Calcium Carbonate Phases

In summary, the conversation discusses the two common crystal forms of calcium carbonate, Calcite and Aragonite, and their respective Gibbs energy, volume, and entropy at room temperature and atmospheric pressure. The question asks which phase is more stable and at what pressure the second phase becomes more stable. The solution requires understanding the relationship between Gibbs energy and stability, as well as using the mathematical definition of Gibbs energy to determine the pressure at which the switch occurs.
  • #1
Sebastian
17
1

Homework Statement



Calcium carbonate ([itex]\mathrm{CaCO_3}[/itex]) has two common crystal forms: Calcite and Aragonite. In room temperature and atmospheric pressure, the Calcite has Gibbs energy -1128.8 kJ/mol, volume 36.93 cm^3/mol and entropy 92.9 J/K/mol, and the Aragonite has Gibbs energy -1127.8 kJ/mol, volume 34.14 cm^3/mol and entropy 88.7 J/K/mol. Under these conditions, which is the stable phase, and under what pressure in room temperature will the second phase be more stable?

Homework Equations



We study with "Thermal Physics" by Kittel.

The Attempt at a Solution



I don't even know how to start. Any help will be appreciated. Thanks!
 
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  • #2
"Which is the stable phase?"

This one can be answered with no arithmetic. You just need to know how the Gibbs energy is related to stability. Look this up if you don't know. And remember it -- this relation is basically why we use Gibbs energy at all!

"Under what pressure in room temperature will the second phase be more stable?"

The idea here is that if you change the pressure, the Gibbs energy will change, and so the phase that was not stable before is now stable. You need to find the pressure at which the switch occurs (or in other words, the pressure at which the two Gibbs energies become equal to each other). To answer this question, look up the mathematical definition of Gibbs energy, plug in what you know, and do the algebra. You should assume that changing the pressure on a solid does not (appreciably) change the entropy or the internal energy.
 

1. What are the different phases of calcium carbonate?

Calcium carbonate has three main phases: calcite, aragonite, and vaterite. Calcite is the most stable and abundant phase, while aragonite and vaterite are less stable and found in smaller quantities.

2. How do these phases form?

The formation of calcium carbonate phases is influenced by a variety of factors, including temperature, pressure, and the presence of other minerals. For example, calcite is commonly formed in marine environments, while aragonite and vaterite are often found in freshwater environments.

3. What are the properties of each phase?

Each calcium carbonate phase has distinct physical and chemical properties. Calcite is a transparent mineral with a Mohs hardness of 3, while aragonite is translucent and has a Mohs hardness of 3.5-4. Vaterite is the least stable and has a Mohs hardness of 3-4.

4. Can these phases transform into each other?

Yes, calcium carbonate phases can transform into one another through a process called polymorph conversion. This can occur naturally over time or through external factors such as changes in temperature and pressure.

5. How is calcium carbonate used in different phases?

Calcium carbonate is used in a variety of industries and applications, and the specific phase is often chosen based on its properties. For example, calcite is commonly used in the production of cement and paper, while aragonite is used in the manufacturing of pearls and jewelry. Vaterite is less commonly used due to its instability.

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