Equilibrium - chemistry (quiz 3 question1)

Question

Consider the following equilibrium reaction in a closed gas container with a constant volume: (= is the arrow that goes both ways) H2(g)+I2(g)=2HI(g) At some point, an amount of deuterium gas (D2) is added to the gasholder. Deuterium (D) is an isotope of hydrogen (H). Which statement is correct?

A. The equilibrium shifts to the right because of the pressure increase in the gasholder.

B. There is no shift of the equilibrium.

C. The balance shifts to the left.

D. None of the previous statements are correct.

The answer is D,

 could you explain why A is wrong? I thought, initially both sides have equal gas moles (=at equilibrium at this point), but when D2 (gas) is added, then that increases the number of gas moles on the left side, causing it to shift the equilibrium to the right.

Answer 1

Your reasoning would be correct if, in fact, D2 were identical to H2, but it isn't. So, to my way of thinking, adding D2 would be the same as adding say, argon (an inert gas). It shouldn't take part in the reaction, so option A would not be correct.