Chem 1102 – Semester 2, 2010

PHYSICAL STATES AND PHASE DIAGRAMS

Lecture 26: •  Bonding forces •  Non-bonding forces •  Allotropes

Lecture 27: •  Gases, Liquids and Solids •  Enthalpy changes •  Phase Diagrams •  Supercritical Fluids •  2 Component Systems •  Alloys

Physical States and Phase Diagrams

Gases, Liquids and Solids

1.   Assumes both the volume and shape of its container.

2.   It is compressible 3.   Flows readily 4.   Diffusion within a gas occurs

rapidly 5.  

1.   Assumes the shape of a portion of the container it occupies

2.   Does not expand to fill its container

3.   Is virtually incompressible 4.   Flows readily 5.   Diffusion within a liquid occurs

slowly

1.   Retains its own shape and volume

2.   Is virtually incompressible 3.   Does not flow 4.  

State of a substance depends on the interplay of kinetic energies of its particles versus the intermolecular forces between them.

Micro (μικρός, small) to macro (μaκρός, large) level

NaCl Electrostatic forces-

ionic solid

Cane Sugar (Sucrose) H-bonding-molecular

solid

Quartz (SiO2) Covalent bonds-network solid (2

elements)

Diamond

Covalent bonds-network solid (1

element)

Water H-bonding-molecular

solid

Naphthalene (C10H8) London dispersion

forces-molecular solid

Benzoic acid

London dispersion forces & h-bonding-

molecular solid

Pyrite (FeS2), Beryl (Be3Al2(SiO3)6)

Covalent solid-network solid (2 or more

elements)

ΔH values–the heat required for changes of phase

Heat supplied (endothermic)

(exothermic)

Condensation

Vaporisation Fusion

Freezing

H2O(l) H2O(s) ΔH= -ΔH0fus = -6.02 H2O(g) H2O(l) ΔH= -ΔH0

vap = -40.7

H2O(l) H2O(g) ΔH0vap = 40.7

H2O(s) H2O(l) ΔH0fus = 6.02 ΔH (kJ/mol)

ΔH Values for some Common Substances Recall Intermolecular Forces: Stronger forces ⇒ higher melting points and boiling points.

Similarly, the stronger the intermolecular force, the higher the ΔH values.

Vapour pressure

-Any collection of molecules has a distribution of kinetic energies. -At constant P, two factors affect escape of molecules from liquid to gas phase: strength of intermolecular forces and T.

B.p. as a function of P

Normal Melting & Boiling Points

•  Normal melting point: melting point when pressure = 1 atm (101.3 kPa).

•  Normal boiling point: temperature where vapour pressure of liquid = 1 atm.

Phase Diagrams

•  In a one-component system (e.g. CO2): 3 phases: solid, liquid, gas. 3 two-phase equilibria: liquid-gas, solid-liquid, solid-gas (boundary lines). 1 three-phase equilibrium: all three phases co-exist: triple point

Construction of a Phase Diagram (Step 1) For one-component systems: •  Plot vapour pressure of solid as a function of temperature (T). •  Line represents unique pressure where both phases exist in a state of dynamic

equilibrium at given T, i.e. co-existence of solid and gas phases.

solid

gas

Temperature

Pre

ssur

e

liquid

gas

Construction of a Phase Diagram (Step 2)

Temperature

Pre

ssur

e

•  Plot vapour pressure of liquid as function of T.

•  Line represents unique pressure where both

phases exist in a state of dynamic equilibrium at

given T, i.e. co-existence of gas and liquid

phases.

•  Plot melting point of solid as function of pressure. •  Line represents unique pressure where both phases exist in a state

of dynamic equilibrium at given T, i.e. co-existence of solid and liquid phases.

liquid solid

Construction of a Phase Diagram (Step 3)

Temperature

Pre

ssur

e

Phase Diagram

Temperature

Pre

ssur

e

solid

gas

liquid

•  From the collection and collation of these data at different Ps and Ts it is possible to produce a phase diagram.

•  Phase diagram is unique for a given substance. •  More complex diagram for systems made from >1

component.

