1. Heat in Changes of State
Prentice-Hall Chapter 17.3
Dr. Yager

2. Objectives
Classify the enthalpy change that occurs when a substance melts, freezes, boils, condenses, or dissolves.
Solve for the enthalpy change that occurs when a substance melts, freezes, boils, condenses, or dissolves.

4. Enthalpy changes accompany changes in state.

5. Heats of Fusion and Solidification
The quantity of heat absorbed by a melting solid is the same as the quantity of heat released when the liquid solidifies.
The molar heat of fusion (∆Hfus) is the heat absorbed by one mole of a solid substance as it melts to a liquid at a constant temperature.
The molar heat of solidification (∆Hsolid) is the heat lost when one mole of a liquid solidifies at a constant temperature.

6. Heats of Fusion and Solidification
The quantity of heat absorbed by a melting solid is the same as the quantity of heat released when the liquid solidifies.
∆Hfus = –∆Hsolid

9. How many kilojoules of heat are required to melt a 10
How many kilojoules of heat are required to melt a 10.0-g popsicle at 0oC? Assume the popsicle has the same molar mass and heat of fusion as water.
moles of water = 10.0 g/(18 g/mol) = 0.56 mol
∆Hfus = 6.01 kJ/mol (melting is endothermic)
∆H = (0.56 mol)(6.01 kJ/mol)
= 3.34 kJ

10. How many kilojoules of heat are required to melt a 10
How many kilojoules of heat are required to melt a 10.0-g popsicle at -20oC? Assume the popsicle has the same molar mass and heat of fusion as water.
moles of water = 10.0 g/(18mg/mol) = 0.56mol
∆Hfus = 6.01 kJ/mol (melting is endothermic)
First raise the temperature to 0oC, then melt:
∆Htotal = ∆Hraise the temp + ∆Hfus
= (10.0 g)[(2.1 J/(g·oC)](20oC) + (.56mol) x
(6.01 kJ/mol)
∆Htotal = 0.42 kJ kJ = kJ

12. Enthalpy changes accompany changes in state.

13. Heats of Vaporization and Condensation
The quantity of heat absorbed by a vaporizing liquid is the same as the quantity of heat released when the vapor condenses.
The amount of heat necessary to vaporize one mole of a given liquid is called its molar heat of vaporization (∆Hvap).
The amount of heat released when 1 mol of vapor condenses at the normal boiling point is called its molar heat of condensation (∆Hcond).

14. Heats of Vaporization and Condensation
The quantity of heat absorbed by a vaporizing liquid is the same as the quantity of heat released when the vapor condenses.
∆Hvap = –∆Hcond

17. How many kilojoules of heat are absorbed when 0
How many kilojoules of heat are absorbed when 0.46 g of chloroethane (C2H5Cl, bp 12.3oC) vaporizes at its normal boiling point? The molar heat of vaporization of chloroethane is 26.4 kJ/mol.
moles of C2H5Cl = 0.46 g/(64.5 g/mol)
= 7.1 x 10-3 mol
∆H = (26.4 kJ/mol)) x (7.1 x 10-3 mol) = 0.19 kJ

18. Heat of Solution
During the formation of a solution, heat is either released or absorbed.
The enthalpy change caused by dissolution of one mole of substance is the molar heat of solution (∆Hsoln).

19. When ammonium nitrate crystals and water mix inside the cold pack, heat is absorbed as the crystals dissolve.

21. How many moles of NH4NO3(s) must be dissolved in water so that 88
How many moles of NH4NO3(s) must be dissolved in water so that 88.0 kJ of heat is absorbed from the water?
∆Hsol = 25.7 kJ/mol
moles of NH4NO3 = (88.0 kJ)/(25.7 kJ/mol)
= 3.42 mol

22. 1. The molar heat of condensation of a substance is the same, in magnitude, as its molar heat of
formation.
fusion.
solidification.
vaporization.

23. 1. The molar heat of condensation of a substance is the same, in magnitude, as its molar heat of
formation.
fusion.
solidification.
vaporization.

24. 2. The heat of condensation of ethanol (C2H5OH) is 43. 5 kJ/mol
2. The heat of condensation of ethanol (C2H5OH) is 43.5 kJ/mol. As C2H5OH condenses, the temperature of the surroundings
stays the same.
may increase or decrease.
increases.
decreases.

25. 2. The heat of condensation of ethanol (C2H5OH) is 43. 5 kJ/mol
2. The heat of condensation of ethanol (C2H5OH) is 43.5 kJ/mol. As C2H5OH condenses, the temperature of the surroundings
stays the same.
may increase or decrease.
increases.
decreases.

26. 3. Calculate the amount of heat absorbed to liquefy 15
3. Calculate the amount of heat absorbed to liquefy 15.0 g of methanol (CH3OH) at its melting point. The molar heat of fusion for methanol is 3.16 kJ/mol.
1.48 kJ
47.4 kJ
1.52  103 kJ
4.75 kJ

27. 3. Calculate the amount of heat absorbed to liquefy 15
3. Calculate the amount of heat absorbed to liquefy 15.0 g of methanol (CH3OH) at its melting point. The molar heat of fusion for methanol is 3.16 kJ/mol.
1.48 kJ
47.4 kJ
1.52  103 kJ
4.75 kJ

28. How much heat (in kJ) is absorbed when 50 g of NH4NO3(s), 0
How much heat (in kJ) is absorbed when 50 g of NH4NO3(s), moles, are dissolved in water?
Hsoln = kJ/mol
12.85 kJ
13.1 kJ
25.7 kJ
1285 kJ

29. How much heat (in kJ) is absorbed when 50 g of NH4NO3(s), 0
How much heat (in kJ) is absorbed when 50 g of NH4NO3(s), moles, are dissolved in water?
Hsoln = kJ/mol
12.85 kJ
13.1 kJ
25.7 kJ
1285 kJ