Chapter 11 Notes IIIHeat at State Changes

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• Enthalpy (H)—the internal heat Enthalpy (H)—the internal heat content of a substance. content of a substance.

• At constant pressure, the At constant pressure, the change in enthalpy (change in enthalpy (H) is the H) is the same as heat lost/gained.same as heat lost/gained.

• H=mCH=mCppTT

Heat and State ChangesHeat and State Changes• It takes energy for a substance to It takes energy for a substance to

change phases.change phases.• The amount of heat absorbed by The amount of heat absorbed by

one mole of a substance going from one mole of a substance going from solid to liquid is the molar heat of solid to liquid is the molar heat of fusion (fusion (HHfusfus).).

Heat and State ChangesHeat and State Changes•The amount of heat absorbed by The amount of heat absorbed by

one mole of a substance going one mole of a substance going from liquid to gas is the molar from liquid to gas is the molar heat of vaporization (heat of vaporization (HHvapvap).).

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?

HHfusfus=6.02 kJ/mol=6.02 kJ/mol

HHvapvap=40.7 kJ/mol=40.7 kJ/mol

Practice ProblemPractice Problem• How much heat does it take to raise the How much heat does it take to raise the

temperature of 1.3g of ice at -26.0temperature of 1.3g of ice at -26.0ooC to water C to water at 76.0at 76.0ooC?C?

CCiceice=2.09J/g=2.09J/gooCC

HHfusfus=6.02 kJ/mol=6.02 kJ/mol

HHvapvap=40.7 kJ/mol=40.7 kJ/mol

Practice ProblemPractice Problem• How much heat does it take to change How much heat does it take to change

22.7g of solid ethanol (C22.7g of solid ethanol (C22HH66O) from -O) from -114.3114.3ooC (its freezing point) to ethanol C (its freezing point) to ethanol vapor at 78.5vapor at 78.5ooC (its boiling point)?C (its boiling point)?

HHfusfus=4.60 kJ/mol=4.60 kJ/mol

HHvapvap=43.5 kJ/mol=43.5 kJ/mol

CCethanolethanol = 2.4 J/g = 2.4 J/gooCC

Practice ProblemPractice Problem• How much required to heat a 3.55g How much required to heat a 3.55g

block of ice at -62block of ice at -62ooC to water vapor at C to water vapor at 115115ooC?C?

HHfusfus=6.02 kJ/mol (make corrections!)=6.02 kJ/mol (make corrections!)

HHvapvap=40.7 kJ/mol=40.7 kJ/mol

CCsteamsteam=2.03 J/g=2.03 J/gooCC

CCiceice=2.09J/g=2.09J/gooCC