Change in Pressure

[Stevin1H]

Can someone explain the concept of how a decrease in Pressure will favor the production of the side with more moles of gas in a reaction and how an increase in pressure will favor the side with fewer moles of gas?

[Chem_Mod]

This is a short cut for changing pressure by reducing V, reaction going toward the direction with less moles of gases will help reduce the effect of increasing the pressure. The more standard approach will be looking at the effect of concentration change

[gwynlu1L]

It also helps to remember that concentration = n (moles)/L (volume). Since moles do not change, a change in volume would change the concentrations. If it helps to understand the short cuts, you can try plugging in the new concentration values you'd get from changing the volume into the Q equation.