Textbook Q 5I.15

[Ivy Nguyen 3I]

"When solid NH4HS and 0.400 mol NH3 were placed in a vessel of volume 2.0 L at 24 °C, the equilibrium
NH4HS(s) <--> NH3 + H2S, for which Kc = 1.6*10^-4, was reached. What are the equilibrium concentrations of
NH3 and H2S?"
In the solution pictured below, the NH3 and H2S have the "+x" but typically we see that the right side has the "-x" so I was wondering how do we determine which way the reaction is going and which side should have the +x or -x?

[Mikayla 2G]

When an initial concentration is 0, it is highly likely that the change in concentration is going to be +x because it is not possible to have a negative concentration.
So whichever side that has a 0 initial concentration for one of the components is most likely to be the +x side

[Kaitlin Eblen 1I]

Hi Ivy!
I believe we look for which way the reaction will shift to determine which side has "+/- x". So although we have typically be given some initial amount of reactant (ex: 0.2 M of NH4Cl) and designated the left as "-x", this is because the reaction will shift to the right as products are formed (not because the reactants are always "-x"). In this problem, we are told that there is both some initial products and initial reactant. However, since there is an initial 0 moles of H2S, we know that the reactant has to shift right (towards the products). Let me know if that answers your question.

[Kimia Rategh 2A]

Hi! The way you know which side has the -x or +x has to do with how the reaction will react to address the change that has been brought upon it. Equilibrium reactions always strive to reach equilibrium and if a change is brought upon it, then it will react in the way that will bring it back to equilibrium. Therefore, if we are adding products, then the reaction would have to favor the reactants to reestablish equilibrium by balancing out the ratio. Therefore, you would put +x for the reactants since their formation is being favored and a -x for the products. Similarly, if reactants are added to the reaction, the reaction would shift to the right, or favor the production of products. Thus you would put +x on the products side and -x on the reactants side.

Hope this helps!

[Allison Li 2F]

We can figure out which side has the +x or -x by looking at which initial conditions are given. Because the right side has an initial concentration of 0, that side will have the +x. The reaction will proceed forward to create more products.