What is Effect of Pressure on Equilibrium?

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Grade Level:

Class 12

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Definition

What is it?

The effect of pressure on equilibrium describes how changing the pressure of a system at equilibrium (especially for reactions involving gases) can shift the reaction to favour either the reactants or products. This shift happens to reduce the stress caused by the pressure change, following Le Chatelier's Principle.

Simple Example

Quick Example

Imagine you have a pressure cooker making dal. When you increase the pressure inside, the dal cooks faster because the chemical reactions happen more efficiently. Similarly, in a chemical reaction involving gases, increasing pressure can push the reaction towards the side that produces fewer gas molecules, just like how a crowded train compartment tries to make space.

Worked Example

Step-by-Step

Let's consider the reaction: N2(g) + 3H2(g) <=> 2NH3(g) (Haber Process for Ammonia)

Step 1: Identify the number of moles of gas on the reactant side. Here, 1 mole of N2 + 3 moles of H2 = 4 moles of gas.
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Step 2: Identify the number of moles of gas on the product side. Here, 2 moles of NH3 = 2 moles of gas.
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Step 3: Analyze the effect of increasing pressure. According to Le Chatelier's Principle, the system will try to relieve the stress. To do this, it will shift towards the side with fewer gas moles.
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Step 4: Compare moles: Reactant side has 4 moles, Product side has 2 moles. The product side has fewer moles.
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Step 5: Conclude the shift. Therefore, increasing pressure will shift the equilibrium to the right (towards the products), favouring the formation of ammonia.
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Step 6: Analyze the effect of decreasing pressure. If pressure is decreased, the system will shift towards the side with more gas moles to increase the pressure. In this case, it would shift to the left (towards reactants).

Why It Matters

Understanding pressure's effect on equilibrium is crucial in many industries. Chemical engineers use this principle to design efficient factories for making fertilisers (like urea for Indian farms) or plastics. It's also vital in medicine for understanding gas exchange in the lungs and even in space technology for designing life support systems.

Common Mistakes

MISTAKE: Thinking pressure changes affect reactions with only solids or liquids. | CORRECTION: Pressure changes significantly affect equilibrium ONLY when gases are involved, especially when there's a change in the total number of gas moles.

MISTAKE: Confusing the direction of shift – assuming increased pressure always shifts to products. | CORRECTION: The shift depends on which side (reactants or products) has fewer gas moles. Increased pressure shifts to the side with fewer gas moles, decreased pressure shifts to the side with more gas moles.

MISTAKE: Not considering the stoichiometry (mole ratios) of the balanced equation. | CORRECTION: Always calculate the total number of gaseous moles on both the reactant and product sides from the balanced chemical equation before determining the shift.

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) because there are 3 moles of gas on the reactant side and 2 moles on the product side.

QUESTION: Consider the reaction H2(g) + I2(g) <=> 2HI(g). If the pressure is increased, how will the equilibrium shift? | ANSWER: The equilibrium will not shift. There are 2 moles of gas on the reactant side and 2 moles on the product side, so there is no change in the number of gas moles.

QUESTION: A chemical plant produces nitric oxide using the reaction N2(g) + O2(g) <=> 2NO(g). If the plant wants to increase the yield of NO, should they operate at high pressure or low pressure? Explain. | ANSWER: Neither high nor low pressure will affect the yield of NO. There are 2 moles of gas on the reactant side and 2 moles on the product side. Since the number of gaseous moles is equal on both sides, a change in pressure will not shift the equilibrium.

MCQ

Quick Quiz

For the reaction PCl5(g) <=> PCl3(g) + Cl2(g), what will happen if the pressure is decreased?

The equilibrium will shift to the left

The equilibrium will shift to the right

The equilibrium will not shift

The reaction will stop

The Correct Answer Is:

Decreasing pressure shifts the equilibrium towards the side with more gas moles. In this reaction, the reactant side has 1 mole of gas, and the product side has 2 moles of gas (1 PCl3 + 1 Cl2). So, the shift is to the right.

Real World Connection

In the Real World

In India, industrial processes like the production of sulphuric acid (used in fertilisers and car batteries) involve gas-phase reactions. Engineers at companies like Reliance Industries or Tata Chemicals carefully control pressure in large reactors to ensure the maximum desired product is formed, making the process cost-effective and efficient.

Key Vocabulary

Key Terms

EQUILIBRIUM: A state where the rates of forward and reverse reactions are equal, and concentrations of reactants and products remain constant. | LE CHATELIER'S PRINCIPLE: If a system at equilibrium is subjected to a change, it will adjust itself to counteract the effect of the change. | MOLES: A unit of amount of substance, often represented by 'mol'. | REACTANTS: Starting materials in a chemical reaction. | PRODUCTS: Substances formed as a result of a chemical reaction.

What's Next

What to Learn Next

Now that you understand how pressure affects equilibrium, you should explore 'What is Effect of Temperature on Equilibrium?' and 'What is Effect of Concentration on Equilibrium?'. These concepts, combined with pressure, complete your understanding of Le Chatelier's Principle and how to control chemical reactions.