The shells of marine organisms contain calcium carbonate, CaCO_3, largely in a c

Question

The shells of marine organisms contain calcium carbonate, CaCO_3, largely in a crystalline form known as calcite. A second crystalline form of calcium carbonate is known as aragonite. Some physical and thermodynamic properties of calcite and aragonite are given below. (EDR 6.43) a. Based on the thermodynamic data given, would you expect an isolated sample of calcite at T=298 K and P=1.00 bar to convert to aragonite, given sufficient time? Explain. b. Suppose the pressure applied to an isolated sample of calcite is increased. Can the pressure be increased to the point that isolated calcite will be converted to aragonite? Explain. c. What pressure must be achieved to induce the conversion of calcite to aragonite at T=298 K? Assume both calcite and aragonite are incompressible at T=298 K. d. Can isolated calcite be converted to aragonite at P=1.00 atm if the temperature is increased? Explain.

Explanation / Answer

a.

Calcite = Argonite

dG = Argonite - Calcite = (-1127.7) - (-1128.8) = 1.1 kJ/mol

Overall reaction has a possitive dG value, meaning this is not favoured toward spontanous reation, i.e. it will not turn to argnoite

b.

If we increase Pressure, then we will favour argonite formation, since it is much more denser, therefore the change in prressure faovur dG towards negative value; that is, Argonite formation from calcite

c.

Apply:

dG2 - dG1 = dV*(P2-P1)

P2 = (dG2 - dG1)/(dV) + P1

P1 = 1 atm so... get dV and dG2-dG1

dV = Vargonite - Vcalcite = 1/(2.93*10^6) - 1/(2.71*10^6) = -2.770*10^-8 m3/mol

dG2-dG1=  (-1127.7) - (-1128.8) = 1.1 kJ/mol = 1100 J/mol

so..

substitute data

P2 = (dG2 - dG1)/(dV) + P1

P2 = (1100 )/( -2.770*10^-8) + 101325

P2 = 39711090010.7 Pa = 391917.98 atm

d.

Not likely to convert, since this is mainly dP*dV dependant and not dT, so increase of T is not likely to work

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