What is the Effect of Pressure on Equilibrium?

S6-SA4-0267

Grade Level:

Class 10

AI/ML, Physics, Biotechnology, Space Technology, Chemistry, Engineering, Medicine

Definition

What is it?

The effect of pressure on equilibrium describes how changing the pressure of a system can shift the balance between reactants and products in a reversible reaction. According to Le Chatelier's Principle, a system at equilibrium will try to counteract any change imposed on it, including pressure changes.

Simple Example

Quick Example

Imagine you are trying to fit too many people into a small auto-rickshaw. If more people try to get in (like increasing pressure), the auto-rickshaw will try to make space by pushing some people out or making them sit closer, trying to balance the crowd. Similarly, a chemical reaction tries to reduce the 'crowd' (pressure) by moving towards the side with fewer gas molecules.

Worked Example

Step-by-Step

Consider the reaction: N2(g) + 3H2(g) <=> 2NH3(g)
---Step 1: Identify the number of gas moles on the reactant side. Here, 1 mole of N2 and 3 moles of H2, so total = 1 + 3 = 4 moles of gas.
---Step 2: Identify the number of gas moles on the product side. Here, 2 moles of NH3, so total = 2 moles of gas.
---Step 3: Compare the moles. Reactants have 4 moles, products have 2 moles.
---Step 4: If pressure is increased, the system will shift to reduce the pressure. It does this by favouring the side with fewer gas moles.
---Step 5: Since the product side (2 moles) has fewer gas moles than the reactant side (4 moles), increasing pressure will shift the equilibrium to the right, forming more NH3.
---Step 6: If pressure is decreased, the system will shift to increase the pressure, favouring the side with more gas moles. In this case, it would shift to the left, forming more N2 and H2.
Answer: For N2(g) + 3H2(g) <=> 2NH3(g), increasing pressure shifts equilibrium to the right (products), and decreasing pressure shifts it to the left (reactants).

Why It Matters

Understanding pressure's effect on equilibrium is crucial in chemical engineering for designing efficient industrial processes, like making ammonia for fertilizers or sulfuric acid. It helps scientists in fields like biotechnology optimize reaction conditions and is even considered in space technology for life support systems where pressure control is vital.

Common Mistakes

MISTAKE: Thinking pressure changes affect all reactions equally. | CORRECTION: Pressure changes only significantly affect reactions involving gases, and only if there's a change in the total number of gas moles between reactants and products.

MISTAKE: Confusing the effect of adding an inert gas with increasing pressure by reducing volume. | CORRECTION: Adding an inert gas at constant volume does not change the partial pressures of reactants/products, so it doesn't shift equilibrium. Increasing pressure by reducing volume DOES change partial pressures and shifts equilibrium.

MISTAKE: Assuming increasing pressure always favours the product side. | CORRECTION: The shift depends on which side has fewer gas moles. Increasing pressure favours the side with fewer gas moles, whether it's reactants or products.

Practice Questions

Try It Yourself

QUESTION: For the reaction 2SO2(g) + O2(g) <=> 2SO3(g), what happens to the equilibrium if pressure is increased? | ANSWER: The equilibrium shifts to the right (towards products), as there are 3 moles of gas on the reactant side and 2 moles on the product side.

QUESTION: For the reaction H2(g) + I2(g) <=> 2HI(g), how does an increase in pressure affect the equilibrium? | ANSWER: An increase in pressure has no effect on the equilibrium, because there are 2 moles of gas on both the reactant side and the product side.

QUESTION: In the industrial synthesis of methanol, CO(g) + 2H2(g) <=> CH3OH(g), explain how adjusting pressure can maximize methanol yield. | ANSWER: To maximize methanol yield, the pressure should be increased. The reactant side has 1 mole CO + 2 moles H2 = 3 moles of gas. The product side has 1 mole CH3OH = 1 mole of gas. Increasing pressure shifts the equilibrium towards the side with fewer gas moles, which is the product side, thus increasing methanol production.

MCQ

Quick Quiz

For the reaction PCl5(g) <=> PCl3(g) + Cl2(g), what happens when the pressure is increased?

Equilibrium shifts to the right

Equilibrium shifts to the left

Equilibrium remains unchanged

The reaction stops

The Correct Answer Is:

The reactant side has 1 mole of gas (PCl5), while the product side has 1 mole PCl3 + 1 mole Cl2 = 2 moles of gas. Increasing pressure favours the side with fewer gas moles, which is the reactant side, so the equilibrium shifts to the left.

Real World Connection

In the Real World

In India, chemical plants producing fertilizers often use the Haber process (N2 + 3H2 <=> 2NH3). Engineers at these plants, like those at IFFCO, precisely control high pressure conditions (typically 150-350 atmospheres) to shift the equilibrium towards ammonia production, ensuring a high yield of fertilizer which is crucial for Indian agriculture.

Key Vocabulary

Key Terms

Equilibrium: A state where forward and reverse reaction rates are equal, and concentrations of reactants and products remain constant. | Le Chatelier's Principle: A principle stating that if a change of condition is applied to a system in equilibrium, the system will shift in a direction that relieves the stress. | Moles of Gas: The amount of gaseous substance, directly related to its volume and pressure. | Reversible Reaction: A reaction that can proceed in both forward and reverse directions.

What's Next

What to Learn Next

Now that you understand how pressure affects equilibrium, you should explore 'What is the Effect of Temperature on Equilibrium?'. This will show you another important factor that can shift chemical balance and build your knowledge of Le Chatelier's Principle even further.