Chapter 8: Physical Equilibria

• In this chapter, we are going to discuss the equilibrium between phases of matter and the thermodynamics behind the process

• What can we use this for?– Environmental Sciences: Toxin cleanup,

wastewater purification– Biochemistry/Biology: Gas exchange, blood flow– Materials Science: Colloids, Biomaterials

Equilibrium

• Remember:

i. At equilibrium, the forward rate of change is equal to the reverse raate

ii. G=0

iii. A phase change is the change of matter from one state to another

Solid -> liquid -> gas -> liquid -> solid

Vapor Pressure

If we place some water vapor in the headspace above the Hg in a barometer, it will exert pressure on the Hg

This pressure is proportional to the amount of water we put into the space UNTIL we add so much that liquid water starts appear on the surface of the Hg.

No more H2O vapor can form

There is an equilibrium between the 2 states

Vapor Pressure

•The pressure exerted on the surface of the Hg increased until we started to get liquid appearing on the Hg

•At this point, the rate of vaporization equals the rate of condensation and the pressure measure is called the Vapor Pressure

At a fixed temperature, as long as some liquid is present, the vapor exerts a characteristic pressure regardless of the amount of liquid present

The vapor pressure of a given phase of a substance is the pressure exerted by its vapor when the vapor is in dynamic equilibrium with the condensed phase

Vapor Pressure

Vapor Pressure

The vapor pressure of a liquid @ a given temperature is expected to be low and its enthalpy of vaporization high if the intermolecular forces are strong

8.3: Variation of Vapor Pressure with Temperature

• What determines the vapor pressure of a liquid?

The Intermolecular Forces

or

How easily the molecules can escape and enter the gas phase

Variation of Vapor Pressure with Temperature

Variation of Vapor Pressure with Temperature

• We can model this behaviour with the Clausius-Clapeyron Equation:

€

lnP2

P1

⎛

⎝ ⎜

⎞

⎠ ⎟=

ΔHvapo

Rln

1

T1

−1

T2

⎛

⎝ ⎜

⎞

⎠ ⎟

The Vapor Pressure Increases with increasing temperature

Or

The more molecules move, the more they go into the gas phaseThe higher the Hvap, the more

energy required to vaporize the molecule

8.4: Boiling• What happens when we boil a liquid (let’s say H2O) at

atmospheric pressure?• At 1 atm and 100°C, the water boils throughout the

volume– You see bubbles coming from all over/throughout the pot,

not just vapor coming off the surface

• This is the normal boiling point of water– The Temperature at which the vapor pressure equals 1 atm

• When the atmospheric/outside pressure is greater than 1 atm, the boiling point is ____?– Conversely, at lower pressures, the boiling point is ____

Boiling

Boiling Occurs when the vapor pressure of a liquid is equal to the

atmospheric pressure

Strong Intermolecular forces usually lead to higher normal boiling points

8.5: Freezing and Melting

• Remember: In a liquid, the molecules can slip past each other and in a solid, they can’t

• At the freezing temperature, the liquid and solid phases are in equilibrium

• The melting temperature is the same as the freezing temperature, it just depends on which way you look at it (are you trying to melt a solid or freeze a liquid?)

• The higher the pressure, the higher the melting (freezing) point. Why is this?– Less pressure, less inhibition of the molecules to eject

into the gas phase• Water is unique

– Higher pressures equals a lower melting point

8.6: Phase Diagrams

• A phase diagram plots the pressure versus temperature for a given sample

• We use them to determine the state of a sample at given conditions

• Phase boundaries separate the different phases and represent where the phases are in equilibrium with each other

Triple point: Where 3 phase lines meet.

All 3 phases are in equilibrium

Representative Phase Diagrams

Sulfur has two solid forms, rhombic and monoclinic

They differ in the way they are packed.

There are 3 triple points

Representative Phase Diagrams

There are at least 10 kinds of ice.

8.7: Critical PropertiesWhat happens when we move to point C on the phase curve.

As the temperature and pressure increase, the liquid and vapor phases remain in equilibrium, but the density of the gas phase increases

At 218 atm and 374°C, the interface between liquid and vapor phases disappears

8.7: Critical Properties

Critical Temp (Tc) or Critical Pressure (Pc) reached

8.7: Critical Properties

A gas can be liquified at any pressure or temperature BELOW Tc or Pc

Above the critical point, the material becomes a Supercritical Fluid

•Many commercial uses of these