Advanced Chemical Principles and Molecular Dynamics · 6th Year · Chemical Equilibrium · Summer Term

Effect of Concentration and Pressure Changes

Apply Le Châtelier's principle to predict how changing the concentration of a substance or the overall pressure affects the position of equilibrium.

TL;DR:This topic explores the responsive nature of chemical reactions at equilibrium. We will investigate how these balanced systems react to disturbances, using Le Châtelier's principle to predict the outcomes.

• NCCA Curriculum Specifications
• Leaving Certificate Chemistry Syllabus: Physical Chemistry - Chemical Equilibrium

About This Topic

This topic delves into Le Châtelier's principle, a cornerstone of the Leaving Certificate Chemistry syllabus, focusing specifically on how systems at equilibrium respond to changes in concentration and pressure. It builds upon students' prior knowledge of reaction kinetics and introduces the concept of equilibrium as a dynamic state, not a static one. A key learning outcome is for students to differentiate between factors that shift the position of equilibrium (concentration, pressure) and the single factor that alters the value of the equilibrium constant, Kc (temperature).

The core of the topic involves analysing why a change in temperature affects the rates of the forward and reverse reactions to different extents, thereby changing the ratio of products to reactants at equilibrium and thus the value of Kc. For exothermic reactions, an increase in temperature favours the endothermic reverse reaction, decreasing Kc. Conversely, for endothermic reactions, an increase in temperature favours the forward reaction, increasing Kc. This understanding is crucial for explaining the conditions used in significant Irish and global industrial applications, such as the Haber process, providing a real-world context for these abstract principles.

Key Questions

1. Analyse the effect of adding more reactant to a system at equilibrium.
2. Explain why changing pressure only affects equilibria involving gases.
3. Compare the effect of increasing pressure on reactions with different numbers of gas moles on each side.

Learning Objectives

• State Le Châtelier's principle and use it to predict the effect of changes in concentration, pressure, and temperature on a system at equilibrium.
• Explain why a catalyst increases the rate at which equilibrium is reached but does not affect the position of equilibrium.
• Distinguish between factors that shift the equilibrium position and the unique effect of temperature on the value of the equilibrium constant, Kc.
• Justify the conditions of temperature and pressure used in the Haber process as a compromise between rate, yield, and cost.
• Interpret concentration-time graphs for reactions at equilibrium, including the effects of imposed changes on the system.

Key Vocabulary

Le Châtelier's Principle	States that if a stress is applied to a system at equilibrium, the system will shift in a direction that tends to relieve that stress.
Dynamic Equilibrium	A state in a reversible reaction where the rate of the forward reaction equals the rate of the reverse reaction.
Equilibrium Constant (Kc)	The ratio of the mathematical product of the concentrations of the products to that of the reactants for a reaction at equilibrium, with each concentration raised to the power of its stoichiometric coefficient.
Exothermic	A reaction that releases heat energy into the surroundings, resulting in a negative enthalpy change (ΔH).
Endothermic	A reaction that absorbs heat energy from the surroundings, resulting in a positive enthalpy change (ΔH).

Watch Out for These Misconceptions

Common Misconception

Adding a catalyst shifts the equilibrium to the right to make more product.

What to Teach Instead

A catalyst increases the rate of both the forward and reverse reactions equally. This means the system reaches equilibrium faster, but the final position of equilibrium and the value of Kc are unchanged.

Common Misconception

Equilibrium means the reactions have stopped.

What to Teach Instead

Equilibrium is a dynamic state. The forward and reverse reactions are still occurring, but their rates are equal, so there is no overall change in the concentrations of reactants and products.

Common Misconception

Any change that shifts the equilibrium position also changes the value of Kc.

What to Teach Instead

Changes in concentration or pressure will shift the position of equilibrium to counteract the change, but the ratio of products to reactants at the new equilibrium (Kc) remains the same. Only a change in temperature will alter the actual value of the equilibrium constant.

Active Learning Ideas

Simulation Game

The Cobalt(II) Chloride Equilibrium

A classic teacher demonstration where a pink solution of hydrated cobalt(II) ions is gently heated, causing it to turn blue as the equilibrium shifts. Cooling the solution in an ice bath reverses the change, visually and memorably demonstrating Le Châtelier's principle in action with temperature.

15 min·Whole Class

Simulation Game

Pressure Syringe Simulation

Using a sealed syringe containing a mixture of nitrogen dioxide (brown) and dinitrogen tetroxide (colourless) gas, students observe the colour change when the plunger is pushed in (increasing pressure) and pulled out. This provides a tangible link between pressure changes and shifts in gaseous equilibria.

20 min·Small Groups

Simulation Game

Industrial Process Case Study

Students work in groups to research an industrial process like the Haber process or the Contact process. They must explain why specific temperatures and pressures are used, justifying the conditions as a compromise between reaction rate, equilibrium yield, and economic cost.

45 min·Small Groups

Real-World Connections

• The Haber process for manufacturing ammonia, essential for producing agricultural fertilisers.
• The Contact process for producing sulfuric acid, a vital industrial chemical used in batteries, detergents, and pigments.
• The equilibrium between oxygen and haemoglobin in the bloodstream, which is sensitive to oxygen pressure changes between the lungs and body tissues.
• The carbonation of fizzy drinks, where the equilibrium between dissolved CO₂ and carbonic acid is dependent on pressure.
• Controlling emissions from car exhausts using catalytic converters, which speed up the attainment of equilibria that convert harmful gases into less harmful ones.

Assessment Ideas

Discussion Prompt

Use 'Think-Pair-Share' activities where students are given a specific equilibrium system and a change (e.g., increase in pressure). They individually predict the outcome, discuss with a partner, and then share their reasoning with the class.

Quick Check

Incorporate a multi-part question into a topic test based on a novel equilibrium system. Students must predict the effects of various changes and explain the unique impact of temperature on Kc, potentially including a calculation.

Quick Check

Provide students with past Leaving Certificate exam questions on the topic. They can attempt the questions and then mark their own work using the official marking scheme to identify areas of weakness.

Frequently Asked Questions

Why is temperature the only thing that changes Kc?

The value of Kc is determined by the relative rates of the forward and reverse reactions. Temperature changes these rates, but it affects the endothermic and exothermic directions to different extents. This changes the ratio of product to reactant concentrations at equilibrium, thus changing Kc. Concentration and pressure changes are counteracted by the system in a way that restores this original ratio.

How do I remember which way the equilibrium shifts for an exothermic reaction when it's heated?

For an exothermic reaction, heat is a product (Reactants ⇌ Products + Heat). According to Le Châtelier's principle, if you add heat (increase the temperature), the system will try to remove it by favouring the reverse, endothermic reaction. Therefore, the equilibrium shifts to the left.

If high pressure is expensive, why use it in the Haber process?

The forward reaction (N₂ + 3H₂ ⇌ 2NH₃) involves a decrease in the number of moles of gas (from 4 to 2). High pressure shifts the equilibrium to the right, favouring the side with fewer moles, which significantly increases the yield of ammonia. This increased yield justifies the high cost of maintaining the pressure.