Final answer:
A decrease in pressure will shift the equilibrium to the left for reaction B) 2SO3(g) ⇌ 2SO2(g) + O2(g), because Le Chatelier's Principle states that the system will favor the direction that increases the number of moles of gas when pressure is decreased.
Step-by-step explanation:
For which of the following reactions will a decrease in pressure shift the equilibrium to the left? The answer is B) 2SO3(g) ⇌ 2SO2(g) + O2(g). According to Le Chatelier's Principle, if the pressure of an equilibrium system is decreased, the system will adjust to increase pressure. This is generally accomplished by shifting towards the side that has a greater number of moles of gas. Looking at reaction B, when the pressure is decreased, the system will shift towards the production of more moles of gas, which is to the left: the decomposition of 2SO3 into 2SO2 and O2. This reaction increases from 2 to 3 moles of gas, and hence the equilibrium will shift left.