Endothermic Reactions
Lesson notes
Overview
Products have more energy than reactants: the reaction absorbs heat and feels cold. This topic is rated intermediate and is number 29 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 endothermic reaction the products have more energy than the reactants, so the reaction takes in heat from the surroundings, which feel colder, and ΔH > 0. Photosynthesis, thermal decomposition, and the dissolving of ammonium nitrate in instant cold packs are examples. More energy is needed to break the bonds in the reactants than is released when new bonds form.
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 “Exothermic Reactions”, the topic before it in Chemistry. Once you are comfortable, continue with “Temperature and Reaction Rate”.
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 “Endothermic 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 Endothermic Reactions?
Products have more energy than reactants: the reaction absorbs heat and feels cold. It is an intermediate topic in Chemistry.
How do I use the Endothermic 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 “Exothermic Reactions”, 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 →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.
Le Chatelier’s Principle
Disturb an equilibrium and it shifts to partly undo the change. Change the rate constants to see it.