Chemistry · General chemistry II · Worked example
Predict the shift when a reactant is added
For H₂(g) + I₂(g) ⇌ 2HI(g), Kc = 50.0 at a certain temperature. An equilibrium mixture has [H₂] = 0.100 M, [I₂] = 0.200 M and [HI] = 1.00 M. H₂ is added until [H₂] = 0.300 M. Which way does the reaction shift, and what are the new equilibrium concentrations?
Check the starting point
Before the addition Q equals K, as it must at equilibrium.
Find Q after the addition
Q is now 16.7, less than K = 50.0, so the reaction runs forward and uses up some of the added H₂.
Set up the ICE table
If x mol/L of H₂ reacts, [H₂] = 0.300 − x, [I₂] = 0.200 − x and [HI] = 1.00 + 2x. Substituting into K and expanding gives a quadratic.
Solve for x
The quadratic formula gives x = 0.0788 or x = 0.552. The larger root would make [H₂] negative, so x = 0.0788.
Read the new equilibrium
[H₂] = 0.221 M, [I₂] = 0.121 M and [HI] = 1.16 M. Some of the added H₂ was used up but not all of it: the shift only partly undoes the change.
Result
The reaction shifts forward. At the new equilibrium [H₂] = 0.221 M, [I₂] = 0.121 M and [HI] = 1.16 M.
Your turn
Starting from the original equilibrium, HI is removed until [HI] = 0.500 M. Which way does the reaction shift?
Show the answer and explanation
Forward, making more HI.
Q = 0.500² ÷ ((0.100)(0.200)) = 12.5, less than K = 50.0, so the reaction runs forward to replace some of the HI.
Keep exploring
Equilibrium & ICE tables opens with the disturbed mixture and Kc = 50.0. It reports Q = 16.7 and a forward shift, and its ICE table gives the new concentrations.
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