ICE table. How does one know if the x is positive or negative for the products and reactants? this question is contradicting what i thought because the x is negative in the reactants yet one reactant starts with zero concentration.

See explanation.

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##.