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Reactions Involving Pure Condensed Phases and a Gas Phase (Chap. 12)
In the last section we looked at reactions between gases– Control of the atmosphere is important for materials
engineering» Oxidation» Sulphidation» Reduction
In this section we will now look at reactions with pure liquids and solids with the gases
Ellingham diagrams are the basis for understanding the behaviour of:– Metals and oxides– Metals and sulphides– Other systems
1
Equilibria between Pure Condensed Phases and Gas (Chap. 12.2)
Consider the equilibrium between a metal M and its oxide, MO, both solid in this case
The metal and oxide will be in equilibrium with their vapors, and again we are using this as an indicator of activity in the solids
Therefore, a gas phase equilibrium (just like in the last section of the course) is established:
2
Equilibria between Pure Condensed Phases and Gas (Chap. 12.2)
ΔG˚ means that this is the free energy change for reactants and products in their standard state (1 atm at the temperature of interest)
The gaseous reactants and products are not generally in their standard state at equilibrium– The metals and oxides are in equilibrium with their saturated
vapour pressures– The vapour pressures of metals and oxides are low– PO2 is low as we will see later when we look at data
3
Equilibria between Pure Condensed Phases and Gas (Chap. 12.2)
The next step is to relate the free energy of the vapours to the free energy of the solid phases, here just the metal
Metal and oxide molar volumes are insensitive to pressure in most situations, so the integral term is negligible
Now we make a subtle, but important, change in the standard state of the condensed phase, it is now just the pure species at the temperature of interest, and we do not have to specify 1 atm pressure (insensitive to pressure)
Standard state of O2 is still 1 atm at temperature of interest4
Equilibria between Pure Condensed Phases and Gas (Chap. 12.2)
Returning to the free energy equilibrium condition for the gases:
We can substitute in our expressions for the difference in free energy of the vapour and the solid
This is the standard free energy change of the reaction
Metal and oxide are in their standard states (pure at temperature) (their activities are unity) and O2 at 1 atm at temperature
5
Equilibria between Pure Condensed Phases and Gas (Chap. 12.2)
Example of reduction of cuprous oxide; how low does PO2 need to be to reduce it
PO2 = 2.38 x 10-50 at 25˚C PO2 = 2.06 x 10-15 at 500˚C PO2 = 8.35 x 10-7 at 1000˚C PO2 = 0.60 at 2000˚C
6
Equilibria between Pure Condensed Phases and Gas (Chap. 12.2)
Decomposition of carbonates and hydroxides– Limestone CaCO3 decomposed or “calcined” to lime, CaO for
cement and slag forming components– Aluminum hydroxide Al(OH)3 calcined to Al2O3 in the
production of aluminum
Example of MgCO3
7
Equilibria between Pure Condensed Phases and Gas (Chap. 12.2)
8
Equilibria between Pure Condensed Phases and Gas (Chap. 12.2)
Rotary kiln for calcination of aluminum hydroxide, similar to those used for limestone calcination in the cement industry
9
Ellingham Diagrams (Ch. 12.4)
In most cases the free energy of formation of the oxides is a linear function of temperature
A is ΔH˚ and B is -ΔS˚ Convention is that the reactions are all based on 1 mole of
O2 on the left hand side, so the reactions are written as oxidation
10
Ellingham Diagrams (Ch. 12.4)
Example of silver oxidation
PO2 = 1 atm at 462 K, above this, oxide unstable
11
Ellingham Diagrams (Ch. 12.4)
Slope is positive – because ΔS˚ is negative– because there is a large decrease in volume – 1 mole of O2 on the left, none on the right– Change in the volume of metal and oxide are relatively small
Consider the oxidation of A to AO
The entropy of the gas is much greater than the condensed phases, so
The slopes of the curves in the Ellingham diagrams are all about the same, due to “loss” of 1 mole of gas
12
Ellingham Diagrams (Ch. 12.4) ΔH˚ does not change very much with temperature (unless
there is a phase change (Chap. 12.5)) Consider Co and Mn: MnO much more stable than CoO due
to ΔH˚ for MnO is more negative
13
Ellingham Diagrams (Ch. 12.4)
In some cases lines on the Ellingham diagram can intersect because the slopes are slightly different, e.g. NiO & CoO
AO more stable than BO2 at high temperature
14
15
Ellingham Diagrams (Ch. 12.4)
Nomograph for PO2 is very convenient– Lines on ΔG˚ vs. T for various PO2
– Change in ΔG˚ going from 1 atm at T to PO2 at T
16
Ellingham Diagrams (Ch. 12.4)
Very convenient superimpose on Ellingham diagram to read off PO2 at the intersection
Just the same as calculating from:
17
Ellingham Diagrams (Ch. 12.4)
Nomographic scale is placed around the outside of the diagram
Convenient for using a ruler
Note you must start at 0 K and 0 kJ for these lines
18
Effect of Phase Transformations (Chap. 12.5)
Phase changes result in elbows, consider melting of A
New ΔH˚ is more negative because melting endothermic
New ΔS˚ is also more negative, slope more positive
Note should be AO2(s) in figure
19
Effect of Phase Transformations (Chap. 12.5)
Consider case of oxide melting:
Add reactions New ΔH˚ is more positive
(smaller)
New ΔS˚ is also smaller, so slope smaller
20
Effect of Phase Transformations (Chap. 12.5)
Shape depends on which melts at lower temperature
21
Effect of Phase Transformations (Chap. 12.5)
“Elbows” are larger for vaporization because of much larger changes in enthalpy and entropy
Add the 2 reactions
22
KJSJH
FeClFeCl
ob
ob
gl
/46.97
500,126)(2)(2
+=∆
=∆
=
TTG
FeClClFeo
gs
78.33000,160)46.9768.63(600,126400,286
)(22)(
−−=−++−=∆
=+
Sulphides
In kcal
23
Fluorides
24
Chlorides (I)
25
Chlorides (II)
26
Bromides &Iodides
27
Nitrides
28