Chemistry · General chemistry II · Worked example
Predict the direction of a reaction with Q
At a certain temperature, Kc = 50.0 for H₂(g) + I₂(g) ⇌ 2HI(g). A mixture contains 0.100 M H₂, 0.200 M I₂ and 0.500 M HI. Is it at equilibrium? If not, which way does it shift?
Write Q
Q has the same form as K: the product concentration over the reactant concentrations, each raised to its coefficient.
Substitute the current concentrations
Use the concentrations in the mixture now, not equilibrium values.
Compare Q with K
Q = 12.5 is less than K = 50.0, so the mixture holds too little HI. The reaction runs forward, using up H₂ and I₂, until Q rises to K.
Result
It is not at equilibrium. Q = 12.5 < K = 50.0, so the reaction shifts forward and more HI forms.
Your turn
Another mixture holds 0.0100 M H₂, 0.0100 M I₂ and 0.100 M HI. Which way does it shift?
Show the answer and explanation
In reverse: some HI decomposes.
Q = (0.100)²/((0.0100)(0.0100)) = 100, which is greater than K = 50.0, so the reaction runs in reverse until Q falls to K.
Keep exploring
In Equilibrium & ICE tables, switch the same mixture to an ICE table. It settles at 0.602 M HI, 0.0488 M H₂ and 0.149 M I₂, where Q equals 50.0.
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Compare Q and K in Equilibrium & ICE tables Check Q in Math Open worked example on a board Chemistry formulas: equilibrium and acidsYour existing work stays on this device. Examples open as editable copies.