19.4
Les diagrammes pV
Le diagramme pV, qui est un graphique de la pression en fonction du volume du gaz étudié, est utile pour décrire certains aspects de la substance. Lor…
Pour un gaz parfait, la pression multipliée par le volume est une constante si la température et le nombre de moles sont constants. Le graphique de la pression en fonction du volume aboutit à une hyperbole appelée isotherme pV. Pour différentes températures constantes, l’ensemble des diagrammes pV est tracé.
Les gaz non parfaits se comportent comme un gaz parfait à des températures plus élevées. Si la température d’un gaz non parfait descend à une température critique, la courbe pV montre une région avec des pentes nulles.
Supposons que la température soit encore abaissée, la région ayant une pente nulle augmente. Ces régions représentent un changement de volume sans aucun changement de pression.
Pour l’isotherme T2, la région A-B de l’isotherme représente la phase de liquéfaction. Le gaz est à l’état de vapeur jusqu’au point A et en phase liquide au-delà du point B.
Pour les isothermes dont la température est inférieure à T2, la liquéfaction commence à des pressions plus basses et à des volumes plus élevés.
Pour les isothermes au-dessus des températures critiques, il n’y a pas de liquéfaction. La température critique est une propriété du gaz étudié pour une pression critique donnée.
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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.