[dc17]

The question is as the title says. Lets say, for example, one of the reactants (H2) has a partial pressure of 0 atm at a certain point. My task is to figure out the Q value. And with that, I could describe which way the reaction would shift.

But if the denominator includes 0, the entire denominator itself would be 0, and how is that possible?

TLDR: if Q=P_A/P_B and P_B=0, what is Q?

[Corribus]

When one of the products has a concentration of zero, this would imply that the reaction quotient approaches infinity. In turn this implies that the instantaneous driving force for the forward reaction is also infinite since the limit of ln(x) approaches infinity as x approaches infinity. (By converse, when a reactant has a concentration of zero, the reaction quotient approaches zero.)

Do note that concentration is really only a surrogate for the chemical activity under the assumption that the chemical activity is equal to the concentration under certain conditions. The equations you are used to frequently break down or are not mathematically processable at extreme limits. E.g., what does an infinite driving force mean in a practical sense? Numerically, very little - but I would interpret it to mean that when the product has zero concentration, as it often does initially, the ratio of the forward instantaneous rate to the backward instantaneous rate is as large as it can be - or is practically infinite. This is because when there is zero product present, there can be no backward reaction since there is nothing to react. This makes intuitive sense but can be difficult to express mathematically due to the fact that infinity is more of a concept than a number than can be processed using a practical calculation.