Chapter 4 Physical transformations of pure substance For the pure substances, a phase diagram is a map of the pressure and temperatures at which each phase is the most stable. Chemical potential (µ): a property that is at the centre of discussion of phase transitions (physical reactions) and chemical reactions. 4.1 The stability of phases: A phase of a substance is a form of matter that is uniform throughout in chemical composition and physical state. (solid, liquid, gas allotropes.)
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Chapter 4 Physical transformations of pure substance For the pure substances, a phase diagram is a map of the pressure and temperatures at which each phase.
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Chapter 4 Physical transformations of pure substance
For the pure substances, a phase diagram is a map of the pressure and
temperatures at which each phase is the most stable.
Chemical potential (µ): a property that is at the centre of discussion of
phase transitions (physical reactions) and chemical reactions.
4.1 The stability of phases:
A phase of a substance is a form of matter that is uniform throughout
in chemical composition and physical state. (solid, liquid, gas allotropes.)
A phase transition, the spontaneous conversion of one phase into
another phase, occurs at a characteristic temperature for a given pressure.
The transition temperature, Ttrs, is the temperature at which the two phase
are in equilibrium and Gibbs energy is minimized at the prevailing pressure.
A transition that is predicted from th
ermodynamics to be spontaneous may occ
ur too slowly to be significant in practice.
Exp. C(Diamond) C (graphite)
is too slow to be detected under ambient
condition.
Thermodynamically unstable phase that
persist because the transition is kinetically
hindered are called metastable phase.
4.2 Phase boundaries
The lines separating the regions, which are called phase boundaries,
shows the values of p and T at which two phases coexist in equilibrium.
Vapor pressure: the pressure of a vapor in equilibrium with the liquid.
Sublimation vapor pressure: the vapor pressure of the solid phase.
The vapor pressure of a substance increases with the increase of
temperature because at higher temperature the Boltzmann distribution
populates more heavily the sates of higher energy .
(a). Critical points and boiling points
When liquid is heated in an open vessel, at the temperature at which its
vapor pressure would be equal to the external pressure, vaporization can
occur throughout the bulk of the liquid. The condition of free vaporization
throughout the liquid is called boiling.
Boiling Temperature: the temperature at which the vapor pressure of a
liquid is equal to the external pressure.
At 1.0 atm: normal boiling point, Tb; At 1.0 bar, standard boiling point.
Boiling does not occur when a liquid in a closed vessel. Instead,
the vapor pressure, and the density of the vapor rise continuously as
increase of temperature.
The temperature at which the surface d
isappears is the critical temperature, Tc.
The vapor pressure at the critical temp
erature is called the critical pressure, pc.
At and above the Tc, a single uniform
phase called a supercritical fluid fills the c
ontainer and interface no longer exist.
At T > Tc, no liquid phase.
The Tc is the upper limit for liquid.
(b). Melting point and triple points
Melting point: the temperature at which, under a specified pressure, the liquid and
solid phases coexist in equilibrium. (Freezing point; liquid solid)
At p = 1 atm, normal freezing (melting) point, Tf
At p = 1 bar, standard freezing (melting) point.
Triple point (T3): a point three phase boundaries
meet. (three phases coexist in
equilibrium)
T3 occurs at a single definite p and T
characteristic of the substance.
thermodynamic temperature scale
The T3 marks the lowest pressure at which a
liquid phase of a substance can exist.
4.3 Three typical phase diagrams
(a)Water
The solid-liquid boundary has a
negative slope, which means a
decrease of volume on melting.
(dp/dT) < 0
At high pressures, different
structure forms of ice come into
stability as the hydrogen bonds
between H2O modified by
pressure.
Hydrogen-bonding structure in ice and water:
Ice (d = 0.9 cm3/g) Water (d = 1.0 cm3/g)
The volume decreases on melting. (ΔVm < 0)
(b) Carbon dioxide:
The positive slope of the solid-liqu
id boundary. (dp/dT >0)
The volume increases on melting.
The triple point lies above 1 atm. T
o obtain the liquid, it is necessary to
exert a pressure > 5.11 atm.
When a 67 atm CO2 squirts(噴出 )
to 1 atm, only the snow-like solid CO2
was obtained
(c). Helium
He behaves unusually at low temperatur
e.
The solid and gas phase are never in
equilibrium however low the temperature.
Solid helium can be obtained by applyin
g pressure (Helium is too light).
4He (I = 0), has a liquid-liquid phase tran
sition at its -line.
Helium-I phase behaves like a normal li
quid, Helium-II is a superfluid (no viscosit
y).
Phase Stability and Phase Transitions
4.4 The thermodynamic criterion of equilibrium
Chemical potential µ = Gm, µ is a measure of potential that a substance
for bring about physical or chemical change in a system.