Enthalpy Diagrams
Simple Explanation
An enthalpy diagram (also called a reaction energy diagram) plots enthalpy on the vertical axis and shows reactants and products as horizontal lines at their respective enthalpy levels β the vertical gap between them, drawn as an arrow, represents ΞH.
Why Do We Need It?
These diagrams make it immediately visual whether a reaction is exothermic (products drawn lower than reactants) or endothermic (products drawn higher), and are used constantly to summarize a reaction's energetics at a glance.
Why Does This Work?
Placing reactants and products at heights proportional to their actual enthalpy directly represents the physical quantity being described β the vertical distance between the two levels is literally |ΞH|, and the direction of the arrow (down for exothermic, up for endothermic) shows the direction of net energy flow.
Real-Life Example
Combustion reaction diagrams in textbooks
Every textbook description of burning fuel shows an enthalpy diagram with reactants high and products low.
Combustion is strongly exothermic, so fuel + oxygen (reactants) is always drawn well above carbon dioxide + water (products), with a large downward arrow labelled with the (negative) ΞH of combustion.
Practice
On an enthalpy diagram, the products are drawn ABOVE the reactants. What does this tell you about the reaction?
MediumCommon mistake
Confusing an enthalpy diagram with a reaction-progress (activation energy) diagram β an enthalpy diagram only shows the start and end levels and ΞH; it doesn't show the energy barrier (activation energy) the reaction has to climb over first, which is a separate concept covered in reaction kinetics.
Quick Review
- Enthalpy diagrams plot reactants and products at their enthalpy levels.
- The vertical gap between them represents ΞH.
- Products lower than reactants = exothermic; higher = endothermic.