Use the phase diagram for methane, CH4, shown in Figure
11.30 to answer the following questions. (a) What are the
approximate temperature and pressure of the critical point?
(b) What are the approximate temperature and pressure of
the triple point? (c) Is methane a solid, liquid, or gas at 1
atm and 0 °C? (d) If solid methane at 1 atm is heated while
the pressure is held constant, will it melt or sublime?
(e) If methane at 1 atm and 0 °C is compressed until a phase
change occurs, in which state is the methane when the
compression is complete?
The Correct Answer and Explanation is :
iturn0image0turn0image3turn0image4turn0image5Analyzing the phase diagram of methane (CH₄) provides insights into its behavior under varying temperature and pressure conditions. Here’s a detailed examination based on the provided diagram:
(a) Critical Point:
The critical point of methane is the temperature and pressure above which it cannot exist as a liquid, regardless of pressure. From the phase diagram, the critical point is approximately at a temperature of 190.6 K (−82.6°C) and a pressure of 46.0 bar. citeturn0search3
(b) Triple Point:
The triple point is where solid, liquid, and gas phases coexist in equilibrium. For methane, this occurs at approximately 90.67 K (−182.48°C) and 0.117 bar. citeturn0search3
(c) State at 1 atm and 0°C:
At 1 atm (1.013 bar) and 0°C (273.15 K), methane exists as a gas. This is because these conditions are well above its boiling point at 1 atm, which is −161.5°C (111.65 K). citeturn0search3
(d) Heating Solid Methane at 1 atm:
If solid methane at 1 atm is heated, it will first transition from solid to gas without passing through a liquid phase, a process known as sublimation. This is because the pressure is below methane’s triple point pressure of 0.117 bar, where direct sublimation occurs upon heating.
(e) Compressing Methane at 1 atm and 0°C:
Starting with methane gas at 1 atm and 0°C, compressing it while maintaining the temperature will eventually lead to liquefaction. This is because increasing pressure at a constant temperature above the critical temperature can transition the gas into a liquid phase.
Understanding methane’s phase diagram is crucial for applications involving its storage and transport. For instance, liquefied natural gas (LNG) is methane cooled below its boiling point at atmospheric pressure to facilitate transportation. Additionally, knowledge of its phase behavior aids in designing equipment for industries utilizing methane as a fuel or feedstock.
For a visual explanation, you might find this video helpful:
videoPractice Exercise p 448 Phase Diagramturn0search5