critical pressure

Examples of critical pressure in the following topics:

• Supercritical Fluids

  • A supercritical fluid is a substance at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist.
  • On the other hand, high enough pressures (above the critical pressure) would not allow a sample to stay in the pure gaseous state.
  • The critical point of a binary mixture can be estimated as the arithmetic mean of the critical temperatures and pressures of the two components,
  • Thus, above the critical temperature a gas cannot be liquified by pressure.
  • At slightly above the critical temperature (310 K), in the vicinity of the critical pressure, the line is almost vertical.

• Interpreting Phase Diagrams

  • Phase diagrams can also be used to explain the behavior of a pure sample of matter at the critical point.
  • General observations from the diagram reveal that certain conditions of temperature and pressure favor certain phases of matter.
  • The critical point, which occurs at critical pressure (Pcr) and critical temperature (Tcr), is a feature that indicates the point in thermodynamic parameter space at which the liquid and gaseous states of the substance being evaluated are indistinguishable.
  • At temperatures above the critical temperature, the kinetic energy of the molecules is high enough so that even at high pressures the sample cannot condense into the liquid phase.
  • A typical phase diagram illustrating the major components of a phase diagram as well as the critical point.

• Three States of Matter

  • The highest temperature at which a particular liquid can exist is called its critical temperature.
  • A liquid can be converted to a gas through heating at constant pressure to the substance's boiling point or through reduction of pressure at constant temperature.
  • A gas at a temperature below its critical temperature can also be called a vapor.
  • It can also exist in equilibrium with a liquid (or solid), in which case the gas pressure equals the vapor pressure of the liquid (or solid).
  • A supercritical fluid (SCF) is a gas whose temperature and pressure are greater than the critical temperature and critical pressure.

• The Structure and Properties of Water

  • It is in dynamic equilibrium between the liquid and gas states at 0 degrees Celsius and 1 atm of pressure.
  • Water is primarily a liquid under standard conditions (25 degrees Celsius and 1 atm of pressure).
  • When water achieves a specific critical temperature and a specific critical pressure (647 K and 22.064 MPa), the liquid and gas phases merge into one homogeneous fluid phase that shares properties of both gas and liquid.
  • Phase diagrams help describe how water changes states depending on the pressure and temperature.
  • The three phases of water – liquid, solid, and vapor – are shown in temperature-pressure space.

• Real Gases

  • Equations other than the Ideal Gas Law model the non-ideal behavior of real gases at high pressures and low temperatures.
  • At a certain point of combined low temperature and high pressure, real gases undergo a phase transition from the gaseous state into the liquid or solid state.
  • Real-gas models must be used near the condensation point of gases (the temperature at which gases begin to form liquid droplets), near critical points, at very high pressures, and in other less common cases.
  • The graph below depicts how the compressibility factor varies with increasing pressure for a generalized graph.
  • According to the Ideal Gas Equation, PV=nRT, pressure and volume should have an inverse relationship.

• Pressure and Free Energy

  • Gibbs free energy measures the useful work obtainable from a thermodynamic system at a constant temperature and pressure.
  • When a system changes from an initial state to a final state, the Gibbs free energy (ΔG) equals the work exchanged by the system with its surroundings, minus the work of the pressure force.
  • Gibbs energy (also referred to as ∆G) is also the chemical potential that is minimized when a system reaches equilibrium at constant pressure and temperature.
  • As such, it is a convenient criterion of spontaneity for processes with constant pressure and temperature.
  • Therefore, Gibbs free energy is most useful for thermochemical processes at constant temperature and pressure.

• Osmotic Pressure

  • Osmotic pressure is the pressure needed to nullify the effects of osmosis and is directly influenced by the amount of solute in the system.
  • The height difference between the two sides can be be converted into pressure to find the osmotic pressure exerted on the solution by the pure solvent.
  • Osmotic pressure is the pressure that needs to be applied to a solution to prevent the inward flow of water across a semipermeable membrane.
  • Osmotic pressure can also be explained as the pressure necessary to nullify osmosis.
  • The osmotic pressure is the pressure required to achieve osmotic equilibrium.

• Vapor Pressure of Nonelectrolyte Solutions

  • When a solute is added to a solvent, the vapor pressure decreases.
  • The decrease in entropy difference lowers the vapor pressure.
  • Raoult's law states that the vapor pressure of an ideal solution is dependent on the vapor pressure of the pure solvent and the mole fraction of the component present in the solution.
  • For an ideal solution, equilibrium vapor pressure is given by Raoult's law:
  • Calculate the vapor pressure of a nonelectrolyte solution using Raoult's law

• Changes in Volume and Pressure

  • This principle can be applied to changes in temperature, concentration, volume, and pressure.
  • As can be seen, a reduction in volume yields an increase in the pressure of the system, because volume and pressure are inversely related.
  • In order to compensate for the increasing pressure and decreasing volume of the system, the new equilibrium state will favor conditions that decrease the pressure in the system.
  • A schematic of the pressure due to the collisions of gas particles with the walls of a vessel
  • Evaluate the effect of pressure on the equilibrium of a chemical reaction

• Dalton's Law of Partial Pressure

  • Dalton's Law of Partial Pressure states the total pressure exerted by a mixture of gases is equal to the sum of the partial pressure of each individual gas.
  • Dalton's Law (also called Dalton's Law of Partial Pressures) states that the total pressure exerted by the mixture of non-reactive gases is equal to the sum of the partial pressures of individual gases.
  • What is the total pressure inside the container?
  • What is the partial pressure of He?
  • We now define the partial pressure of each gas in the mixture to be the pressure of each gas as if it were the only gas present.