You and a friend are studying for a chemistry exam. What if your friend says, “A

Question

You and a friend are studying for a chemistry exam. What if your friend says, “Adding an inert gas to a system of gaseous components at equilibrium never changes the equilibrium position”? How do you explain to your friend that this holds true for a system at constant volume but is not necessarily true for a system at constant pressure? When would it hold true for a system at constant pressure?

If I add an inert gas, the pressure of the system should change. If the pressure changes, why the equilibrium position doesn't change?

Explanation / Answer

Let us have a reaction of    aA <--->. bB

let an inert gas represented by X, is added since inert gas doent nor react X will appear both sides

aA + X <--> bB + X

we see that addition of X will increase the total pressure of the system ( pressure is proportional to number of gas moles) . But the total pressure change doensot effect the equilibrium since equilibrium depends on partial pressure of gases which doenot change when inert gas is added at constant volume.

Let the presssure be constant hence volume changes. In this case clearly addition of gas changes the partial pressure of gas. Example if we had 1 atm total pressure then we have pA + pB = 1atm when inert gas is not added. When inert gas is added we get pA + pB + pX = 1atm . Thus partial pressures of A and B are changed and thus equilibrium position changed .

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