Reaction Types and Energy Diagrams

Overview

This lecture explains exothermic and endothermic reactions, how to represent them with energy profile diagrams, and the concept of activation energy.

Exothermic reactions transfer energy from reacting molecules to the surroundings, increasing the temperature.
Examples include combustion (burning), certain oxidation reactions, and neutralization.
On an energy profile diagram, products have less energy than reactants.
The difference in energy between reactants and products represents energy released to the surroundings.
Exothermic reactions are used in hand-warmers and self-heating cans of food or drinks.

Endothermic reactions absorb energy from the surroundings, decreasing the temperature.
An example is thermal decomposition.
On an energy profile diagram, products have more energy than reactants.
The difference in energy between reactants and products shows how much energy is absorbed.

Both exothermic and endothermic reactions show a rise to a peak in energy on the diagram.
The vertical difference between reactants and products shows energy released (exothermic) or absorbed (endothermic).
You may be asked to label energy changes on these diagrams in exams.

Activation energy is the minimum energy particles need to collide and react.
It is shown as the energy from reactants up to the peak of the energy profile diagram.
Both exothermic and endothermic reactions require activation energy to start.

Exothermic reaction — A chemical reaction that releases energy to the surroundings.
Endothermic reaction — A chemical reaction that absorbs energy from the surroundings.
Energy profile diagram — A graph showing energy changes during a chemical reaction.
Activation energy — The minimum energy needed for particles to react.

Practice drawing and labeling energy profile diagrams for exothermic and endothermic reactions.
Learn examples of each type of reaction for exams.
Review thermal decomposition and neutralization in detail in future lessons.