 Solution

ID: past-exam-of-the-year-2-of-the-earth-sciences-geology-course-of-the-university-of-oxford/2022/paper-1/8/ii/solution

Subtract reaction (b) from reaction (a) and divide by two. In the direction chosen here, the resulting balanced conversion is

NaAlSi O_{4} + 2 H_{4} Si O_{4}^{0} = NaAlS i_{3} O_{8} + 4 H_{2} O .

Thus nepheline plus dissolved silica converts to albite. Additivity of the Gibbs energy of reaction, equivalently Hess's law, gives

lo g_{10} K_{c} = \frac{1}{2} (lo g K_{a} - lo g K_{b}) = \frac{1}{2} (6.88 + 15.18) = 11.03.

For the reverse reaction the answer is

- 11.03

; the reaction direction must accompany the equilibrium constant.

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