Advanced Chemical Principles and Molecular Dynamics · 6th Year

Active learning ideas

Effect of Concentration and Pressure Changes

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

• 15–45 min
• Pairs → Whole Class
• 3 activities

Activity 01

• Simulation Game
• 15 min · Whole Class

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.

Analyse the effect of adding more reactant to a system at equilibrium.

Facilitation Tip

Ask students to write the equation and identify heat as a 'product' or 'reactant' to help them predict the colour changes.

What to look for

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.

• Apply
• Analyze
• Evaluate
• Create
• Social Awareness
• Decision-Making

Activity 02

• Simulation Game
• 20 min · Small Groups

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.

Explain why changing pressure only affects equilibria involving gases.

Facilitation Tip

Ensure students focus on the final, settled colour after the pressure change, not the initial instantaneous change.

What to look for

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.

• Apply
• Analyze
• Evaluate
• Create
• Social Awareness
• Decision-Making

Activity 03

• Simulation Game
• 45 min · Small Groups

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.

Compare the effect of increasing pressure on reactions with different numbers of gas moles on each side.

Facilitation Tip

Provide a structured worksheet to guide their research on yield, rate, cost, and safety considerations.

What to look for

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.

• Apply
• Analyze
• Evaluate
• Create
• Social Awareness
• Decision-Making

A few notes on teaching this unit

Begin with a visual demonstration to anchor the abstract theory in a concrete observation. Use clear, consistent language when applying Le Châtelier's principle, always framing the system's response as 'opposing' the change. Link each principle directly to an industrial example like the Haber process to reinforce its real-world relevance.

Upon completing this topic, students will be able to analyse any equilibrium system and confidently predict how it will shift in response to changes in concentration, pressure, or temperature, and explain why only temperature alters Kc.

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Watch Out for These Misconceptions

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

  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.

• Equilibrium means the reactions have stopped.

  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.

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

  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.

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Methods used in this brief

• Simulation Game