18 – Investigating Effects on the State of Equilibrium (LC 2027) (Leaving Cert Chemistry): Revision Notes

18 – Investigating Effects on the State of Equilibrium

Introduction

This experiment explores how changes in concentration and temperature can affect the position of chemical equilibrium. We'll investigate these effects using the reaction between iron(III) chloride and potassium thiocyanate, which produces a distinctive colour change that makes it easy to observe equilibrium shifts.

The chemical system we're studying involves the following equilibrium:

$\text{Fe}^{3+} + \text{CNS}^{-} \rightleftharpoons \text{Fe(CNS)}^{2+}$

(yellow) (colourless) (red)

In this reaction, we use iron(III) chloride (FeCl₃) as our source of Fe³⁺ ions and potassium thiocyanate (KCNS) as our source of CNS⁻ ions. When these combine, they form a complex ion called Fe(CNS)²⁺, which gives the solution its characteristic red colour.

The equilibrium system

The equilibrium can be written more completely as:

$\text{FeCl}_3 + \text{CNS}^{-} \rightleftharpoons \text{Fe(CNS)}^{2+} + 3\text{Cl}^{-}$

(yellow) (colourless) (red)

This colour change from yellow to red makes it perfect for observing how Le Chatelier's Principle works in practice. When the equilibrium lies to the left (favouring reactants), the solution appears yellow. When it shifts to the right (favouring products), the solution turns red.

Part (a): Investigating the effect of concentration

Procedure

The first part of this experiment examines how adding different substances affects the equilibrium position:

Observations and explanations

Key principle

These observations demonstrate Le Chatelier's Principle perfectly: when you disturb an equilibrium by changing concentrations, the system responds by shifting in the direction that opposes the change.

Part (b): Investigating the effect of temperature

Understanding the reaction type

Before examining temperature effects, it's important to know that this equilibrium reaction is exothermic in the forwards direction:

$\text{Fe}^{3+} + \text{CNS}^{-} \rightleftharpoons \text{Fe(CNS)}^{2+} + \text{heat}$

This means heat is released when the red complex forms, and heat is absorbed when it breaks down.

Procedure

Observations and explanations

Temperature-equilibrium relationship

This experiment beautifully demonstrates how temperature affects equilibrium position:

Practical applications

Understanding equilibrium shifts has many real-world applications:

• Industrial processes: Optimising reaction conditions for maximum yield
• Environmental chemistry: Understanding how temperature changes affect natural chemical cycles
• Biological systems: Many enzyme reactions involve equilibrium shifts

Exam tips

• Colour memory: Remember yellow = reactants, red = products
• Le Chatelier's rule: The equilibrium always shifts to oppose the change
• Temperature effects: Know whether the reaction is exothermic or endothermic
• Practice explaining: Be able to explain observations using Le Chatelier's Principle