[Ben Cohen]

Consider the equation

A (g) + B (g) C (g) + D(g)

If there is no A or B present, then Q is "undefined" and the reaction will proceed left. If there is no C or D present, Q is zero and the reaction will proceed right. However, if there is no B or C, which way will the reaction proceed?

To give a real example,

CH₄ + 2O₂  2H₂O + CO₂.

Thanks in advance!

[Raphael]

Look at your example.

CH₄ + 2O₂  2H₂O + CO₂.

If you take the oxygen out of the equation will you really get the water and CO2? is there still a reaction going on?

[curiouscat]

However, if there is no B or C, which way will the reaction proceed?

Won't proceed anywhere.