19.4
pV-Diagrams
The pV diagram, which is a graph of pressure versus volume of the gas under study, is helpful in describing certain aspects of the substance. When the…
For an ideal gas, pressure times volume is a constant if the temperature and number of moles are constant. The plot of pressure versus volume results in a hyperbola called a pV isotherm. For different constant temperatures, the set of pV diagrams is plotted.
Non-ideal gases behave like an ideal gas at higher temperatures. If the temperature of a non-ideal gas reduces to critical temperature, the pV curve shows a region with zero slopes.
Suppose the temperature is lowered further, the region having zero slope increases. These regions represent a change in volume without any change in pressure.
For isotherm T2, the region A-B on the isotherm represents the liquification phase. The gas is in a vapor state until point A and in a liquid phase beyond point B.
For isotherms with temperatures lower than T2, liquification starts at lower pressures and higher volumes.
For isotherms above the critical temperatures, no liquification takes place. The critical temperature is a property of the gas under study for a given critical pressure.
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Q1: What does a pV diagram show about an ideal gas?
A pV diagram plots pressure versus volume for a gas. For an ideal gas at constant temperature and moles, pressure times volume remains constant, producing a hyperbolic curve called a pV isotherm. This relationship helps visualize how pressure and volume change inversely as described by the ideal gas equation.
Q2: Why does a pV curve deviate from a hyperbola at lower temperatures?
At high temperatures and low pressures, pV isotherms remain hyperbolic. As temperature decreases, the curve deviates from this shape. At the critical temperature, the pV curve displays a point with zero slope. Below this temperature, a region of zero slope emerges where volume changes without pressure changes, indicating phase transitions.
Q3: What happens in the flat region of a pV isotherm below the critical temperature?
The flat region represents the liquid-gas transition where liquid and gas coexist. Volume can increase as liquid vaporizes or decrease as gas condenses without changing pressure or temperature. This region shows combinations of pressure and volume where both phases exist simultaneously during vaporization or liquification.
Q4: What is the critical temperature and why does it matter?
The critical temperature is the maximum temperature at which a substance can exist as a liquid. Above this temperature, no liquification occurs regardless of pressure. The critical temperature is a property specific to each gas at a given critical pressure, marking the point where the liquid-gas distinction disappears.
Q5: How do non-ideal gases differ from ideal gases on a pV diagram?
Non-ideal gases behave like ideal gases at higher temperatures but deviate at lower temperatures. When cooled to the critical temperature, their pV curves show regions with zero slope, indicating phase transitions. This behavior reflects intermolecular forces absent in ideal gas models, causing real gases to liquefy under appropriate conditions.
Q6: What does the steep left portion of a pV isotherm represent?
The steep left portion represents the liquid phase, which is nearly incompressible. A slight decrease in volume requires a significant increase in pressure. This steep slope reflects the liquid's resistance to compression, contrasting sharply with the flat transition region and the gentler slope of the gas phase on the right.
Q7: What is a supercritical fluid and when does it form?
A supercritical fluid forms when a gas exists above the critical temperature at sufficiently high pressure. At these conditions, the gas has the density of a liquid but will not condense into a distinct liquid phase. Supercritical fluids exhibit unique properties intermediate between gases and liquids, making them valuable in industrial applications.