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Updated Mar 27, 2026
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GCSE AQA Energy Changes Revision Notes
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Energy Changes in Chemical Reactions
Think about lighting a bonfire - the flames give off heat and light, making you feel warmer. This happens because energy is always conserved in chemical reactions, but it can move between the reaction and its surroundings.
When a reaction releases energy to the surroundings, we call it exothermic. The product molecules end up with less energy than the reactants because they've given some away. You'll see this in combustion (like that bonfire), oxidation reactions, and neutralisation between acids and bases.
Endothermic reactions work the opposite way - they absorb energy from the surroundings, making things feel colder. Thermal decomposition and the reaction between citric acid and sodium hydrogencarbonate are perfect examples. Those instant cold packs for sports injuries? They're using endothermic reactions!
Key Point: Exothermic reactions heat things up (think hand warmers), whilst endothermic reactions cool things down (think sports injury packs).
Reaction Profiles and Bond Energy Calculations
Here's something brilliant: you can actually calculate exactly how much energy a reaction will transfer! Reaction profiles show you the energy journey - they display the activation energy (the minimum energy needed for particles to react) and whether products end up higher or lower in energy than reactants.
Breaking bonds always requires energy input, whilst forming new bonds always releases energy. The overall energy change depends on which process needs more energy. In exothermic reactions, more energy comes out from making new bonds than goes in to break old ones.
To calculate energy changes, you'll use bond energies from data tables. Add up all the energy needed to break reactant bonds, then subtract all the energy released when product bonds form. If your answer is negative, the reaction is exothermic; if positive, it's endothermic.
Top Tip: Remember the phrase "Breaking Bonds Requires Energy, Making Bonds Releases Energy" - this will help you get calculations right every time.
Cells, Batteries and Fuel Cells
Your phone, laptop, and TV remote all depend on chemical reactions to produce electricity. Simple cells work by connecting two different metals in an electrolyte - the voltage depends on which metals and electrolyte you choose.
Non-rechargeable batteries (like alkaline ones) stop working when their reactants run out - you bin them and buy new ones. Rechargeable batteries are cleverer because you can reverse their chemical reactions by plugging them into the mains, restoring the original reactants.
Hydrogen fuel cells offer an exciting alternative. They combine hydrogen gas with oxygen to produce electricity, with water as the only waste product. They're more efficient than batteries and refuel quickly, but hydrogen refueling stations are still rare and expensive.
Future Focus: Whilst rechargeable batteries dominate today's market, hydrogen fuel cells could power tomorrow's cars and homes with zero harmful emissions.
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THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE Knowunity AI. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
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This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
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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 S
iOS 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.
Samantha Klich
Android user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
iOS user
Best app on earth! no words because it’s too good
Thomas R
iOS user
Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
Android user
This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
Android user
I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE Knowunity AI. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user
GCSE AQA Energy Changes Revision Notes
G
G
@gurneet
Ever wondered how hand warmers get hot or why your phone battery eventually dies? Chemical reactions are constantly transferring energy around us, and understanding how this works is crucial for GCSE Chemistry. You'll master energy changes in reactions and discover... Show more
Energy Changes in Chemical Reactions
Think about lighting a bonfire - the flames give off heat and light, making you feel warmer. This happens because energy is always conserved in chemical reactions, but it can move between the reaction and its surroundings.
When a reaction releases energy to the surroundings, we call it exothermic. The product molecules end up with less energy than the reactants because they've given some away. You'll see this in combustion (like that bonfire), oxidation reactions, and neutralisation between acids and bases.
Endothermic reactions work the opposite way - they absorb energy from the surroundings, making things feel colder. Thermal decomposition and the reaction between citric acid and sodium hydrogencarbonate are perfect examples. Those instant cold packs for sports injuries? They're using endothermic reactions!
Key Point: Exothermic reactions heat things up (think hand warmers), whilst endothermic reactions cool things down (think sports injury packs).
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Reaction Profiles and Bond Energy Calculations
Here's something brilliant: you can actually calculate exactly how much energy a reaction will transfer! Reaction profiles show you the energy journey - they display the activation energy (the minimum energy needed for particles to react) and whether products end up higher or lower in energy than reactants.
Breaking bonds always requires energy input, whilst forming new bonds always releases energy. The overall energy change depends on which process needs more energy. In exothermic reactions, more energy comes out from making new bonds than goes in to break old ones.
To calculate energy changes, you'll use bond energies from data tables. Add up all the energy needed to break reactant bonds, then subtract all the energy released when product bonds form. If your answer is negative, the reaction is exothermic; if positive, it's endothermic.
Top Tip: Remember the phrase "Breaking Bonds Requires Energy, Making Bonds Releases Energy" - this will help you get calculations right every time.
Sign up to see the contentIt's free!
Access to all documents
Improve your grades
Join milions of students
Cells, Batteries and Fuel Cells
Your phone, laptop, and TV remote all depend on chemical reactions to produce electricity. Simple cells work by connecting two different metals in an electrolyte - the voltage depends on which metals and electrolyte you choose.
Non-rechargeable batteries (like alkaline ones) stop working when their reactants run out - you bin them and buy new ones. Rechargeable batteries are cleverer because you can reverse their chemical reactions by plugging them into the mains, restoring the original reactants.
Hydrogen fuel cells offer an exciting alternative. They combine hydrogen gas with oxygen to produce electricity, with water as the only waste product. They're more efficient than batteries and refuel quickly, but hydrogen refueling stations are still rare and expensive.
Future Focus: Whilst rechargeable batteries dominate today's market, hydrogen fuel cells could power tomorrow's cars and homes with zero harmful emissions.
We thought you’d never ask...
What is the Knowunity AI companion?
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.
Where can I download the Knowunity app?
You can download the app from Google Play Store and Apple App Store.
Is Knowunity really free of charge?
That's right! Enjoy free access to study content, connect with fellow students, and get instant help – all at your fingertips.
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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 S
iOS 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.
Samantha Klich
Android user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
iOS user
Best app on earth! no words because it’s too good
Thomas R
iOS user
Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
Android user
This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
Android user
I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE Knowunity AI. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user
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 S
iOS 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.
Samantha Klich
Android user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
iOS user
Best app on earth! no words because it’s too good
Thomas R
iOS user
Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
Android user
This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
Android user
I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE Knowunity AI. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user