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25 Dec 2025
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Enthalpy Changes in WJEC A-Level Chemistry Unit 3 Explored
Nikita
@nikitaxo8
Enthalpy Changes and Energy Calculations
Enthalpy (ΔH) is simply the energy change that happens during chemical reactions. When ΔH is negative, the reaction gives out heat (exothermic) - think of burning wood or hand warmers. When ΔH is positive, the reaction absorbs heat (endothermic) - like the cooling effect in instant ice packs.
You'll use the formula ΔH = -mcΔT/n constantly in calculations. Here, 'm' is the mass of your solution, 'c' is the specific heat capacity, 'ΔT' is temperature change, and 'n' is moles of your limiting reactant. Always remember to use the smallest number of moles!
Standard conditions are crucial for comparing different reactions fairly. These are 1 ATM pressure, 298K (25°C), and 1 mol dm⁻³ for solutions. Think of these as the "referee's rules" that make sure everyone's playing the same game.
Quick Tip: Negative ΔH = heat out (exothermic), Positive ΔH = heat in (endothermic)
Types of Enthalpy Changes You Need to Know
There are loads of different enthalpy changes, but each one has a specific definition. Enthalpy of combustion is when exactly one mole burns completely in oxygen - perfect for comparing fuels. Enthalpy of formation shows the energy when one mole of compound forms from its elements in their natural states.
Hess' Law is your best friend for tricky calculations. It states that the total enthalpy change doesn't depend on the route you take - just like climbing a mountain, the height difference is the same whether you take the direct path or go round the scenic route.
For ionic compounds, you'll encounter lattice formation and lattice breaking . These are opposites of each other, which makes perfect sense when you think about it.
Exam Tip: Always check if enthalpy changes should be positive or negative - it's an easy way to spot mistakes in your calculations!
Measuring Enthalpy and Factors Affecting Lattice Energy
Calorimetry uses simple equipment - a polystyrene cup, thermometer, and your reaction mixture. The main problem is heat loss, so you'll need a lid with insulating material and take several temperature readings to get accurate starting temperatures.
Lattice energy measures how strongly ions are held together in ionic compounds. Two main factors affect this strength: ionic size and charge on the ion. Smaller ions pack closer together, creating stronger attractions and higher lattice energies.
Charge density (charge divided by ionic radius) is the key concept here. Higher charge density means stronger electrostatic forces between ions. This explains why MgO has a much higher melting point than NaCl - Mg²⁺ and O²⁻ have higher charges than Na⁺ and Cl⁻.
Real-World Connection: This is why salt (NaCl) dissolves easily in water but limestone (CaCO₃) doesn't - the lattice energy differences are huge!
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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
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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
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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
Enthalpy Changes in WJEC A-Level Chemistry Unit 3 Explored
Nikita
@nikitaxo8
Enthalpy changes are absolutely everywhere in chemistry - from the warmth you feel when hand warmers activate to the energy released when petrol burns in car engines. Understanding these energy transfers will help you predict whether reactions will happen and... Show more
Enthalpy Changes and Energy Calculations
Enthalpy (ΔH) is simply the energy change that happens during chemical reactions. When ΔH is negative, the reaction gives out heat (exothermic) - think of burning wood or hand warmers. When ΔH is positive, the reaction absorbs heat (endothermic) - like the cooling effect in instant ice packs.
You'll use the formula ΔH = -mcΔT/n constantly in calculations. Here, 'm' is the mass of your solution, 'c' is the specific heat capacity, 'ΔT' is temperature change, and 'n' is moles of your limiting reactant. Always remember to use the smallest number of moles!
Standard conditions are crucial for comparing different reactions fairly. These are 1 ATM pressure, 298K (25°C), and 1 mol dm⁻³ for solutions. Think of these as the "referee's rules" that make sure everyone's playing the same game.
Quick Tip: Negative ΔH = heat out (exothermic), Positive ΔH = heat in (endothermic)
Types of Enthalpy Changes You Need to Know
There are loads of different enthalpy changes, but each one has a specific definition. Enthalpy of combustion is when exactly one mole burns completely in oxygen - perfect for comparing fuels. Enthalpy of formation shows the energy when one mole of compound forms from its elements in their natural states.
Hess' Law is your best friend for tricky calculations. It states that the total enthalpy change doesn't depend on the route you take - just like climbing a mountain, the height difference is the same whether you take the direct path or go round the scenic route.
For ionic compounds, you'll encounter lattice formation and lattice breaking . These are opposites of each other, which makes perfect sense when you think about it.
Exam Tip: Always check if enthalpy changes should be positive or negative - it's an easy way to spot mistakes in your calculations!
Measuring Enthalpy and Factors Affecting Lattice Energy
Calorimetry uses simple equipment - a polystyrene cup, thermometer, and your reaction mixture. The main problem is heat loss, so you'll need a lid with insulating material and take several temperature readings to get accurate starting temperatures.
Lattice energy measures how strongly ions are held together in ionic compounds. Two main factors affect this strength: ionic size and charge on the ion. Smaller ions pack closer together, creating stronger attractions and higher lattice energies.
Charge density (charge divided by ionic radius) is the key concept here. Higher charge density means stronger electrostatic forces between ions. This explains why MgO has a much higher melting point than NaCl - Mg²⁺ and O²⁻ have higher charges than Na⁺ and Cl⁻.
Real-World Connection: This is why salt (NaCl) dissolves easily in water but limestone (CaCO₃) doesn't - the lattice energy differences are huge!
We thought you’d never ask...
What is the Knowunity AI companion?
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Where can I download the Knowunity app?
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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