Quick chemistry question (Le Chatalier's Principle)

nice question.

what we're looking at when we say 2 moles --> 1 mole is that 2 moles would take up more volume, while 1 mole would take up less. So if you increase pressure, you want to decrease volume and it shifts right. Lower pressure, and you want to increase volume and shift it left.

When you add nitrogen while maintaining a constant volume, you are not changing the partial pressures of A+B relative to AB at all (partial pressure is (mol*temp*R)/L - none of these are changing). So there will be no effect on equilibrium because no shifts due to volume change are needed.

When you add nitrogen while maintaining a constant pressure, however, the increase in the total moles of gas causes the volume of the whole system to increase. To compensate for this increased volume, the reaction must shift to the side with more moles (to the left). (This makes sense for an ideal gas because the helium molecules are assumed to have no inherent volume. A(g), B(g) and AB(g) are free to fill up the whole container.)