19.4
pV 图
pV 图是所研究气体的压力与体积的关系图，有助于描述物质的某些方面。 当物质的行为类似于理想气体时，理想气体方程描述了其压力和体积之间的关系。 在 pV 图上，通常会绘制等温线，该曲线显示 p 是 V 的函数，分子数量和温度固定。 那么，对于理想气体，气体的压力与其体积的乘积是恒定的。 例如，气体的…
对于理想气体，当温度和物质的量保持不变时，压力与体积的乘积为常数。压力与体积关系的图像是一条称为 pV 等温线的双曲线。对于不同的恒定温度，可绘制出一组 pV 图。
非理想气体在较高温度下表现得像理想气体。当非理想气体的温度降低至临界温度时，其 pV 曲线会出现一个斜率为零的区域。
假设温度进一步降低，斜率为零的区域会增大。这些区域代表体积发生变化，而压力保持不变。
对于等温线 T2，等温线上的 A-B 区域表示液化阶段。气体在到达 A 点之前处于蒸气状态，在超过 B 点之后则处于液态。
对于温度低于 T2 的等温线，液化在较低的压力和较高的体积下开始。
对于高于临界温度的等温线，不会发生液化。临界温度是给定临界压力下所研究气体的特性。
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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.