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Dec 7, 2025
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Energy changes in chemical reactions are absolutely crucial for understanding... 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 specifically built for the needs of students. Based on the millions of content pieces we have on the platform we can provide truly meaningful and relevant answers to students. But its not only about answers, the companion is even more about guiding students through their daily learning challenges, with personalised study plans, quizzes or content pieces in the chat and 100% personalisation based on the students skills and developments.
You can download the app in the Google Play Store and in the Apple App Store.
That's right! Enjoy free access to study content, connect with fellow students, and get instant help – all at your fingertips.
App 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
I think it’s very much worth it and you’ll end up using it a lot once you get the hang of it and even after looking at others notes you can still ask your Artificial intelligence buddy the question and ask to simplify it if you still don’t get it!!! In the end I think it’s worth it 😊👍 ⚠️Also DID I MENTION ITS FREEE YOU DON’T HAVE TO PAY FOR ANYTHING AND STILL GET YOUR GRADES IN PERFECTLY❗️❗️⚠️
Thomas R
iOS user
Knowunity is the BEST app I’ve used in a minute. This is not an ai review or anything this is genuinely coming from a 7th grade student (I know 2011 im young) but dude this app is a 10/10 i have maintained a 3.8 gpa and have plenty of time for gaming. I love it and my mom is just happy I got good grades
Brad T
Android user
Not only did it help me find the answer but it also showed me alternative ways to solve it. I was horrible in math and science but now I have an a in both subjects. Thanks for the help🤍🤍
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
I found this app a couple years ago and it has only gotten better since then. I really love it because it can help with written questions and photo questions. Also, it can find study guides that other people have made as well as flashcard sets and practice tests. The free version is also amazing for students who might not be able to afford it. Would 100% recommend
Aubrey
iOS user
Best app if you're in Highschool or Junior high. I have been using this app for 2 school years and it's the best, it's good if you don't have anyone to help you with school work.😋🩷🎀
Marco B
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 app is phenomenal down to the correct info and the various topics you can study! I greatly recommend it for people who struggle with procrastination and those who need homework help. It has been perfectly accurate for world 1 history as far as I’ve seen! Geometry too!
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
I think it’s very much worth it and you’ll end up using it a lot once you get the hang of it and even after looking at others notes you can still ask your Artificial intelligence buddy the question and ask to simplify it if you still don’t get it!!! In the end I think it’s worth it 😊👍 ⚠️Also DID I MENTION ITS FREEE YOU DON’T HAVE TO PAY FOR ANYTHING AND STILL GET YOUR GRADES IN PERFECTLY❗️❗️⚠️
Thomas R
iOS user
Knowunity is the BEST app I’ve used in a minute. This is not an ai review or anything this is genuinely coming from a 7th grade student (I know 2011 im young) but dude this app is a 10/10 i have maintained a 3.8 gpa and have plenty of time for gaming. I love it and my mom is just happy I got good grades
Brad T
Android user
Not only did it help me find the answer but it also showed me alternative ways to solve it. I was horrible in math and science but now I have an a in both subjects. Thanks for the help🤍🤍
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
I found this app a couple years ago and it has only gotten better since then. I really love it because it can help with written questions and photo questions. Also, it can find study guides that other people have made as well as flashcard sets and practice tests. The free version is also amazing for students who might not be able to afford it. Would 100% recommend
Aubrey
iOS user
Best app if you're in Highschool or Junior high. I have been using this app for 2 school years and it's the best, it's good if you don't have anyone to help you with school work.😋🩷🎀
Marco B
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 app is phenomenal down to the correct info and the various topics you can study! I greatly recommend it for people who struggle with procrastination and those who need homework help. It has been perfectly accurate for world 1 history as far as I’ve seen! Geometry too!
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.
Access to all documents
Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
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 specifically built for the needs of students. Based on the millions of content pieces we have on the platform we can provide truly meaningful and relevant answers to students. But its not only about answers, the companion is even more about guiding students through their daily learning challenges, with personalised study plans, quizzes or content pieces in the chat and 100% personalisation based on the students skills and developments.
You can download the app in the Google Play Store and in the Apple App Store.
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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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)
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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
I think it’s very much worth it and you’ll end up using it a lot once you get the hang of it and even after looking at others notes you can still ask your Artificial intelligence buddy the question and ask to simplify it if you still don’t get it!!! In the end I think it’s worth it 😊👍 ⚠️Also DID I MENTION ITS FREEE YOU DON’T HAVE TO PAY FOR ANYTHING AND STILL GET YOUR GRADES IN PERFECTLY❗️❗️⚠️
Thomas R
iOS user
Knowunity is the BEST app I’ve used in a minute. This is not an ai review or anything this is genuinely coming from a 7th grade student (I know 2011 im young) but dude this app is a 10/10 i have maintained a 3.8 gpa and have plenty of time for gaming. I love it and my mom is just happy I got good grades
Brad T
Android user
Not only did it help me find the answer but it also showed me alternative ways to solve it. I was horrible in math and science but now I have an a in both subjects. Thanks for the help🤍🤍
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
I found this app a couple years ago and it has only gotten better since then. I really love it because it can help with written questions and photo questions. Also, it can find study guides that other people have made as well as flashcard sets and practice tests. The free version is also amazing for students who might not be able to afford it. Would 100% recommend
Aubrey
iOS user
Best app if you're in Highschool or Junior high. I have been using this app for 2 school years and it's the best, it's good if you don't have anyone to help you with school work.😋🩷🎀
Marco B
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 app is phenomenal down to the correct info and the various topics you can study! I greatly recommend it for people who struggle with procrastination and those who need homework help. It has been perfectly accurate for world 1 history as far as I’ve seen! Geometry too!
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
I think it’s very much worth it and you’ll end up using it a lot once you get the hang of it and even after looking at others notes you can still ask your Artificial intelligence buddy the question and ask to simplify it if you still don’t get it!!! In the end I think it’s worth it 😊👍 ⚠️Also DID I MENTION ITS FREEE YOU DON’T HAVE TO PAY FOR ANYTHING AND STILL GET YOUR GRADES IN PERFECTLY❗️❗️⚠️
Thomas R
iOS user
Knowunity is the BEST app I’ve used in a minute. This is not an ai review or anything this is genuinely coming from a 7th grade student (I know 2011 im young) but dude this app is a 10/10 i have maintained a 3.8 gpa and have plenty of time for gaming. I love it and my mom is just happy I got good grades
Brad T
Android user
Not only did it help me find the answer but it also showed me alternative ways to solve it. I was horrible in math and science but now I have an a in both subjects. Thanks for the help🤍🤍
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
I found this app a couple years ago and it has only gotten better since then. I really love it because it can help with written questions and photo questions. Also, it can find study guides that other people have made as well as flashcard sets and practice tests. The free version is also amazing for students who might not be able to afford it. Would 100% recommend
Aubrey
iOS user
Best app if you're in Highschool or Junior high. I have been using this app for 2 school years and it's the best, it's good if you don't have anyone to help you with school work.😋🩷🎀
Marco B
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 app is phenomenal down to the correct info and the various topics you can study! I greatly recommend it for people who struggle with procrastination and those who need homework help. It has been perfectly accurate for world 1 history as far as I’ve seen! Geometry too!
Paul T
iOS user
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