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QUESTION

# 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.