17.3. Heat and Changes of State http://www.ck12.org
Lesson Summary
- The molar heat of fusion is the heat absorbed when one mole of a solid melts and is numerically equivalent to
the molar heat of solidification, the heat released when one mole of the liquid freezes. - The molar heat of vaporization is the heat absorbed when one mole of a liquid boils and is numerically
equivalent to the molar heat of condensation, the heat released when one mole of the gas condenses. - Conversion factors can be used to calculate the heat absorbed or released during any change of state. Multiple-
step problems, such as ice being transformed into steam, can be solved using specific heats along with heats
24.3 Fission and Fusion
- The molar heat of solution is the heat absorbed or released when one mole of a solute dissolves in water.
Lesson Review Questions
Reviewing Concepts
- How does the molar heat of fusion of a substance compare to the molar heat of solidification?
- How does the molar heat of vaporization of a substance compare to the molar heat of condensation?
- State the names given to the following changes of state, and classify each as being endothermic or exothermic.
a. Ar(g)→Ar(l)
b. KBr(s)→KBr(l)
c. C 4 H 10 (l)→C 4 H 10 (g)
d. CO 2 (s)→CO 2 (g)
e. Br 2 (l)→Br 2 (s) - Explain why the temperature of ice at 0°C does not initially rise when heat is added to it.
- The molar heat of solution of ammonia is−30.50 kJ/mol. What happens to the temperature of water when
ammonia is added to it?
Problems
- Heats of fusion and vaporization can also be expressed in J/g or kJ/g.
a. Use the molar mass of H 2 O to convert the molar heat of fusion and the molar heat of vaporization of
water to kJ/g.
b. Calculate the energy required to melt 50.0 g of ice at 0°C and to boil 50.0 g of water at 100°C. - Calculate the quantity of heat that is absorbed or released (in kJ) during each of the following processes.
a. 655 g of water vapor condenses at 100°C.
b. 3.25 g of CaCl 2 (s) is dissolved in water.
c. 8.20 kg of water is frozen.
d. 40.0 mL of ethanol is vaporized at 78.3°C (its boiling point). The density of ethanol is 0.789 g/mL. - Various systems are each supplied with 9.25 kJ of heat. Calculate the mass of each substance that will undergo
the indicated process with this input of heat.
a. melt ice at 0°C
b. vaporize water at 100°C
c. dissolve NH 4 NO 3 in water
d. vaporize water originally at 25.0°C