I got a question about noble gas in a equilibrium system?

Question

At constant pressure or/and constant volume
will it shift the equilibrium? Personally i think so- under some conditions
But i dont know why.
Can anyone help me?

Answer 1

At constant pressure it wont. Noble gases are unreactive, thus cannot chemically disturb the equilibrium.

At constant volume, however the noble gas will. Remember that you are introducing more particles in the system, thus at constant volume the pressure will increase. This will shift the equilibrium of the reacting substances to the side with less gas particles to reduce the pressure. This is different, as the gas is PHYSICALLY disturbing the equilibrium.

Need more explanation?

Answer 2

If "at constant pressure" means after the noble gas is added the pressure remains unchanged then there will be no effect on the equilibrium.

However if that isn't the case and adding a noble gas will increase the pressure on the system then the equilibrium may or may not be affected. You need the balanced equation; look at the coefficients for all the GASES in the system. If the total moles of gas are different for reactants and products then the increased pressure will cause a shift in the direction of the least gas. If the moles of reactant gases and moles of product gases are equal then there will be no effect.

Examples:

2H2 + O2 = 2H20 (all gases)
there are 3 moles of reactant gases and 2 moles of product gases. Increased pressure will cause the forward reaction to speed up since that would USE 3 moles of reactant gases and PRODUCE only 2moles of product, thus reducing the pressure.

H2 + Cl2 = 2HCl (all gases)

There are 2 moles of reactant gases and 2 moles of product gases. There will be no effect on this equilibrium.