Question

Consider the reaction shown. Using your knowledge of relative acid-base strengths and equilibrium, determine what you can about the size of K_c for the reaction.

H₃AsO₃(aq) + HSeO₃^-(aq) <=> H₂AsO₃^-(aq) + H₂SeO₃(aq)

	1.	Kc = 1
	2.	Impossible to tell
	3.	Kc < 1
	4.	Kc > 1

Answer 1

H₃AsO₃(aq) + HSeO₃^-(aq) <=> H₂AsO₃^-(aq) + H₂SeO₃(aq)

Electronegativity of Arsenic     = 2.18

Electronegativity of Selenium = 2.55

The strength of the acid is determined by the central atom's electronegativity relative to the surround atoms in the molecule. Because Selenium (Se) is the more electronegative, it draws the bulk of the electrons in the H₂SeO₃ molecule toward itself; because H and Se are on opposite ends of the molecule, Se pulls at the electrons in the H-O bond, thereby weakening it. The weaker the H-O bond, the more easily the H⁺ can ionize in water, and the stronger the acid.

So that means H₂AsSeO₃ is weaker than H₂SeO₄. Thus the above reaction as written is non-spontaneous (The reverse reaction is spontaneous).

So from relation between K_c and :delta:(G⁰_rxn) you get

K_c<1