Quiz 3 Prep Fall 2015 Q#10

[Kim]

"For the reaction at equilibrium, HCl (g) + I₂ (s) <--> HI (g) + Cl₂ (g), how will an increase in the pressure (by decreasing the volume) affect the concentration of Cl2?" the answer is that it decreases but I don't understand why :(
thanks in advance!

[Lyndon_Bui_3J]

Short method: There is one mole of gas on the left (reactant) side and 2 moles of gas on the right (product side). Increasing pressure shift reaction to the side with less moles of gas, which in this case is the left (reactant) side. Thus, the reverse reaction is favored and the concentration of Cl2 (products) will decrease.