Aluminum silicate. Al2SiO5, has three different crystalline forms: kyanite, andalusite, and sillimanite. Because each is stable under a different, set of temperature-pressure conditions, and all are commonly found in metamorphic rocks, these minerals are important indicators of the geologic history of rock bodies.

(a) Referring to the thermodynamic data at the back of this book, argue that at 298 K the stable phase should be kyanite, regardless of pressure.

(b) Now consider what happens at fixed pressure as we vary the temperature. Let ΔG be the difference in Gibbs free energies between any two phases, and similarly for ΔS. Show that the T dependence of ΔG is given by

Although the entropy of any given phase will increase significantly as the temperature increases, above room temperature it is often a good approximation to take ΔS. the difference in entropies between two phases, to be independent of T. This is because the temperature dependence of S is a function of the heat capacity (as we saw in Chapter 3), and the heat capacity of a solid at high temperature depends, to a good approximation, only on the number of atoms it contains.

(c) Taking ΔS to be independent of T, find the range of temperatures over which kyanite, andalusite, and sillimanite should be stable (at 1 bar).

(d) Referring to the room-temperature heat capacities of the three forms of Al2SiO5, discuss the accuracy the approximation ΔS = constant.

Chemistry Lecture 7 Chapter 15: Chemical Equilibrium Dynamics of Chemical Reactions and Microscopic Reversibility Reactions can be reversible and dynamic when rate of forward and reverse reaction are equal reaction arrows in each direction can denote this Equilibrium does not mean same amount of reactant and product...