Yaoling Flashcards
Zeroth law
If two thermodynamic systems are separately in thermal equilibrium with a third, they are also in thermal equilibrium with each other.
That is, if T(A) = T(B), and if T(B) = T(C), then T(A) = T(C)
1-3 laws of thermodynamics
First law: Energy can neither be created nor destroyed, but can only be transformed from one form to another (e.g., mechanical, thermal, electrical, light etc.).
Second law: The total entropy of any isolated thermodynamic system tends to increase over time, approaching a maximum value.
Third law: As a system asymptotically approaches absolute zero of temperature all processes virtually cease, and the entropy of the system asymptotically approaches a minimum value
Note: Thermodynamics ONLY applies to equilibrium situations or processes towards equilibrium
However, thermodynamics still applies for small portions of the rock body. That is, you need to choose or define your system to work with
What is a system?
A region of thermodynamic interest, which is separated from its surroundings by a boundary. Mass, heat and work may be exchanged across the boundary of the system.
We may not choose the entire intrusion as a system, but a very small portion of it as a system: a piece of rock, a thin-section etc. We can be readily convinced that equilibrium has been reached on such a small scale – looking at the thin section under microscope etc.
A system can be:
Isolated - no exchange of mass, heat or work across the boundary;
Closed – exchange of heat and work, but no mass;
Open – exchange of heat and work and mass;
Adiabatic – a closed system with thermally insulated boundaries.
Variables or variables of state
- Variables - Measurable properties of matter which define the state of a system.
- Two types of variables – Extensive variables vs. Intensive variables:
- Extensive variables – depending on the amount of material in the system – the values are additive. For example, volume, mass, energies etc.
- Intensive variables – independent of the amount of material in the system; non-additive, e.g., temperature, pressure, density, composition etc
Other important concepts
- Species – a chemically identified substance (e.g., SiO2, H2O, FeO, K2O etc.).
- Component – the minimum number of species required to completely define a system.
- Phases – a physically distinct, “homogeneous” portion of a system (different minerals, liquids, gas etc.).
The familiar Phase Rule or Gibbs Phase Rule
f + p = C + 2
f = C + 2 – p
p = C + 2 - f
f = degree of freedom, i.e., the maximum number of intensive variables that YOU can change without affecting the equilibrium condition of the system C = number of components or minimum number of species required to fully describe the system p = number of phases (minerals) 2 = refers to the two intensive variables T and P
Gibbs Phase Rule for a unary system
For a one component system, or “unary” system, C = 1, p = 3 -f
If f = 0, p = 3 (invariant point)
If f = 1, p = 2 (univariant line) – reaction line
If f = 2, p = 1 (divariant plane or field)
P = number of minerals Centre point = where all coexist F = 0 = P = 3 = all minerals coexist F = 1 =P = 2 = reaction line between 2 minerals F = 2 =P= 3 = Singular mineral
Gibbs Phase Rule for a binary system
For two component system, or “binary” system, p = C + 2 –f, p = 4 - f
If f = 0, p = 4 (invariant point)
If f = 1, p = 3 (univariant line) – reaction line
If f = 2, p = 2 (divariant plane or field)
Gibbs Phase Rule for a ternary system
For three component system, or “ternary” system, p = C + 2 –f, p = 5 - f
If f = 0, p = 5 (invariant point)
If f = 1, p = 4 (univariant line) – reaction line
If f = 2, p = 2 (divariant plane or field)
First law of thermodynamics:
dU = dQ + dW
Change in total energy = the change in heat energy + work done
dU = dQ - PdV
Work done = constant pressure x change in volume (PdV)
Calculations of Enthalpy Changes:
Enthalpy of formation at standard state -
- delta Hf = enthalpy at formation
- Standard state: T = 298.15 K (25°C), P = 1 bar, one mole of pure substance
- At standard state enthalpy of elements (e.g., H, O, Si, Fe, Mg etc.) are arbitrarily assigned “zero”.
- Use the sum of the enthalpy formation change of the products minus the sum of the enthalpy formation of the reactants
If H > 0, endothermic – needs heat in order for the forward reaction to occur;
If H < 0, exothermic – gives off heat in order for the forward reaction to occur
2nd + 3rd law
Second Law of Thermodynamics:
Verbally - The spontaneous reaction within a system occurs only when the S of a system increases.
Third Law of Thermodynamics:
The entropy of a pure, perfectly crystalline substance is zero at absolute temperature of zero degree (0K) – verbal description of the third law
Pseudosections
- A type of phase diagram that shows the fields of stability of different equilibrium mineral assemblages for a single bulk-rock composition.
- Standard phase diagrams may include many reactions but, depending on its composition, a particular rock may only experience a few (or none) of them.
- Pseudosections only include those reactions that affect a particular composition. Fields on a pseudosection are labeled (specifying the equilibrium mineral assemblage) with the reaction lines unlabeled (although the specific reaction can be deduced).
Lever rule
graphic representation of the abundance of components in a binary mixture is analogous to the position of a fulcrum (pivot) on a lever to the weights at each end of the lever
Peritectic
- Reaction
- A peritectic reaction is a reaction where a solid phase and liquid phase will together form a second solid phase at a particular temperature and composition
