CH3410, Test 4, C98

1.(10 points) A chemist isolates a compound having empirical formula SF. He claims that it contains cations of formula S₄⁴⁺, and fluoride anions.

a. Propose a reasonable structure for the cation, consistent with the usual criteria for judging structure.

b. The chemist proposes that he should be able to synthesize a compound with empirical formula NaSi, containing anions having formula Si₄^4-. What do you think?

2. (10 points) Estimate the enthalpy change for the thermite reaction:

Al(s) + Fe₂O₃(s) ® Fe(s) + Al₂O₃(s)

(Hint: set up a Born-Haber cycle)