3. Phase diagram of a chemically homogenous system

p-T phase diagram, characteristics of phase equilibria, triple point, critical point, vapor pressure, boiling, vapor concept

 

A chemically homogeneous system's p-T phase diagram (pressure-temperature phase diagram) visually represents the phases of matter (solid, liquid, and gas) under different pressure and temperature conditions.

p-T phase diagram:

p (Pa)

 

A képen szöveg, diagram, sor, Betűtípus látható Automatikusan generált leírás

p-T Phase Diagram Overview

  • The diagram consists of curves that separate different phases (solid, liquid, gas).

  • Axes:

    • The x-axis typically represents temperature (T).

    • The y-axis represents pressure (p).

·       Characteristics of phase equilibria

o   Phase boundary lines:

§  the lines on the phase diagram that separate the different phases
                       (solid, liquid, gas)

  • Transitions between phases occur without a change in temperature or pressure for a given composition.

§  -each line represents a set of conditions where two phases exist in equilibrium

§    Solid-Liquid Boundary: The melting/freezing line where solid and liquid coexist. (melting point, m)

§   Liquid-Gas Boundary: The boiling/condensation line where liquid and gas coexist. (boiling point, t)

§   Solid-Gas Boundary: The sublimation/deposition line where solid and gas coexist. (s)

o   Triple point:

§  the unique point where all three phases coexist in equilibrium

§  it is a fundamental characteristic of a pure substance

§  Example: Water's triple point occurs at 0.01°C and 611.657 Pa

o   Critical point:

§  The end point of the gas-liquid boundary

§  beyond this point the liquid and gas phases become indistinguishable and result in a single supercritical fluid phase

§  Example: Water's critical point occurs at 374°C and 22.1 MPa.


Vapor pressure:

·       the pressure exerted by a vapor in equilibrium with its liquid or solid phase at a given temperature.

·       increases with temperature, since more molecules have sufficient energy to evaporate

Boiling

·       Boiling temperature:

o   when vapor pressure equals the external pressure acting on the liquid

·       Boiling point:

o   the temperature at which pressure is the atmospheric pressure and boiling occurs

Vapor

·       the gaseous phase of a substance that is liquid or solid under normal conditions

·       it can be condensed into the liquid or solid form by increasing the pressure or decreasing the temperature

Sublimation

·       a lot of materials can sublime, the most common are the ones having molecular lattice (Camphor, Iodine, water (lyophilization))

·       when a material turns into the gaseous phase from the solid phase without forming any liquid

Formula for tension/vapor pressure:

p*=p*0e-Qsz/RT

p*=tension/vapor pressure
p*0= preexponential
e= Euler number (same as the base of ln) (2,718)
Qsz= heat of sublimation
R= gas constant (8,314)
t=temperature (in Kelvin)