Of state. Contents and Concepts 1. Comparison of Gases, Liquids, and Solids Changes of State

11 States of Matter; Liquids and Solids

FIGURE 11.9

Heating curve for water Heat is being added at a constant rate to a system containing water. Note the flat regions of the curve. When heat is added during a phase transition, the temperature does not change. The heat needed for the melting of a solid is called the heat of fusion (or enthalpy of fusion) and is denoted _H_fus. For ice, the heat of fusion is 6.01 kJ per mole..

H ₂O_(s) H₂O_(l); _H_fus= 6.01 kJ/mol

T he heat needed for the vaporization of a liquid is called the heat of vaporization (or enthalpy of vaporization) and is denoted _H_vap. At 100_C, the heat of vaporization of water is 40.7 kJ per mole

H ₂O_(l) H₂O_(g); _H_vap= 40.7 kJ/mol

The fact that heat is required for vaporization can be demonstrated by evaporating a dish of water inside a vessel attached to a vacuum pump. With the pressure kept low, water evaporates quickly enough to freeze the water remaining in the dish.

. Note that much more heat is required for vaporization than for melting. Melting needs only enough energy for the molecules to escape from their sites in the solid. For vaporization, enough energy must be supplied to break most of the intermolecular attractions. A refrigerator relies on the cooling effect accompanying vaporization. The mechanism contains an enclosed gas that can be liquefied under pressure, such as ammonia or 1,1,1,2-tetrafluoroethane (NFC-134a), CH2FCF3. As the liquid is allowed to evaporate, it absorbs heat and thus cools its surroundings (the interior space of the refrigerator). Gas from the evaporation is recycled to a compressor and then to a condenser, where it is liquefied again. Heat leaves the condenser, going into the surrounding air.

Example 11.1 Calculating the Heat Required for a Phase Change of a Given Mass of Substance A particular refrigerator cools by evaporating liquefied dichlorodifluoromethane, CCl2F2. How many kilograms of this liquid must be evaporated to freeze a tray of water at 0_C toice at 0_C? The mass of the water is 525 g, the heat of fusion of ice is 6.01 kJ/mol, and the heat of vaporization of dichlorodifluoromethane is 17.4 kJ/mol.