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 oC (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