In these cases, the transition from the solid to the gaseous state requires an intermediate liquid state. But at temperatures below that of the triple point, a decrease in pressure will result in a phase transition directly from the solid to the gaseous.
Also, at pressures below the triple point pressure, an increase in temperature will result in a solid being converted to gas without passing through the liquid region. For some substances, such as carbon and arsenic, sublimation is much easier than evaporation. This is because the pressure of their triple point is very high and it is difficult to obtain them as liquids. The solid has such high vapor pressures that heating leads to a substantial amount of direct vaporization even before the melting point is reached.
The process of sublimation requires additional energy and is therefore an endothermic change. The enthalpy of sublimation also called heat of sublimation can be calculated as the sum of the enthalpy of fusion and the enthalpy of vaporization. The cooling effect in a refrigerator is achieved by a cycle of condensation and vaporization of the nontoxic compound CCl 2 F 2 Freon As shown in Figure 5, the refrigerator contains 1 an electrically-powered compressor that does work on Freon gas, and 2 a series of coils that allow heat to be released outside on the back of the refrigerator or absorbed from inside the refrigerator as Freon passes through these coils.
This is a schematic diagram of the major functional components of a refrigerator. The major features include a compressor containing Freon CCl 2 F 2 gas, an external heat-exchange coil on the outside back of the refrigerator in which the Freon passes and condenses, an expansion valve, and a heat-exchange coil inside the insulated compartment of the refrigerator blue in which the Freon is vaporized, absorbing heat from inside the refrigerator and thus lowering its temperature.
Figure 6 below traces the phase transitions of Freon and their associated heat-exchange events that occur during the refrigeration cycle. The steps of the refrigeration cycle are described below the figure. The numbers in the figure correspond to the numbered steps below. This diagram shows the major steps in the refrigeration cycle.
For a description of each step indicated by the green numbers , see the numbered steps below. In this figure, blue dots represent Freon gas, and solid blue areas represent liquid Freon. Small arrows indicate the direction of heat flow into or out of the refrigerator coils. Please click on the pink button below to view a QuickTime movie showing an animation of the refrigeration cycle shown in the figure above and described below. Click the blue button below to download QuickTime 4.
Outside of the refrigerator, the electrically-run compressor does work on the Freon gas, increasing the pressure of the gas. As the pressure of the gas increases, so does its temperature as predicted by the ideal-gas law. Next, this high-pressure, high-temperature gas enters the coil on the outside of the refrigerator. Heat q flows from the high-temperature gas to the lower-temperature air of the room surrounding the coil.
This heat loss causes the high-pressure gas to condense to liquid, as motion of the Freon molecules decreases and intermolecular attractions are formed. Hence, the work done on the gas by the compressor causing an exothermic phase transition in the gas is converted to heat given off in the air in the room behind the refrigerator.
If you have ever felt the coils on the back of the refrigerator, you have experienced the heat given off during the condensation of Freon. Next, the liquid Freon in the external coil passes through an expansion valve into a coil inside the insulated compartment of the refrigerator. Now, the liquid is at a low pressure as a result of the expansion and is lower in temperature cooler than the surrounding air i.
Since heat is transferred from areas of greater temperature to areas of lower temperature, heat is absorbed from inside the refrigerator by the liquid Freon, causing the temperature inside the refrigerator to be reduced. The absorbed heat begins to break the intermolecular attractions of the liquid Freon, allowing the endothermic vaporization process to occur. When all of the Freon changes to gas, the cycle can start over.
The cycle described above does not run continuously, but rather is controlled by a thermostat. When the temperature inside the refrigerator rises above the set temperature, the thermostat starts the compressor. Once the refrigerator has been cooled below the set temperature, the compressor is turned off. This control mechanism allows the refrigerator to conserve electricity by only running as much as is necessary to keep the refrigerator at the desired temperature. Refrigerators are essentially heat engines working in reverse.
Whereas a heat engine converts heat to work, reverse heat engines convert work to heat. In the refrigerator, the heat that is generated is transferred to the outside of the refrigerator. To cool the refrigerator, a "working substance", or "coolant", such as Freon is required. The refrigerator works by a cycle of compressing and expanding the Freon, combined with phase transitions between the gaseous and liquid phases of Freon.
Work is done on the Freon by a compressor, and the Freon then releases heat to the air outside of the refrigerator as it undergoes the exothermic condensation from a gas to a liquid. To regenerate the gaseous Freon for compression, the Freon passes through an internal coil, where it undergoes the endothermic vaporization from the liquid phase to the gaseous phase. This endothermic process causes the Freon to absorb heat from the air inside the refrigerator, cooling the refrigerator.
Louis, MO Figure 1 In a heat engine, an input of heat causes an increase in the temperature of the working substance, allowing the working substance to perform work. Figure 2 In a reverse heat engine, a work input is converted to a heat output.
Gases : Gases can ionize into plasma, condense into liquids, or undergo deposition into solids. Gases form from the sublimation of solids, vaporization of liquids, and recombination of plasma. Plasma : Plasma can recombine to form a gas. Plasma most often forms from ionization of a gas, although if sufficient energy and enough space are available, it's presumably possible for a liquid or solid to ionize directly into a gas. Phase changes aren't always clear when observing a situation. For example, if you view the sublimation of dry ice into carbon dioxide gas, the white vapor that is observed is mostly water that is condensing from water vapor in the air into fog droplets.
Multiple phase changes can occur at once. For example, frozen nitrogen will form both the liquid phase and the vapor phase when exposed to normal temperature and pressure. Actively scan device characteristics for identification. Use precise geolocation data. Select personalised content. Create a personalised content profile. Measure ad performance. Select basic ads. Create a personalised ads profile.
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Share Flipboard Email. Anne Marie Helmenstine, Ph. Chemistry Expert. Helmenstine holds a Ph. She has taught science courses at the high school, college, and graduate levels. Facebook Facebook Twitter Twitter.
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