[thepackage]

imagine you had 2 containers.
1 container had a 2L mixture of SO₂, SO₃ and O₂, at equilibrium.
the second container is 1L and contains O₂ only.

the concentration of O₂ for both these containers is 1M. The concentration of other components are not specified

The 2 containers are connected by a valve as shown in the image attached

When the tap is opened, and the substances from both compartments are allowed to mix, which way will the equilibirum shift, for the reaction 2SO₂+O₂ 2SO₃

i said the equilibrium would shift to the left after a overall decrease in the system's pressure, so the backward reaction would be favoured to produce more molecules and partially restore pressure.

however, from another perspective, if O₂ concentration remains constant but SO₂ and SO₃ decrease, then this will affect the concentration fraction. This will make Q<K_c because your O2 concentration remains the same, making the denominator larger, hence reducing Kc. So this would lead to a net forward reaction?

[Borek]

When simple rules of thumb give contradicting predictions it is time to do the real math.