Advanced Chemical Principles and Molecular Dynamics · 6th Year · Rates of Reaction · Summer Term

Reaction Profile Diagrams

Learn to draw and interpret energy level diagrams that show the energy changes during a reaction, including activation energy and enthalpy change.

TL;DR:Let's map out the energy journey of a chemical reaction. These diagrams show us the 'hill' molecules must climb to transform and whether they release or absorb energy overall.

• NCCA Curriculum Specifications
• Leaving Certificate Chemistry Syllabus: Physical Chemistry - Rates of Reaction

About This Topic

Reaction profile diagrams are a cornerstone of the Leaving Certificate Chemistry syllabus, bridging the topics of chemical kinetics and thermodynamics. This visual tool is essential for students to grasp how energy changes over the course of a reaction. For the Irish curriculum, these diagrams provide a clear, graphical representation of abstract concepts like activation energy (Ea) and enthalpy change (ΔH), which are frequently examined in Section B of the Leaving Cert paper. Understanding these diagrams allows students to not only classify reactions as exothermic or endothermic but also to explain the mechanism of catalysis, a key industrial and biological principle.

Teaching this topic effectively involves moving beyond rote memorisation of the diagram's shape. It requires students to interpret the meaning of each part of the curve: the initial energy of the reactants, the peak representing the unstable activated complex, and the final energy of the products. By comparing catalysed and uncatalysed pathways on the same axes, students can concretely see how catalysts provide an alternative, lower-energy route, thereby increasing the reaction rate without altering the overall thermodynamics. This foundational knowledge is crucial for further study in chemistry and related science fields at third level.

Key Questions

1. Analyse a reaction profile to determine if a reaction is exothermic or endothermic.
2. Compare the reaction profiles for a reaction with and without a catalyst.
3. Explain the meaning of the activated complex in a reaction profile.

Learning Objectives

• Draw and accurately label reaction profile diagrams for exothermic and endothermic reactions.
• Identify reactants, products, activation energy (Ea), enthalpy change (ΔH), and the activated complex on a diagram.
• Explain the function of a catalyst in terms of providing an alternative reaction pathway with a lower activation energy.
• Compare and contrast the reaction profiles for catalysed and uncatalysed reactions.
• Deduce whether a reaction is exothermic or endothermic by analysing its reaction profile diagram.

Key Vocabulary

Reaction Profile Diagram	A graph showing the change in potential energy of a chemical system as a reaction progresses from reactants to products.
Activation Energy (Ea)	The minimum energy which colliding particles must have for a reaction to occur.
Enthalpy Change (ΔH)	The heat change of a reaction carried out at constant pressure.
Activated Complex	An unstable, high-energy arrangement of atoms formed at the transition state, which is the peak of the activation energy barrier.
Catalyst	A substance that alters the rate of a chemical reaction without being consumed in the reaction itself.

Watch Out for These Misconceptions

Common Misconception

A catalyst adds energy to the reaction to make it go faster.

What to Teach Instead

A catalyst does not provide energy. It provides an alternative reaction pathway with a lower activation energy, meaning more particles have sufficient energy to react upon collision.

Common Misconception

Exothermic reactions are always fast, and endothermic reactions are always slow.

What to Teach Instead

The speed of a reaction is determined by its activation energy, not its enthalpy change. A highly exothermic reaction, like the rusting of iron, can be very slow if it has a high activation energy.

Common Misconception

The activated complex is a stable intermediate product.

What to Teach Instead

The activated complex is a highly unstable, temporary arrangement of atoms that exists at the peak of the energy barrier. It immediately breaks down to form either products or reactants.

Active Learning Ideas

Concept Mapping

Whiteboard Reaction Stories

In pairs, students are given cards with data for different reactions (e.g., ΔH = -92 kJ/mol, Ea = 150 kJ/mol). They must draw the corresponding reaction profile diagram on a mini whiteboard, labelling all key features.

20 min·Pairs

Concept Mapping

Catalyst Sketch Challenge

Provide students with a pre-drawn reaction profile for an uncatalysed reaction. Their task is to accurately draw and label the reaction profile for the same reaction in the presence of a catalyst on the same set of axes.

15 min·Individual

Concept Mapping

Human Energy Profile

Use a chair or a stack of books as an 'energy barrier'. Students physically walk over the barrier to model a reaction, representing the activation energy needed to get from 'reactants' on one side to 'products' on the other.

10 min·Whole Class

Real-World Connections

• Enzymes in the human body, such as amylase in saliva, act as biological catalysts to speed up the breakdown of food at body temperature.
• Catalytic converters in car exhausts use catalysts like platinum and rhodium to accelerate the conversion of toxic gases into less harmful ones.
• The Haber process uses an iron catalyst to produce ammonia for fertilisers, making the reaction economically viable at lower temperatures and pressures.
• Instant cold packs used for sports injuries contain chemicals that undergo an endothermic reaction when mixed, absorbing heat from the surroundings.
• The hardening of cement is an exothermic process, releasing a significant amount of heat as the chemical reactions proceed.

Assessment Ideas

Quick Check

Use a 'show-me' activity with mini whiteboards. Present an unlabelled diagram and ask students to circle the activation energy or calculate the enthalpy change.

Quick Check

In a topic test, provide students with energy values for reactants, products, and the activated complex, and require them to draw and fully label the corresponding reaction profile diagram.

Quick Check

Provide a 'traffic light' worksheet where students colour-code their confidence level (red, amber, green) against each of the learning objectives for the topic.

Frequently Asked Questions

Can activation energy be negative?

No, activation energy is always a positive value. It represents the minimum energy barrier that must be overcome for a reaction to proceed, so it's always an energy input relative to the reactants.

Does a catalyst change the amount of product formed?

No, a catalyst does not affect the position of equilibrium or the overall yield of a reaction. It only increases the rate at which equilibrium is reached.

What's the difference between the 'transition state' and the 'activated complex'?

For the Leaving Certificate course, these terms can be used interchangeably. The transition state refers to the point of highest energy on the reaction profile, and the activated complex is the chemical species that exists at that point.