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Energy Changes & Phase Changes

Heating & Cooling Curves. Energy Changes & Phase Changes. energy needed to heat/cool matter!. All P/C changes accompanied by energy changes. phase changes are physical changes: sometimes energy is absorbed (endothermic) sometimes energy is released (exothermic)

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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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