 J Measure average kinetic energy with a thermometer and call it temperature How can we measure heat if things don’t heat and cool the same way?

  21. Specific Heat Capacity Think of it as heating ability… The amount of heat energy that is required to raise the temperature of one gram of a substance by one degree Celsius is called the specific heat capacity, or simply the specific heat, of that substance. Water, for instance, has a specific heat of 1.0 calorie per gram degree Celsius [1.0 cal / (g x o C)]. Water becomes our standard for studying energy.

  22. Specific Heat Capacity Water 1 calorie 100 cal So: It takes 1 calorie of heat to raise 1 gram of water 1 degree How many calories are released if 1 gram of water cools 1 degree? How many calories are needed to heat 1 gram of water at OoC to boiling (100oC)?

  23. HChemLab Lab Burners and BTUs

  24. BTU Inquiry Lab Tips Rate of BTU production be lab burner. 1 BTU = Nrg to increase 1 lb of water @39.1oF, 1oF Convert measurable metric masses and temperatures into BTU’s and predict the time to get a hypothetical rate (BTU/sec)

  25. E N T H A L P Y q = U + P V q = heat gained or lost U = internal energy P = Pressure V = volume See Energy notes

  26. Spark 09/27/2011 How much energy is needed to melt a 10 lb bag of party ice at 0o C? (heat of fusion = 334 J/g or 80 cal/g) There are 2.2 lbs/1 kg • Hfus - heat of fusion • The amount of heat needed to change one gram of a solid into a liquid at its melting point.

  27. E N T H A L P Y Hfus - heat of fusion As energy is added to a solid at its melting point, ALL the energy is used to increase the kinetic energy of the molecules during the phase change.

  28. E N T H A L P Y Hfus - heat of fusion The amount of heat needed to change one gram of a solid into a liquid at its melting point.

  29. Every phase change is accompanied by a change in the energy of the system.

  30. As the temperature of a solid increases, its particle vibration increases.

  31. When a solid melts, its particles are freed to move with respect to one another.

  32. This melting process is called fusion.

  33. The increased freedom of motion of the molecules or ions comes at a price...

  34. measured by the heat of fusion, or enthalpy of fusion, DHfus

  35. solid/liquid phase change q = m Hfus q = heat gained or lost m = mass DHfus = heat of fusion

  36. E N T H A L P Y Hfus H2O = 334 J/g Hfus lead = 24.72 J/g

  37. As the temperature of a liquid increases, the molecules move with increasing energy.

  38. When increasing temperature, the concentration of gas-phase molecules just above the surface of the liquid increases.

  39. These gas-phase molecules exert a pressure on the liquid called vapor pressure.

  40. This vapor pressure increases with temperature until it equals the atmospheric pressure.

  41. This is the boiling point of the liquid.

  42. The energy required to cause this transition..

  43. is called the heat of vaporization or enthalpy of vaporization, DHvap

  44. E N T H A L P Y As energy is added to a liquid at its boiling point, ALL the energy is used to increase the kinetic energy of the molecules during the phase change.

  45. E N T H A L P Y Hvap- heat of vaporization The amount of heat needed to change one gram of a liquid into a gas at its boiling point.

  46. liquid/gas phase change q = m Hvap q = heat gained or lost m = mass DHvap = heat of vaporization

  47. E N T H A L P Y HvapH2O = 2260 J/g

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