The Impact of Pressure on Chemical Equilibrium: Case Study of PCl5 Decomposition

Understanding the behavior of a system under changing conditions is crucial in various fields, including chemistry and chemical engineering. A prime example of such a system involves the gaseous decomposition of PCl5 into PCl3 and Cl2. The chemical reaction can be represented as follows:

PCl5? ? PCl3 Cl2

In this article, we will explore how the equilibrium of the reaction is affected when the pressure of the system is decreased. By leveraging Le Chatelier's principle, we can predict the direction in which the equilibrium will shift.

Counting the Moles of Gas

To understand how the pressure affects the equilibrium, we first need to count the moles of gas on each side of the reaction equation. This step is crucial as it helps us determine the direction of the shift.

Left Side: Reactants

On the left side of the reaction, the only component is PCl5. This means that the number of moles of gas on the left side is 1.

Right Side: Products

On the right side, the products are PCl3 and Cl2. Together, these two products have a total of 2 moles of gas.

The Effect of Decreasing Pressure on Equilibrium

According to Le Chatelier's principle, if a gaseous system at equilibrium is subjected to a change in pressure, the equilibrium will shift towards the side with fewer moles of gas to counteract the change and restore the pressure.

In this reaction, the right side of the equation has 2 moles of gas, while the left side has only 1 mole. Therefore, when the pressure is decreased, the equilibrium will shift towards the right side to counteract the reduction in pressure.

Explanation of Le Chatelier’s Principle

Le Chatelier's principle states that if a dynamic equilibrium is disturbed by changing the conditions such as concentration, temperature, or pressure, the position of equilibrium moves to counteract the change. In this specific case, the decrease in pressure will cause the equilibrium to shift towards the side with more moles of gas.

As a result, the equilibrium will favor the formation of PCl3 and Cl2 to counteract the decrease in pressure. This will lead to an increase in the concentrations of these products until a new equilibrium is established.

Additional Considerations

It's important to note that if the reactants and products are all in the solid state, with only chlorine being a gas at standard temperature and pressure (STP), the equilibrium would shift towards producing more chlorine gas to create the necessary pressure.

Moreover, in a closed system, the pressure would naturally increase as the reaction progresses due to stoichiometry. A decrease in pressure would then mitigate this effect by shifting the equilibrium towards the right.

Conclusion

In conclusion, decreasing the pressure of the system will shift the equilibrium towards the right, favoring the formation of PCl3 and Cl2. This results in an increase in their concentrations until a new equilibrium is achieved.

Therefore, it is clear that decreasing the pressure will push the equilibrium to the right, promoting the conversion of PCl5 into PCl3 and Cl2.