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17 Jan 2026
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Understanding Lattice Enthalpy in Chapter 22 - OCR A-Level Chemistry
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Lattice Enthalpy - The Stability Secret
Think of ionic compounds as incredibly strong fortresses - they're almost impossible to break down because of the powerful electrostatic bonds holding everything together.
Lattice enthalpy is the energy change when one mole of an ionic compound forms from its gaseous ions under standard conditions. Picture potassium and chloride ions floating around as gases, then suddenly snapping together to form solid KCl - that releases a whopping 711 kJ/mol of energy!
The key thing to remember is that lattice enthalpy values are always negative because forming ionic compounds is exothermic. Energy gets released as those oppositely charged ions attract each other with tremendous force.
💡 Quick Tip: The more negative the lattice enthalpy, the more stable the ionic compound. It's like measuring how tightly the ions are hugging each other!
Born-Haber Cycles - The Indirect Route
Here's the thing about lattice enthalpy - you can't measure it directly in a lab, which is where Born-Haber cycles become your best friend. They're like taking a scenic route to reach the same destination.
Enthalpy change of formation is when one mole of a compound forms from its elements in their standard states. For example, solid magnesium plus solid iodine creating magnesium iodide. This process is typically exothermic.
Enthalpy change of atomisation breaks elements down into individual gaseous atoms. Think of it as completely vaporising and separating everything - like turning solid sodium into floating sodium atoms in the gas phase.
💡 Memory Hook: Formation brings things together (usually exo), while atomisation breaks them apart (always endo). Easy!
Ionisation Energy and Electron Affinity
Now we're getting to the real action - creating those charged ions that make ionic compounds possible.
First ionisation energy is the energy needed to rip one electron away from each atom in a mole of gaseous atoms, creating 1+ ions. This always requires energy input (endothermic) because you're fighting against the nucleus's attraction to its electrons.
First electron affinity works in the opposite direction - it's the energy change when gaseous atoms each grab an extra electron to form 1- ions. This typically releases energy (exothermic) because atoms often want that extra electron.
The beauty is in the balance: metals lose electrons relatively easily (lower ionisation energies), while non-metals love gaining electrons (more negative electron affinities).
💡 Exam Insight: Always check the direction of your arrows in equations - they tell you whether energy is being absorbed or released!
Multiple Charges - Going Further
Real chemistry gets more complex when ions carry charges greater than 1+ or 1-. That's where successive ionisation energies and electron affinities come into play.
For metals forming 2+ ions (like magnesium), you'll need both first and second ionisation energies. The second one is always much larger because you're removing an electron from an already positive ion - imagine trying to take something away from someone who's already holding on tighter!
For non-metals forming 2- ions (like oxygen), the second electron affinity is actually endothermic. Adding that second electron to an already negative ion requires energy input because like charges repel each other.
Understanding these successive values helps explain why some ionic compounds are more common than others - it's all about the energy costs involved.
💡 Pattern Spotter: Notice how successive ionisation energies increase dramatically, while successive electron affinities can flip from exo to endo!
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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.
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Anna
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Best app on earth! no words because it’s too good
Thomas R
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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
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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
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Sudenaz Ocak
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Greenlight Bonnie
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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.
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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
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Understanding Lattice Enthalpy in Chapter 22 - OCR A-Level Chemistry
Ever wondered why salt crystals are so incredibly stable and hard to break apart? It's all down to lattice enthalpy- the massive amount of energy released when ions come together to form ionic compounds. Understanding this concept, along with... Show more
Lattice Enthalpy - The Stability Secret
Think of ionic compounds as incredibly strong fortresses - they're almost impossible to break down because of the powerful electrostatic bonds holding everything together.
Lattice enthalpy is the energy change when one mole of an ionic compound forms from its gaseous ions under standard conditions. Picture potassium and chloride ions floating around as gases, then suddenly snapping together to form solid KCl - that releases a whopping 711 kJ/mol of energy!
The key thing to remember is that lattice enthalpy values are always negative because forming ionic compounds is exothermic. Energy gets released as those oppositely charged ions attract each other with tremendous force.
💡 Quick Tip: The more negative the lattice enthalpy, the more stable the ionic compound. It's like measuring how tightly the ions are hugging each other!
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Born-Haber Cycles - The Indirect Route
Here's the thing about lattice enthalpy - you can't measure it directly in a lab, which is where Born-Haber cycles become your best friend. They're like taking a scenic route to reach the same destination.
Enthalpy change of formation is when one mole of a compound forms from its elements in their standard states. For example, solid magnesium plus solid iodine creating magnesium iodide. This process is typically exothermic.
Enthalpy change of atomisation breaks elements down into individual gaseous atoms. Think of it as completely vaporising and separating everything - like turning solid sodium into floating sodium atoms in the gas phase.
💡 Memory Hook: Formation brings things together (usually exo), while atomisation breaks them apart (always endo). Easy!
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Ionisation Energy and Electron Affinity
Now we're getting to the real action - creating those charged ions that make ionic compounds possible.
First ionisation energy is the energy needed to rip one electron away from each atom in a mole of gaseous atoms, creating 1+ ions. This always requires energy input (endothermic) because you're fighting against the nucleus's attraction to its electrons.
First electron affinity works in the opposite direction - it's the energy change when gaseous atoms each grab an extra electron to form 1- ions. This typically releases energy (exothermic) because atoms often want that extra electron.
The beauty is in the balance: metals lose electrons relatively easily (lower ionisation energies), while non-metals love gaining electrons (more negative electron affinities).
💡 Exam Insight: Always check the direction of your arrows in equations - they tell you whether energy is being absorbed or released!
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Multiple Charges - Going Further
Real chemistry gets more complex when ions carry charges greater than 1+ or 1-. That's where successive ionisation energies and electron affinities come into play.
For metals forming 2+ ions (like magnesium), you'll need both first and second ionisation energies. The second one is always much larger because you're removing an electron from an already positive ion - imagine trying to take something away from someone who's already holding on tighter!
For non-metals forming 2- ions (like oxygen), the second electron affinity is actually endothermic. Adding that second electron to an already negative ion requires energy input because like charges repel each other.
Understanding these successive values helps explain why some ionic compounds are more common than others - it's all about the energy costs involved.
💡 Pattern Spotter: Notice how successive ionisation energies increase dramatically, while successive electron affinities can flip from exo to endo!
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?
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Is Knowunity really free of charge?
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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
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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
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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