What will happen when at a constant temperature, more iodide ions, \({{\text{I}}^ - }\), are added to the equilibrium below?

\[{{\text{I}}_{\text{2}}}{\text{(s)}} + {{\text{I}}^ - }{\text{(aq)}} \rightleftharpoons {\text{I}}_3^ - {\text{(aq)}}\]

A.     The amount of solid iodine decreases and the equilibrium constant increases.

B.     The amount of solid iodine decreases and the equilibrium constant remains unchanged.

C.     The amount of solid iodine increases and the equilibrium constant decreases.

D.     The amount of solid iodine increases and the equilibrium constant remains unchanged.

B

One G2 comment stated that the SL syllabus only requires students to consider Kc from the equation for a homogeneous reaction as outlined in AS 7.2.1. The respondent stated that the equilibrium as written involves two phases, (s) and (aq), and hence this would have confused candidates. 52% of candidates however did get the question correct and the question proved to be a reasonably good discriminator also, with a discrimination index of 0.45.