Ever wondered why some reactions make things hot whilst others...
Chemistry61 views·Updated 20 Aug 2026·3 pagesUnderstanding Exothermic and Endothermic Reactions
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Maya Ahad@mayaahad
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Energy Transfer in Reactions
Here's something brilliant about the universe - energy is always conserved during chemical reactions. The total amount of energy before and after a reaction stays exactly the same, it just moves around.
When a reaction transfers energy to its surroundings, the product molecules end up with less energy than the reactants. Think of it like this - the "missing" energy hasn't disappeared, it's escaped into the surroundings as heat.
Exothermic reactions are the heat-givers. They transfer energy to the surroundings, making everything around them warmer. Remember: exothermic means energy "exits" the reaction. You'll see this in combustion, oxidation reactions, and neutralisation.
Endothermic reactions do the opposite - they're energy takers. They absorb energy from the surroundings, making things cooler. Think "endothermic = energy enters" the reaction. Thermal decomposition and some fizzy reactions work this way.
Real-world tip: Self-heating coffee cans use exothermic reactions, whilst sports injury ice packs rely on endothermic ones!
2
of 3
Reaction Profiles
Reaction profiles are like energy maps that show what happens during chemical reactions. They're dead useful for understanding why some reactions need a kick-start whilst others happen easily.
The most important concept here is activation energy - this is the minimum energy that particles need to actually react when they collide. Think of it as the energy hill that reactants must climb before they can transform into products.
You can instantly tell if a reaction is exothermic or endothermic by looking at the energy levels. In exothermic reactions, products sit lower than reactants on the energy scale because energy has been released. For endothermic reactions, it's the opposite - products are higher because they've absorbed energy.
These profiles show you three key things: how much energy reactants and products have, the activation energy barrier, and the overall energy change. Once you can read these diagrams, predicting reaction behaviour becomes much easier.
Exam tip: If products are lower than reactants on the profile, it's exothermic. Higher means endothermic!
3
of 3
Calculating Energy Changes
Understanding bond energies is your key to predicting whether reactions will heat up or cool down. Every chemical reaction involves two main energy steps that you need to master.
First, energy must be supplied to break the bonds in reactants - this always requires energy input. Then, when new bonds form in the products, energy gets released. The balance between these two steps determines the overall energy change.
Here's the magic formula: Energy change = Energy needed to break bonds - Energy released forming bonds. If your answer is negative, you've got an exothermic reaction. A positive answer means endothermic.
This calculation method lets you predict reaction behaviour before you even do the experiment. It's particularly handy for working out whether a reaction will be useful for heating or cooling applications.
Calculation reminder: Negative result = exothermic (energy released), Positive result = endothermic (energy absorbed)!
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Chemistry61 views·Updated 20 Aug 2026·3 pagesUnderstanding Exothermic and Endothermic Reactions
M
Maya Ahad@mayaahad
Ever wondered why some reactions make things hot whilst others cool them down? Energy changes in chemical reactions are happening all around you - from the warmth of hand warmers to the cooling effect of sports injury packs. Understanding how...
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of 3
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Energy Transfer in Reactions
Here's something brilliant about the universe - energy is always conserved during chemical reactions. The total amount of energy before and after a reaction stays exactly the same, it just moves around.
When a reaction transfers energy to its surroundings, the product molecules end up with less energy than the reactants. Think of it like this - the "missing" energy hasn't disappeared, it's escaped into the surroundings as heat.
Exothermic reactions are the heat-givers. They transfer energy to the surroundings, making everything around them warmer. Remember: exothermic means energy "exits" the reaction. You'll see this in combustion, oxidation reactions, and neutralisation.
Endothermic reactions do the opposite - they're energy takers. They absorb energy from the surroundings, making things cooler. Think "endothermic = energy enters" the reaction. Thermal decomposition and some fizzy reactions work this way.
Real-world tip: Self-heating coffee cans use exothermic reactions, whilst sports injury ice packs rely on endothermic ones!
2
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Reaction Profiles
Reaction profiles are like energy maps that show what happens during chemical reactions. They're dead useful for understanding why some reactions need a kick-start whilst others happen easily.
The most important concept here is activation energy - this is the minimum energy that particles need to actually react when they collide. Think of it as the energy hill that reactants must climb before they can transform into products.
You can instantly tell if a reaction is exothermic or endothermic by looking at the energy levels. In exothermic reactions, products sit lower than reactants on the energy scale because energy has been released. For endothermic reactions, it's the opposite - products are higher because they've absorbed energy.
These profiles show you three key things: how much energy reactants and products have, the activation energy barrier, and the overall energy change. Once you can read these diagrams, predicting reaction behaviour becomes much easier.
Exam tip: If products are lower than reactants on the profile, it's exothermic. Higher means endothermic!
3
of 3Sign up to see the content. It's free!
- Access to all documents
- Improve your grades
- Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Calculating Energy Changes
Understanding bond energies is your key to predicting whether reactions will heat up or cool down. Every chemical reaction involves two main energy steps that you need to master.
First, energy must be supplied to break the bonds in reactants - this always requires energy input. Then, when new bonds form in the products, energy gets released. The balance between these two steps determines the overall energy change.
Here's the magic formula: Energy change = Energy needed to break bonds - Energy released forming bonds. If your answer is negative, you've got an exothermic reaction. A positive answer means endothermic.
This calculation method lets you predict reaction behaviour before you even do the experiment. It's particularly handy for working out whether a reaction will be useful for heating or cooling applications.
Calculation reminder: Negative result = exothermic (energy released), Positive result = endothermic (energy absorbed)!
We thought you’d never ask...
Our AI Companion is a student-focused AI tool that offers more than just answers. Built on millions of Knowunity resources, it provides relevant information, personalised study plans, quizzes, and content directly in the chat, adapting to your individual learning journey.
You can download the app from Google Play Store and Apple App Store.
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Students love us — and so will you.
4.6/5App Store
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The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan SiOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
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