Question

Problem 5.28. Calcium carbonate, CaCO3, has two common crystae forms, calcite and aragonite. Thermodynamic data for these phases can be found at the back of this book (a) Which is stable at earth's surface, calcite or aragonite? (b) Calculate the pressure (still at room temperature) at which the other phase should become stable. All of the values in this table are for one mole of material at 298 K and 1 bar. Following the chemical formula is the form of the substance, either solid (s), liquid (1), gas (g), or aqueous solution (aq). When there is more than one common solid form, the mineral name or crystal structure is indicated. Data for aqueous solutions are at a standard concentration of 1 mole per kilogram water. The enthalpy and Gibbs free energy of formation, AjH and A/G, represent the changes in H and G upon forming one mole of the material starting with elements in their most stable pure states (e.g., C (graphite), 02 (g), etc.). To obtain the value of ΔΗ or ΔG for another reaction, subtract of the reactants from Af of the products. For ions in solution there is an ambiguity in dividing thermodynamic quantities between the positive and negative ions; by convention, H+ is assigned the value zero and all others are chosen to be consistent with this value. Data from Atkins (1998), Lide (1994), and Anderson (1996). Please note that, while these data are sufficiently accurate and consistent for the examples and problems in this textboolk, not all of the digits shown are necessarily significant; for research purposes you should always consult original literature to determine experimental uncertainties Substance (form) ДјН (kJ) fG (k) S (J/K) Cp (J/K) V (cm3) Al (s) Al2SiOs (kyanite) -2594.29 -2443.88 83.81 121.7 44.09 Al2SiO5 (andalusite)-2590.272442.66 93.22 122.7251.53 Al2SiOs (sillimanite) -2587.76 -2440.99 96.11124.5249.90 28.33 24.35 9.99 Ar (g) C (graphite) 0 0 154.84 20.79 0 1.895 74.81 84.68 103.85 277.69 1273 110.53 -393.51 -699.65 -691.99 5.74 2.38 50.72186.26 32.82 229.60 23.49 269.91 160.7 212 137.17197.67 394.36 213.74 187.4 91.2 0 2.900 8.53 6.11 35.31 52.63 73.5 111.46 5.30 3.42 diamond) CH4 (g) C2He (g) C3Hs (g) C2H5OH (1) C6H1206 (glucose) CO (g) CO2 (g) H2CO3 (aq) HCO3 (aq) 174.78 910 58.4 29.14 37.11 623.08 -586.77 553.58 -1128.8 -542.83 1206.9 53.1 92.9 aq CaCO3 (calcite) CaCO3 (aragonite) -1207.1-1127.888.781.25 34.15 CaCl2 (s) 81.88 36.93 -748. I 795.8 104.6 72.59 51.6

Answer 1

calcite is more stable in general than aragonite ,although they convert t each other as temperature and pressure changes.At surface conditions, aragonite spontaneously changes to calcite over geologic time, but at higher pressure aragonite, the denser of two, is the preferred structure.(organisms live in high-energy environment prefer aragonite as it is more stress resistant than calcite)

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