Short Answer 
Le Chatelier’s principle indicates that a system in equilibrium will adjust to counteract changes in conditions, such as volume reduction causing increased pressure. In the reaction involving phosphorus trichloride (PCl3) and chlorine gas (Cl2) forming phosphorus pentachloride (PCl5), reducing the volume results in a shift towards PCl5, which has fewer gas molecules and helps reduce the overall pressure.
Step 1: Understand Le Chatelier’s Principle
Le Chatelier’s principle states that if a system at equilibrium experiences a change in conditions, such as volume, pressure, or temperature, the equilibrium will shift in a direction that counteracts that change. This principle is crucial in predicting how a system will respond to alterations in its environment. Specifically, when the volume of a system is reduced, the pressure increases, prompting a shift to balance the system.
Step 2: Identify the Reaction Components
In the given chemical reaction involving phosphorus trichloride (PCl3) and chlorine gas (Cl2), it’s essential to note the number of gas molecules on each side of the reaction. Since the reaction forms phosphorus pentachloride (PCl5), we can analyze the changes in gas volume:
- Reactants: 1 PCl3 + 1 Cl2 = 2 gas molecules
- Product: 1 PCl5 = 1 gas molecule
Step 3: Apply the Principle to Predict the Shift
Upon reducing the volume of the system, Le Chatelier’s principle suggests that the equilibrium will shift to decrease pressure. Therefore, the system will favor the side with fewer gas molecules to lessen the total volume and thus the pressure:
- Shifts towards PCl5 formation because it has fewer gas molecules (1 vs. 2).
- This shift leads to decreased gas collisions and ultimately lowers the pressure.