Phase Diagram

Temperature

Pre

ssur

e

solid

gas

liquid

•  Critical point. •  The critical T of a substance is

the T above which that substance can no longer exist as a liquid, no matter how much the P is increased.

supercritical fluid

In the same way, the critical P is the P above which the substance can no longer exist as a gas, no matter how high the T is.

Phase Diagram for CO2 •  At 0 °C, 1 atm (of CO2 only):

only gas present. •  from slope of liquid/solid

interface: increase P, more solid formed.

I2

Also sublimes at atm P and room T

Phase Diagram for H2O •  Triple point: three phases co-

exist only at single P and T. •  The Kelvin scale is defined by

273.16 K = temperature at which water is at triple point.

•  The solid-liquid slope for H2O is in the opposite direction to CO2.

•  Why?

P

T

start here (ice)

increase P, ice liquefies

solid liquid

Phase Diagram for N2

Phase Diagram for He

Liquid

Liquid

5.2K, 2.2bar

Triple point 2.2K, 0.05bar

-He II displays the highest thermal conductivity of all known substances -lowest viscosity of all known fluids being about three orders of magnitude smaller than that of air -Many technical applications: e.g. cooling of superconducting magnets of MRI scanners, space-based IR telescopes,…

Triple point

-No triple point between S, L and G phases

Phase Transitions Equilibria between solid, liquid & gas:

ΔHvaporisation ΔSvaporisation

solid

liquid

gas

melting/freezing

vapourisation/ condensation

sublimation/ deposition

ΔHfusion ΔSfusion

ΔHsublimation ΔSsublimation

+ve

+ve

+ve

-ve

-ve

-ve

Some Properties of SCFs

T & P at which boundary between liquid & vapour disappears.

supercritical fluid vapour

liquid

31°C 73atm

E.g. CO2

Supercritical Fluids

Can behave as powerful solvents dissolving a wide range of substances: also a new industrial reaction medium.

•  Easily removed (open valve to release pressure). •  Widely used in industrial separation processes: e.g. removal of caffeine

from coffee beans by supercritical CO2: removes caffeine from beans but leaves flavour and aroma components.

•  Also used in dry-cleaning & as solvent for chemical industry.

Environmentally friendly: CO2 is a harmless solvent. Chemically inert, non-flammable, non-explosive

Two Component Mixtures: Alloys Alloys are solid mixtures made up from 2 or more metals. Alloys usually have different properties from those of their component elements.

Two Component Mixtures: Alloys

Cu3Au β-brass

Two Component Mixtures: Alloys Phase diagram typical for a mixture of two elements or compounds completely miscible in both the solid and liquid states. -Characteristics of components: Hume Rothery Rules (A and B need similar atomic volumes, small Δχ, similar crystal structures).

-Liquid-solid equilibria are strongly dependant on T and composition (very weakly on P). -TA= m.p. of pure A; TB= m.p. of pure B. -Solidus line:

-Liquidus line:

-e.g. Cu-Ni mixture, NiO-MgO mixture.

1 phase

1 phase

2 phases

Pure A on the left to pure B on the right.

Two Component Mixtures: Alloys Phase diagram typical for a mixture of two elements or compounds miscible in the liquid state but completely immiscible in the solid state.

-Liquid and two solid phases exist in equilibrium at the eutectic composition and the eutectic temperature. Also the m.p. of the eutectic alloy is lower than that of the components . e.g. if A is cinnamic acid (m.p. 137ºC) and B is benzoic acid (m.p. 122ºC), the eutectic T is 82ºC at the eutectic c. e.g. NaCl and water: eutectic point at −21.2oC, c: 23.3% salt by mass. -ced: isothermal line. Below ced T, the mixture is entirely solid, consisting of a conglomerate of solid A and solid B. Above ced T, the mixture is either a liquid or a liquid-solid mixture, the composition of which varies.

A small amount of compound B in a sample of compound A lowers its m.p.

1 phase

2 phases

1 phase

Compositions of Common Alloys

Two Component Mixtures: Chocolate/Vanilla

How different compositions and Ts can change the final product. The eutectic line indicates the temperature where the liquid transforms into two types of solids, like chocolate ripple. The solvus line indicates the limit for how much vanilla can be dissolved into the chocolate as a function of temperature.