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Chemistry
29 Nov 2025
214
3 pages
Maya A @maya.ah
Enthalpy changes are fundamental to understanding how energy flows in chemical reactions. These definitions and calculations help you... Show more
You'll encounter loads of enthalpy terms in chemistry, but they all follow the same basic principle energy change per mole. Each definition describes what happens when exactly one mole undergoes a specific process.
Formation enthalpy (ΔHf) tells you the energy change when making a compound from its elements. For example, making sodium oxide from sodium metal and oxygen gas is exothermic - it releases energy. Combustion enthalpy (ΔHc) measures energy released when substances burn completely in oxygen, like methanol burning to produce carbon dioxide and water.
Bond dissociation enthalpy and atomisation enthalpy both involve breaking things apart, so they're endothermic (require energy input). Breaking iodine molecules into separate atoms or turning solid iodine into gaseous atoms both need energy.
Ionisation energies are always endothermic because you're forcing electrons away from atoms. The first ionisation energy removes one electron, whilst the second removes another from an already positive ion - which requires even more energy.
💡 Remember Formation and combustion are usually exothermic, whilst bond breaking and ionisation are always endothermic.
Electron affinity shows a split personality - the first is usually exothermic (atoms actually want that first extra electron), but the second is endothermic because you're forcing another electron onto an already negative ion.
Lattice energies come in two flavours formation and dissociation . Hydration enthalpy describes when gaseous ions dissolve in water, releasing energy as they get surrounded by water molecules.
The key calculation formulas work like algebraic equations. For combustion products minus reactants using standard values. For mean bond enthalpy energy needed to break bonds minus energy released forming new ones.
Entropy and Gibbs free energy determine if reactions actually happen. When ΔG° equals zero, you've found the temperature where a reaction just balances - neither favouring products nor reactants.
🔥 Pro tip Solution enthalpy equals lattice dissociation plus hydration enthalpy - it's like a two-step energy journey from solid to dissolved ions.
Enthalpy change (ΔH) represents the heat energy transferred during chemical reactions at constant pressure. This fundamental concept connects all the specific enthalpy types you've learned.
Whether you're calculating formation enthalpies, combustion reactions, or lattice energies, you're essentially measuring how much energy gets absorbed or released when chemical bonds break and form.
⚡ Key insight All enthalpy calculations follow the same pattern - energy in minus energy out determines whether reactions feel hot or cold to touch.
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 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 key concepts in thermodynamics, focusing on enthalpy definitions and changes. This summary covers enthalpy of formation, combustion, neutralization, and more, providing essential insights for students studying physical chemistry. Ideal for exam preparation and understanding thermodynamic principles.
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.
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
Maya A
@maya.ah
Enthalpy changes are fundamental to understanding how energy flows in chemical reactions. These definitions and calculations help you predict whether reactions will release or absorb energy, making them crucial for A-level chemistry exams.
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You'll encounter loads of enthalpy terms in chemistry, but they all follow the same basic principle: energy change per mole. Each definition describes what happens when exactly one mole undergoes a specific process.
Formation enthalpy (ΔHf) tells you the energy change when making a compound from its elements. For example, making sodium oxide from sodium metal and oxygen gas is exothermic - it releases energy. Combustion enthalpy (ΔHc) measures energy released when substances burn completely in oxygen, like methanol burning to produce carbon dioxide and water.
Bond dissociation enthalpy and atomisation enthalpy both involve breaking things apart, so they're endothermic (require energy input). Breaking iodine molecules into separate atoms or turning solid iodine into gaseous atoms both need energy.
Ionisation energies are always endothermic because you're forcing electrons away from atoms. The first ionisation energy removes one electron, whilst the second removes another from an already positive ion - which requires even more energy.
💡 Remember: Formation and combustion are usually exothermic, whilst bond breaking and ionisation are always endothermic.
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Electron affinity shows a split personality - the first is usually exothermic (atoms actually want that first extra electron), but the second is endothermic because you're forcing another electron onto an already negative ion.
Lattice energies come in two flavours: formation and dissociation . Hydration enthalpy describes when gaseous ions dissolve in water, releasing energy as they get surrounded by water molecules.
The key calculation formulas work like algebraic equations. For combustion: products minus reactants using standard values. For mean bond enthalpy: energy needed to break bonds minus energy released forming new ones.
Entropy and Gibbs free energy determine if reactions actually happen. When ΔG° equals zero, you've found the temperature where a reaction just balances - neither favouring products nor reactants.
🔥 Pro tip: Solution enthalpy equals lattice dissociation plus hydration enthalpy - it's like a two-step energy journey from solid to dissolved ions.
Access to all documents
Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Enthalpy change (ΔH) represents the heat energy transferred during chemical reactions at constant pressure. This fundamental concept connects all the specific enthalpy types you've learned.
Whether you're calculating formation enthalpies, combustion reactions, or lattice energies, you're essentially measuring how much energy gets absorbed or released when chemical bonds break and form.
⚡ Key insight: All enthalpy calculations follow the same pattern - energy in minus energy out determines whether reactions feel hot or cold to touch.
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.
4
Smart Tools NEW
Transform this note into: ✓ 50+ Practice Questions ✓ Interactive Flashcards ✓ Full Mock Exam ✓ Essay Outlines
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 key concepts in thermodynamics, focusing on enthalpy definitions and changes. This summary covers enthalpy of formation, combustion, neutralization, and more, providing essential insights for students studying physical chemistry. Ideal for exam preparation and understanding thermodynamic principles.
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.
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