Exothermic Reactions
Lesson notes
Overview
Products sit lower in energy than reactants, so heat is released to the surroundings. This topic is rated intermediate and is number 28 of 110 in Chemistry. Builds on the basics with relationships, methods and trade-offs.
What you are seeing
The panel above shows particles colliding and reacting as conditions change. The animation is drawn live from the model, so everything you see responds to the sliders. Press Space to pause, use Restart to replay from the start, and change the speed to slow down quick events.
In depth
In an exothermic reaction the products are lower in energy than the reactants, so energy is released to the surroundings, usually as heat, and the temperature of the surroundings rises. The enthalpy change is negative, ΔH < 0. Combustion, respiration, neutralisation and hand warmers are examples. The energy comes from forming stronger bonds in the products than the ones broken in the reactants.
Reading the relationship
The expression "ΔH < 0" is a compact summary of the idea. It uses the symbols Δ, H; name what each one stands for before you start. Then use the sliders to see how changing one quantity changes the others.
A guided exploration
1. Watch the particles with the default conditions and note how often they react. 2. Raise the temperature or concentration and compare. 3. Decide which change speeds the reaction up most, and explain why on the particle level.
Try this
Predict the outcome first, then test extreme slider values to find where the pattern changes or breaks.
Common pitfalls
Keep the particle picture in mind. Mistakes often come from treating a rate, an amount and an equilibrium position as the same thing, or from ignoring units and conditions.
How it connects
It builds on “Catalysts”, the topic before it in Chemistry. Once you are comfortable, continue with “Endothermic Reactions”.
Why it matters
Chemistry links the tiny scale of atoms to the materials, medicines and energy we use every day.
Where you might use it
- Revising “Exothermic Reactions” before a class, exam or certification
- Predicting how fast a reaction runs in a lab or an industrial plant
- Choosing materials, medicines and fuels based on how their atoms bond and react
- Reading the labels and data sheets of cleaning products, batteries and food
Frequently asked questions
What is Exothermic Reactions?
Products sit lower in energy than reactants, so heat is released to the surroundings. It is an intermediate topic in Chemistry.
How do I use the Exothermic Reactions simulation?
The panel at the top shows particles colliding and reacting as conditions change. Move the sliders one at a time, then press Space to pause the animation and look closer.
What does the formula mean?
The expression ΔH < 0 packs the idea into symbols. Name each symbol first, then change one quantity with the sliders and see which others respond.
Who is this topic for?
Builds on the basics with relationships, methods and trade-offs. If the quiz feels hard, reread the lesson notes and try the previous topic first.
What should I learn before this?
Start with “Catalysts”, the previous topic in Chemistry. Each topic in a subject builds on the ones before it.
How do I track my progress?
Press “Mark as learned” on this page. Progress is stored only in this browser and is never sent anywhere. The Browse page can show topics you have learned or still need to learn.
Quick quiz
More in Chemistry
See all →Endothermic Reactions
Products have more energy than reactants: the reaction absorbs heat and feels cold.
Temperature and Reaction Rate
A hotter mixture has more collisions that clear the energy barrier.
Chemical Equilibrium
Forward and reverse reactions reach equal rates and concentrations stop changing.