Part C Flashcards
(13 cards)
Define the most common characteristic of an equilibrium state.
The system is in its lowest free energy state.
The Phase Rule:
Relates the variables acting on a system (P,T etc) to the restrictions on them at Equilibrium.
What is a component? Number of independent variable operators needed to describe all phases of the system.
Variance in the number of intensive variables that need to be specified to fix a state.
Invariant points have no variance, univariant line, one Independent variable can be changed and equilibrium still achieved. In one phase region, 2 parameters can be changed, this is a divariant field.
Binary Phase system
Albite, jadeite, quartz system. Albite = Jadeite + Quartz. 2 component system, 3 phases in spite of the fact that jadeite and quartz both exist beyond the line.
No triple point, just one univariant line.
Phase assemblages: In Ja & Qz region, all compositions along tie line have those two molecules. IN albite region, it can be Al + Ja/Qz.
The Lever Rule for phase composition.
Take Al between Jd(60%) and Qz(100%). Albite os at 69.2%. Composition at a point X between Al and Qz of each substance is
%Al = (Qz-x)100/(Qz-Al)
%Qz=(Ab - x)100/Qz-Al)
Essentially, % of desired molecule = difference of other molecule and X, divided by difference of higher percentage molecule and lower, multiplied by 100.
State Function?
Functions which describe macroscopic physical state of system, can be measured and tabulated, and do not depend on the path to reach their final state.
Internal energy equals.
heat energy due to thermal gradient plus the mechanical work that flows across a system boundary.
Energy equals the capacity of a system to do work.
Energy cannot be created or destroyed, only relative value of energy can be discussed, energy is transformed from one state to another.
Work is -PV, where P is force per unit area.
Therefore U = q - PV.
U=U(V,T)
Enthalpy is the internal energy at a constant pressure.
Eutectic Point
Where liquidus points of two Liquid plus compound both meet at the solidus.
How many phases can coexist in a given P,T system.
if there are n component, phases possible = n in normal ranges, = n+1 on univariant lines, = n+2 at invariant points.
If Pressure or Temperature is fixed, we must subtract 1 from the possible variance number.
In larger, three component systems, phase rule can be used to determine number of invariant points and number of univariant reactions not involving one phase.
What is heat capacity?
Difficulty in increasing temperature of a material by supplying thermal energy.
Hidden and Free Energy?
Hidden energy does not appear as available energy, it is equal to:
TxS where S is entropy.
Stability of phase (lowest most stable) depends on the available (free) energy.
Enthalpy change?
dH=Hprod-Hreac
+ve equals endothermic.
Gibb
Increases with Pressure & decreases with T. Convex upward. Respective derivative of geometric interpretation shows this based on gradient sign.
Reactions strive to make a state a lower G. Reactants and products are in equilibrium is equal.
PvsT phase stability diagram. Line shows equal G.
A phase at higher entropy is stable at higher temperatures and one at higher pressures is stable at lower energy.
At higher P,T,
G = H - TS
ENTROPY NEVER DECREASES
Entropy increases with both internal energy and Volume. This is equated to the GFE graph.
Gibbs and Phase Diagrams
At invariant points, the gibbs free energy of each phase is the same, univariant lines show one deviation.
Clapeyron
DP/DT=DS/DV=Slope of PT reaction.
Entropy is an inverse function of energy.
Going back to albite, quartz, jadeite. Gibbs of all is equal at line and albite is above or below depending on where it is stable.
