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Energy Changes & Phase Changes

Learn about energy changes in heating & cooling curves, phase changes, endothermic & exothermic processes, PE, melting ice, and heating curves of H2O. Discover the amount of energy needed for phase changes and how temperature and phases relate. Explore the concepts of PE, boiling points, and heating/cooling rate effects.

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Energy Changes & Phase Changes

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  1. Heating & Cooling Curves Energy Changes & Phase Changes

  2. energy needed to heat/cool matter!

  3. All P/C changes accompanied by energy changes • phase changes are physical changes: • sometimes energy is absorbed (endothermic) • sometimes energy is released (exothermic) • energy change for given phase change can be measured/calculated

  4. calculate amount E needed to change pure substance: from one single phase to another Q = mcT Q = energy in Joules m = mass in grams C = specific heat capacity T = change in temperature = Tf - Ti

  5. What about phase changes? • more than one phase substance present • melting ice: both H2O(l) & H2O(s) present at same time • temperature is constantso T=0 • even though ice is absorbing heat energy to change phase

  6. 6 possible phase changes

  7. PE: energy of relative position • molecules are always attracted to one another • energy put into system to pull molecules apart from one another • farther apart, the higher their PE

  8. Melting Ice • Ice on hot plate: ice is melting • ice absorbing heat from hot plate and using to change phase • temperature of ice-water mix is constant • heat energy from hot plate going into phase change • heat energy not going to KE of particle is going to PE during phase change

  9. Identify a phase change as endothermic or exothermic endothermic exothermic gas freezing condensation deposition melting (fusion) vaporization sublimation PE liquid solid

  10. Heating & Cooling Curves • way to illustrate energy changes • measure temperature as function of time at constant heating/cooling rate

  11. I II KE↔ III IV V Liquid & Gas Solid & Liquid Solid Liquid Gas KE KE boil pt. KE↔ PE↔ KE PE Temperature PE melt pt. PE↔ PE↔ Time

  12. Melting & Boiling Points • Plateaus = Phase changes = PE changes 2 phases are present, temp is constant • melting point/boiling point: • value always given as exact temp (number with unit: ˚C, K)

  13. What happens to temperature as heat is added at boiling point? • NOTHING … temp doesn’t change until only 1 phase present!

  14. To analyze a heating/cooling curve: • curve: does it go uphill or downhill? • label: phases present in each region • describe what happens to KE in each region • describe what happens to PE in each region • locate melting point and boiling point

  15. Heating curve of H2O Why does it take longer for water to vaporize than it does to melt?

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