CH8 - Energetics Flashcards
(11 cards)
Why is it easier to measure enthalpy changes rather than energy changes?
As it is easier to consider constant pressure rather than constant volume when dealing with solids. At constant pressure, heat input is equal to enthalpy (thus U).
Why was entropy introduced?
In order to define the relative stability of minerals and predict the direction of a mineral reaction
Why do transformations not commonly occur at the thermodynamically defined equilibrium temperature?
As the free energy of the phases is the same. This means a reduction in G is needed to drive transformation i.e. undercooling
If activation energy is low, undercooling will be low
Why is superheating lower than supercooling?
As heating increases atomic mobility
Why is diamond considered to be in a metastable state?
As at low temperatures structural changes are difficult, thus its transformation to graphite at room temperature is insignificant.
If a complex mineral fails to transform to its equilibrium state, it may undergo an easier thermodynamic transformation (e.g. clinopyroxene to orthopyroxene)
What assumptions can be used to simplify the system?
Born Oppenheimer approximation, Harmonic approximation and phonon approximation
Born Oppenheimer approximation
The electronic and nuclear problems are considered separately - calculations related to electronic properties are solved at fixed nuclear coordinates
Harmonic approximation
initial calculation assumes vibrations behave harmonically. Underestimates density of states at high temperatures close to the dissociation point of bonds.
Phonon dispersion approximation
use the lattice rather than the motif to consider phonon displacements of the structure. Also limits the vibrations considered to a reasonable spatial frame
Undercooling
undercooled growth occurs when the liquid temperature is below the interface temperature. This occurs by cooling the liquid below its equilibrium transformation temp / by compositional segregation during solidification process
Hess’ law
Enthalpy change is the same regardless of it the reaction took place in one or several steps, provided reactants and products are the same
