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Chemistry
7 Dec 2025
163
4 pages
Energy changes in chemical reactions are absolutely crucial for understanding chemistry. This topic covers enthalpy - the heat... Show more
Ever wondered why some reactions make things hot whilst others make them cold? It's all about enthalpy - the measure of heat energy present in any chemical system. Energy can never be created or destroyed, only transferred between the reaction system and its surroundings.
Enthalpy change (ΔH) tells you exactly how much energy gets released or absorbed during a chemical reaction. When reactions release energy to the surroundings, they're exothermic and have a negative ΔH value - you'll notice the temperature goes up.
When reactions absorb energy from the surroundings, they're endothermic with a positive ΔH value, causing temperatures to drop. The units are always measured in kJ mol⁻¹.
Quick Tip Remember that exothermic reactions feel warm because they're giving out energy, whilst endothermic reactions feel cool because they're taking energy in from their surroundings.
Picture this methane burning in oxygen - a classic exothermic reaction you see every time someone lights a gas cooker. The activation energy (Ea) represents the minimum energy bump that reactants need to overcome before the reaction kicks off.
In exothermic reactions, energy is needed initially to break existing bonds in the reactants. However, when new bonds form in the products, they release much more energy than was originally put in.
The enthalpy profile diagram shows reactants starting high and products ending up at a lower energy level. The difference between these levels is your negative ΔH value - that's the excess energy that gets released as heat.
Remember Think of exothermic reactions like rolling a ball down a hill - once you give it that initial push (activation energy), it releases loads of energy as it rolls down to a lower level.
Now consider calcium carbonate breaking down when heated - this is endothermic because you need to keep pumping in energy for it to happen. The activation energy barrier still exists, but here's the key difference from exothermic reactions.
In endothermic processes, breaking the original bonds requires more energy than you get back when forming the new bonds. This means the products end up at a higher energy level than the reactants started at.
The enthalpy profile shows this upward energy change clearly - reactants begin low and products finish high. The positive ΔH value represents all that extra energy that had to be absorbed from the surroundings to make the reaction work.
Memory Aid Endothermic reactions are like pushing a boulder uphill - you're constantly putting energy in, and the final position has more stored energy than where you started.
Here's something that trips up loads of students enthalpy changes vary depending on conditions. That's why chemists use standard conditions to ensure everyone's comparing like with like when they quote ΔH values.
Standard conditions are defined as 100 kPa pressure, 298 K temperature (that's 25°C), and 1 mol dm⁻³ for any solutions involved. When enthalpy changes are measured under these conditions, they get the special symbol ΔH⊖.
The standard state refers to the physical state each substance would naturally be in under standard conditions. For example, water's standard state is liquid, whilst oxygen exists as a gas.
Exam Tip Always remember to convert Celsius to Kelvin by adding 273 - this conversion appears in loads of exam questions and it's easy marks if you get it right!
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.
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Explore the key concepts of entropy, including its definition, the second law of thermodynamics, and how to predict and calculate entropy changes in chemical reactions. This summary provides essential insights into the behavior of energy dispersal and disorder in systems, making it a valuable resource for students studying thermodynamics. Type: Summary.
ChemistryUnit 10 of Chemistry Honors (Inorganic)
ChemistryExplore the differences between exothermic and endothermic reactions through detailed energy profiles. This summary covers key concepts such as energy changes, enthalpy change (ΔH), and how to interpret energy diagrams. Ideal for chemistry students looking to understand reaction thermodynamics and energy transfer in chemical processes.
ChemistryUnit 3 chemistry notes (10th grade)
ChemistryApp Store
Google Play
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 THE SCHOOLGPT. 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 THE SCHOOLGPT. 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
Energy changes in chemical reactions are absolutely crucial for understanding chemistry. This topic covers enthalpy - the heat energy involved when bonds break and form during reactions - and why some reactions release heat whilst others absorb it.
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Ever wondered why some reactions make things hot whilst others make them cold? It's all about enthalpy - the measure of heat energy present in any chemical system. Energy can never be created or destroyed, only transferred between the reaction system and its surroundings.
Enthalpy change (ΔH) tells you exactly how much energy gets released or absorbed during a chemical reaction. When reactions release energy to the surroundings, they're exothermic and have a negative ΔH value - you'll notice the temperature goes up.
When reactions absorb energy from the surroundings, they're endothermic with a positive ΔH value, causing temperatures to drop. The units are always measured in kJ mol⁻¹.
Quick Tip: Remember that exothermic reactions feel warm because they're giving out energy, whilst endothermic reactions feel cool because they're taking energy in from their surroundings.
Access to all documents
Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Picture this: methane burning in oxygen - a classic exothermic reaction you see every time someone lights a gas cooker. The activation energy (Ea) represents the minimum energy bump that reactants need to overcome before the reaction kicks off.
In exothermic reactions, energy is needed initially to break existing bonds in the reactants. However, when new bonds form in the products, they release much more energy than was originally put in.
The enthalpy profile diagram shows reactants starting high and products ending up at a lower energy level. The difference between these levels is your negative ΔH value - that's the excess energy that gets released as heat.
Remember: Think of exothermic reactions like rolling a ball down a hill - once you give it that initial push (activation energy), it releases loads of energy as it rolls down to a lower level.
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Now consider calcium carbonate breaking down when heated - this is endothermic because you need to keep pumping in energy for it to happen. The activation energy barrier still exists, but here's the key difference from exothermic reactions.
In endothermic processes, breaking the original bonds requires more energy than you get back when forming the new bonds. This means the products end up at a higher energy level than the reactants started at.
The enthalpy profile shows this upward energy change clearly - reactants begin low and products finish high. The positive ΔH value represents all that extra energy that had to be absorbed from the surroundings to make the reaction work.
Memory Aid: Endothermic reactions are like pushing a boulder uphill - you're constantly putting energy in, and the final position has more stored energy than where you started.
Access to all documents
Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Here's something that trips up loads of students: enthalpy changes vary depending on conditions. That's why chemists use standard conditions to ensure everyone's comparing like with like when they quote ΔH values.
Standard conditions are defined as 100 kPa pressure, 298 K temperature (that's 25°C), and 1 mol dm⁻³ for any solutions involved. When enthalpy changes are measured under these conditions, they get the special symbol ΔH⊖.
The standard state refers to the physical state each substance would naturally be in under standard conditions. For example, water's standard state is liquid, whilst oxygen exists as a gas.
Exam Tip: Always remember to convert Celsius to Kelvin by adding 273 - this conversion appears in loads of exam questions and it's easy marks if you get it right!
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.
That's right! Enjoy free access to study content, connect with fellow students, and get instant help – all at your fingertips.
13
Smart Tools NEW
Transform this note into: ✓ 50+ Practice Questions ✓ Interactive Flashcards ✓ Full Mock Exam ✓ Essay Outlines
Explore the key concepts of entropy, including its definition, the second law of thermodynamics, and how to predict and calculate entropy changes in chemical reactions. This summary provides essential insights into the behavior of energy dispersal and disorder in systems, making it a valuable resource for students studying thermodynamics. Type: Summary.
ChemistryUnit 10 of Chemistry Honors (Inorganic)
ChemistryExplore the differences between exothermic and endothermic reactions through detailed energy profiles. This summary covers key concepts such as energy changes, enthalpy change (ΔH), and how to interpret energy diagrams. Ideal for chemistry students looking to understand reaction thermodynamics and energy transfer in chemical processes.
ChemistryUnit 3 chemistry notes (10th grade)
ChemistryApp Store
Google Play
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 THE SCHOOLGPT. 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 THE SCHOOLGPT. 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
Sociology