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Understanding Heat and State Changes in Chemistry

Explore the impact of temperature on water boiling and ice melting, delve into enthalpy calculations, and solve practice problems related to heat and state changes in chemistry.

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Understanding Heat and State Changes in Chemistry

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  1. Chapter 11 Notes III Heat at State Changes

  2. At what temperature does water boil?

  3. What if you turn the heat on the stove WAY up? Does it change?

  4. What about ice melting? Is it the same or different?

  5. Enthalpy (H)—the internal heat content of a substance. • At constant pressure, the change in enthalpy (DH) is the same as heat lost/gained. • DH=mCpDT

  6. Heat and State Changes • It takes energy for a substance to change phases. • The amount of heat absorbed by one mole of a substance going from solid to liquid is the molar heat of fusion (DHfus).

  7. Heat and State Changes • The amount of heat absorbed by one mole of a substance going from liquid to gas is the molar heat of vaporization (DHvap).

  8. Practice Problem • How much energy does it take to melt 25g of ice? • If you have 250kJ of heat, how much ice can you melt? DHfus=6.02 kJ/mol DHvap=40.7 kJ/mol

  9. Practice Problem • How much heat does it take to raise the temperature of 1.3g of ice at -26.0oC to water at 76.0oC? Cice=2.09J/goC DHfus=6.02 kJ/mol DHvap=40.7 kJ/mol

  10. Practice Problem • How much heat does it take to change 22.7g of solid ethanol (C2H6O) from -114.3oC (its freezing point) to ethanol vapor at 78.5oC (its boiling point)? DHfus=4.60 kJ/mol DHvap=43.5 kJ/mol Cethanol = 2.4 J/goC

  11. Practice Problem • How much required to heat a 3.55g block of ice at -62oC to water vapor at 115oC? DHfus=6.02 kJ/mol (make corrections!) DHvap=40.7 kJ/mol Csteam=2.03 J/goC Cice=2.09J/goC

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