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Britain & the wider world: 1745 -1901
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Aasiyah Rahman
30/11/2025
Chemistry
Phyisical Chemistry- Thermodynamics, enthapy definitions
190
•
30 Nov 2025
•
Aasiyah Rahman
@aasiyahrahman
Thermodynamics is all about energy changes in chemical reactions. Understanding... Show more
Enthalpy change (ΔH) represents the heat energy transferred during a reaction that occurs at constant pressure. When this change is measured under standard conditions (298K and 100kPa), we call it standard enthalpy change (ΔH°). Reactions are classified as either exothermic (negative ΔH, releasing energy) or endothermic (positive ΔH, absorbing energy).
There are several specific types of enthalpy changes you'll need to know. Enthalpy of formation measures the energy change when one mole of a compound forms from its elements in their standard states. Enthalpy of combustion measures energy released when one mole of substance burns completely in oxygen.
The enthalpy of neutralisation describes energy released when an acid and alkali react to form one mole of water. Bond dissociation enthalpy is the energy needed to break all bonds of the same type in one mole of gaseous molecules.
⚡ Think of enthalpy changes like bank transactions: exothermic reactions are like deposits (giving energy to surroundings), while endothermic reactions are withdrawals (taking energy from surroundings).
Other important enthalpy terms include ionisation energy (forming positive ions), electron affinity (forming negative ions), enthalpy of atomisation (forming gaseous atoms), hydration enthalpy (when ions dissolve in water), and solution enthalpy (when a solute completely dissolves).
Every enthalpy change measures a specific type of chemical or physical process. For instance, formation enthalpy (ΔHf) tells us about making compounds from elements, while combustion enthalpy (ΔHc) measures energy released during burning. These values help chemists predict whether reactions will be feasible and how much energy they'll release.
Most formation reactions are exothermic (negative ΔH), meaning they release energy when compounds form. Combustion reactions are almost always exothermic too, which is why burning fuels generates heat. Neutralisation reactions between acids and bases also release energy, with a consistent enthalpy change when water forms.
Breaking bonds always requires energy, which is why bond dissociation and lattice dissociation enthalpies are endothermic (positive ΔH). Similarly, changing states from solid to liquid (fusion) or liquid to gas (vaporisation) requires energy input. Knowing these patterns helps you predict reaction outcomes.
🔍 Remember that exothermic reactions feel hot to touch because they release energy to surroundings, while endothermic reactions feel cold because they absorb energy from surroundings.
Ionisation enthalpies are always endothermic because removing electrons requires energy. The first electron affinity is usually exothermic for non-metals (they want electrons), but the second electron affinity is endothermic (they resist taking another electron due to repulsion).
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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 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
Aasiyah Rahman
@aasiyahrahman
Thermodynamics is all about energy changes in chemical reactions. Understanding enthalpy changes helps us predict whether reactions release or absorb heat energy, and by how much. This topic is crucial for explaining why some reactions feel hot while others cool... Show more
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Enthalpy change (ΔH) represents the heat energy transferred during a reaction that occurs at constant pressure. When this change is measured under standard conditions (298K and 100kPa), we call it standard enthalpy change (ΔH°). Reactions are classified as either exothermic (negative ΔH, releasing energy) or endothermic (positive ΔH, absorbing energy).
There are several specific types of enthalpy changes you'll need to know. Enthalpy of formation measures the energy change when one mole of a compound forms from its elements in their standard states. Enthalpy of combustion measures energy released when one mole of substance burns completely in oxygen.
The enthalpy of neutralisation describes energy released when an acid and alkali react to form one mole of water. Bond dissociation enthalpy is the energy needed to break all bonds of the same type in one mole of gaseous molecules.
⚡ Think of enthalpy changes like bank transactions: exothermic reactions are like deposits (giving energy to surroundings), while endothermic reactions are withdrawals (taking energy from surroundings).
Other important enthalpy terms include ionisation energy (forming positive ions), electron affinity (forming negative ions), enthalpy of atomisation (forming gaseous atoms), hydration enthalpy (when ions dissolve in water), and solution enthalpy (when a solute completely dissolves).
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Improve your grades
Join milions of students
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Every enthalpy change measures a specific type of chemical or physical process. For instance, formation enthalpy (ΔHf) tells us about making compounds from elements, while combustion enthalpy (ΔHc) measures energy released during burning. These values help chemists predict whether reactions will be feasible and how much energy they'll release.
Most formation reactions are exothermic (negative ΔH), meaning they release energy when compounds form. Combustion reactions are almost always exothermic too, which is why burning fuels generates heat. Neutralisation reactions between acids and bases also release energy, with a consistent enthalpy change when water forms.
Breaking bonds always requires energy, which is why bond dissociation and lattice dissociation enthalpies are endothermic (positive ΔH). Similarly, changing states from solid to liquid (fusion) or liquid to gas (vaporisation) requires energy input. Knowing these patterns helps you predict reaction outcomes.
🔍 Remember that exothermic reactions feel hot to touch because they release energy to surroundings, while endothermic reactions feel cold because they absorb energy from surroundings.
Ionisation enthalpies are always endothermic because removing electrons requires energy. The first electron affinity is usually exothermic for non-metals (they want electrons), but the second electron affinity is endothermic (they resist taking another electron due to repulsion).
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.
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Explore the concepts of entropy, Gibbs free energy, and the Born-Haber cycle in this comprehensive AQA A-Level Physical Chemistry resource. This study note includes key definitions, detailed calculations, and practical examples to enhance your understanding of thermodynamic favorability and spontaneity. Ideal for exam preparation and revision.
ChemistryExplore the various types of enthalpy changes in A-Level Chemistry, including reaction, combustion, formation, and lattice energies. This summary covers key concepts such as Hess's Law, standard conditions, and the factors affecting ionic size. Ideal for WJEC Unit 3 students seeking to grasp the fundamentals of thermodynamics in chemistry.
ChemistryExplore the key concepts of enthalpy changes in thermodynamics, including formation, combustion, and bond dissociation enthalpy. This summary covers essential definitions and calculations relevant to AQA Physical Chemistry, providing clarity on exothermic and endothermic reactions, standard states, and Gibbs free energy. Ideal for students preparing for exams.
ChemistryExplore the fundamentals of enthalpy changes in chemical reactions, including exothermic and endothermic processes, standard enthalpy changes, and Hess's law. This summary covers key concepts such as calorimetry, energy changes, and bond enthalpies, providing essential insights for AQA AS Chemistry students.
ChemistryExplore the concept of lattice enthalpy in this detailed summary, covering key topics such as ionization energy, electron affinity, and the Born-Haber cycle. Understand the enthalpy changes associated with ionic compounds and their formation from gaseous ions. Ideal for A-Level Chemistry students preparing for exams.
ChemistryExplore the concepts of bond dissociation energy, enthalpy changes, and calorimetry in this comprehensive study note. Understand exothermic and endothermic reactions, Hess's law, and how to calculate energy changes using q=MCΔT. Ideal for A-level chemistry students looking to master energetics and thermochemistry.
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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 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