Calculate the standard free energy change of the reasction by which fluorite (Ca

Question

Calculate the standard free energy change of the reasction by which fluorite (CaF2) dissociates into ions. The following information may be helpful. Please show your work.

Dissolution reactions involve the uptake of ions intosolution by the hydration of the component ions. Orientation of polar H2O molecules around the ion by reduces its ability to react with an oppositely charged ion and reform a solid mineral phase, and so enhances solubility. The dissolution of a salt, say fluorite — CaF2, is a good example:

CaF2Ca2++ 2F–

In writing this reaction for the dissolution of fluorite the hydration role played by water is not explicitlywritten, but the hydration reactions and their aqueous complex can be approximated by:

Ca2++ 6H2O Ca(H2O)62+F–+ 6H2O F(H2O)6–

Explanation / Answer

You can calculate the standard free energy of dissolution of CaF2 by using the following equation:

dG0 = -RTlnK

CaF2 (s)= Ca2+ + 2F- ....( Ksp at 298K=5.3×10–9)

K = [Ca2+ ].[ F-]2 / [CaF2]

Ksp = [Ca2+ ].[ F-]2

dG0 = -RTlnK

= -RTln{[Ca2+ ].[ F-]2 / [CaF2]}

= -RTln{[Ca2+ ].[ F-]2}.....(As CaF2 is solid )

= -RTln Ksp

= - 8.314 * 298 * ln( 5.3×10–9 )

= 47.21 kJ

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