I am not sure why the answer is missing a very essential point which is teaching the student Le Châtelier's principle . Maybe this is not a grade 12 question!!!!
The student could easily deduce that since the [I_2]=0M the equilibrium will shift toward the direction of making I_2 and therefore to the left and in this case, the ICE table is written as follows:
" " " " " " " " " "H_2(g)+I_2(g)rightleftharpoons 2HI(g)
Initial" " " " " "8M" " "0M" " " " "2.4M
"Change" " " " " " "+x" +x" " " " "-2x
Equilibrium" "8+x" " x" " " "2.4-2x
In this case, ONLY color(green)("positive") values of x will be accepted.
However, the way the answer is written is that he will assume that the equilibrium is shifting to the right and in this case x will be color(green)("positive"), however, if his assumption is wrong, x will be color(red)("negative") and this is the case, that is why the [HI] dropped from 2.4M to 2.2M.
