Final answer:
Decreasing the volume of a container leads to an increase in pressure, affecting the concentration of gases but not solids or liquids. In a chemical equilibrium, the equilibrium will shift to favor the side with fewer moles of gas to reduce pressure.
Step-by-step explanation:
When the volume of a container holding a chemical equilibrium system is decreased, it leads to an increase in pressure. The effect of this change on the concentration of the chemical species depends on their state of matter. For solids and liquids, their concentration remains unaffected by changes in pressure or volume. However, for gases, a decrease in volume results in an increase in their concentration. This is because gases compress under pressure, leading to a higher number of particles per unit volume.
According to Le Chatelier's principle, an increase in pressure (by decreasing volume) will cause the equilibrium to shift in the direction that reduces pressure. This usually favors the reaction that produces fewer moles of gas since a smaller number of gas molecules will result in lower pressure. Therefore, if one side of a reaction has a different number of moles of gas than the other, the equilibrium will shift to accommodate the pressure change.
For example, consider the reaction: 2 SO₂(g) + O₂(g) ⇒ 2 SO₃(g). If the system volume is decreased, the equilibrium will shift towards the side with fewer gas moles, which in this case is the product side (2 mol of SO₃(g)) versus the reactant side (3 mol total: 2 mol of SO₂(g) and 1 mol of O₂(g)).