L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 21: Phase Equilibrium I
Chpt. 21: p.1
PHASE EQUILIBRIUM 相平衡
Phase equilibrium is the most important physical equilibria :
a system.
Example : the equilibrium exists between water and steam
the equilibria exist between the phase of
ONE COMPONENT SYSTEMS
A phase is defined as a portion of matter which is homogenous. Subdivision of a phase will
only produce smaller portions indistinguishable from one another.
Phases are separated from one another by physical boundaries known as phase boundaries.
A phase may consist of two or more substances. A solution is a single-phase substance for example.
Gases always mix freely with one another in any proportion to give a fully homogeneous mixture.
A system may contain one or more phases.
Example:
Solutions and mixture of gases constitute a single phase. The gaseous, liquid and solids
forms of a particular substance consist of three different phases.
A phase diagram is a graph showing the relationships between the solid, liquid and gaseous
phases of a substance as a function of temperature and pressure.
I.
Vapour / Liquid Equilibrium — Qualitative account by Kinetic Theory
Consider a liquid kept in a close container at a particular temperature.
<1> Some of the molecules near the surface can escape from the liquid and accumulate as
vapour phase in the space above. (Explanation
<2>
These vapour molecules will move constantly around, colliding with one another and
with the walls of the container. The vapour molecules with the lose of energy
collision will eventually collide with the liquid phase and so return to it.
<3>
At any given temperature, a state will eventually be reached when the number of
molecules leaving the liquid will equal the number of returning. A dynamic equilibrium
is said to be reached by the system.
after
The vapour pressure of a liquid at a particular temperature is therefore the pressure of
the vapour in equilibrium with the liquid at that temperature.
L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 21: Phase Equilibrium I
Chpt. 21: p.2
Note: Increasing the temperature of the system will increaser number of molecules entering
the vapour phase because more molecules have sufficient kinetic energy to leave
the
liquid.
walls
A new equilibrium is finally reached in with a greater number of molecules in the
vapour phase, resulting in a greater pressure exerted by the gas molecules on the
of the container.
The curve of the graph of the vapour pressure against temperature indicates the
temperatures with their corresponding pressures at which an equilibrium can be
reached by the liquid phase and the vapour phase.
II.
Phase Diagram for a One-component System
Particulars for the diagram
<1>
Each area included by the curves represents the conditions of temperature and pressure under
which a particular phase is stable.
<2>
Each curve represents the conditions of temperature and pressure under which the two phases
coexist in equilibrium.
Example : Curve BD shows the indefinite equilibrium between a ___________ and a
______________ under certain temperature and pressures.
Curve BC shows the indefinite equilibrium between a ______________ and a
L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 21: Phase Equilibrium I
Chpt. 21: p.3
___________ under certain temperatures and pressures.
<3> The triple point - point B
At this point, the three curves intersect, all three phases are in equilibrium with each
other in an isolated system.
<4>
Sublimation curve : curve _________
Vaporization cürve : curve _________
Fusion, curve : curve ________
<5>
Enthalpy (heat) of vaporization
Enthalpy (heat) of sublimation.
Enthalpy (heat) of fusion.
The enthalpy required
to change 1 mole of
the substance from one
state to another
without changing the
temperature
<6> Changes in temperature
(i)
At the pressure (P1) which is above the pressure (PT):
An increase in temperature causes first__________________ at Tm and then
_______________ at Tb.
A decrease in temperatur4 causes first __________________ at Tb and then
___________ at Tm.
(ii)
At the pressure (P2) which is below the pressure (PT):
An increase in temperature causes a phase change from ____________direct to
_________________, and vice versa.
Note: For CO2, PT is 5.1 atmospheres. At atmospheric pressure carbon dioxide sublimes
at -78°C, as its vapor pressure is well below 5.1 atmosphere.
<7> Changes in pressure
Depending on the constant temperature being considered,
(i) An increase in pressure may produce a change of phase from vapor to ____________
or from vapor to _______________.
L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 21: Phase Equilibrium I
(ii)
Chpt. 21: p.4
If the constant temperature is just above the triple point, the substance may go
through all three phases.
Note: Point C is known as the critical point. This represent the highest temperature
(critical temperature) at which vapour can be compressed to a liquid by increase in
pressure alone.
more
Above the critical temperature the phase is called a gas.Its behaviour approximates to
the ideal behaviour of gases. Below the critical temperature the phase should be
properly called a gas.
Exercise1:
The following diagrams illustrate the different phase diagrams of carbon dioxide and water
respectively.
Figure (a) and (b) illustrate the pressure — temperature relationship of carbon dioxide and water
respectively.
What are the different features in fig.(a) and (b)?
Hence explain
(i)
why liquid water is denser than ice?
(ii)
why solid CO2 sublimes rather than melts at r.t.p. ?