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30 Dec 2025
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A-Level Chemistry: Understanding Energetics
C
Chloe
@chloe_qxoy
1 / 4
Understanding Energy Changes in Reactions
Ever wondered why some reactions make things hot whilst others cool them down? Exothermic reactions release energy to their surroundings, making the temperature drop and giving a negative ΔH value. Think of combustion or neutralisation - they're making new bonds and releasing energy.
Endothermic reactions do the opposite - they absorb heat from their surroundings, causing temperature to rise and giving a positive ΔH. Thermal decomposition is a classic example where you're breaking bonds and need energy input.
Enthalpy change (ΔH) measures this heat energy at constant pressure under standard conditions (100kPa pressure, 298K temperature). You'll use the formula q = mcΔT constantly - it's your best friend for calculations. Remember: mass × specific heat capacity × temperature change gives you the energy transferred.
Key Point: Standard conditions are crucial - always check you're using 100kPa and 298K when dealing with standard enthalpy values.
Measuring Energy Changes with Calorimetry
Setting up a simple calorimeter is straightforward but requires precision. You'll measure the mass of your fuel, add a known quantity of water to an insulated container, ignite the fuel, and track temperature changes over time. Don't forget to weigh everything again afterwards!
The main problems you'll face include heat loss (solve with insulation), incomplete reactions (stir thoroughly), and only accounting for water's specific heat capacity. These limitations explain why your results might differ from data book values.
Flame calorimeters are the upgraded version with spiral copper chimneys, enclosed flames, and pure oxygen instead of air. They're much more accurate but also more expensive. For school experiments, polystyrene cups work brilliantly because they're good insulators with low heat capacity.
Pro Tip: Always plot temperature vs time graphs and extrapolate back to find the true temperature change - this accounts for heat loss during mixing.
Hess' Law and Thermochemical Cycles
Hess' Law is your shortcut to finding enthalpy changes you can't measure directly - it states that the total enthalpy change is the same regardless of the route taken. Think of it like climbing a mountain: whether you take one path or several, the height difference remains the same.
Thermochemical cycles let you calculate unknown enthalpy changes using known values. For formation enthalpies, use ΔH = products - reactants. Remember that elements already in their standard states have ΔfH° = 0.
For combustion enthalpies, flip it around: ΔH = reactants - products. Oxygen can't combust in oxygen, so its ΔcH° = 0. These cycles might look complex, but they're just organised ways of adding and subtracting known values.
Memory Aid: Formation goes "products minus reactants", combustion goes "reactants minus products" - opposite directions for opposite processes.
Bond Enthalpies and Energy Calculations
Bond dissociation enthalpy tells you exactly how much energy you need to break one mole of a specific covalent bond into gaseous atoms. However, the same bond behaves differently in different molecules, which is where mean bond enthalpy comes in - it's an average across various compounds.
Your energy calculation formula becomes: ΔH = bonds broken - bonds formed. Breaking bonds is endothermic (positive), whilst forming bonds is exothermic (negative). The overall sign tells you whether your reaction gives out or takes in energy.
Common bond enthalpies you should memorise include C-H , C-F , and H-F . Notice how the H-F bond is particularly strong - fluorine really doesn't like letting go of hydrogen!
Exam Tip: Always draw out the structural formulae and count every single bond - missing just one C-H bond can throw off your entire calculation.
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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.
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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.
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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
A-Level Chemistry: Understanding Energetics
C
Chloe
@chloe_qxoy
Energetics is all about the energy changes that happen during chemical reactions - basically whether reactions give out heat or take it in. You'll need to understand how to measure these energy changes and calculate them using different methods, which... Show more
Understanding Energy Changes in Reactions
Ever wondered why some reactions make things hot whilst others cool them down? Exothermic reactions release energy to their surroundings, making the temperature drop and giving a negative ΔH value. Think of combustion or neutralisation - they're making new bonds and releasing energy.
Endothermic reactions do the opposite - they absorb heat from their surroundings, causing temperature to rise and giving a positive ΔH. Thermal decomposition is a classic example where you're breaking bonds and need energy input.
Enthalpy change (ΔH) measures this heat energy at constant pressure under standard conditions (100kPa pressure, 298K temperature). You'll use the formula q = mcΔT constantly - it's your best friend for calculations. Remember: mass × specific heat capacity × temperature change gives you the energy transferred.
Key Point: Standard conditions are crucial - always check you're using 100kPa and 298K when dealing with standard enthalpy values.
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Measuring Energy Changes with Calorimetry
Setting up a simple calorimeter is straightforward but requires precision. You'll measure the mass of your fuel, add a known quantity of water to an insulated container, ignite the fuel, and track temperature changes over time. Don't forget to weigh everything again afterwards!
The main problems you'll face include heat loss (solve with insulation), incomplete reactions (stir thoroughly), and only accounting for water's specific heat capacity. These limitations explain why your results might differ from data book values.
Flame calorimeters are the upgraded version with spiral copper chimneys, enclosed flames, and pure oxygen instead of air. They're much more accurate but also more expensive. For school experiments, polystyrene cups work brilliantly because they're good insulators with low heat capacity.
Pro Tip: Always plot temperature vs time graphs and extrapolate back to find the true temperature change - this accounts for heat loss during mixing.
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Hess' Law and Thermochemical Cycles
Hess' Law is your shortcut to finding enthalpy changes you can't measure directly - it states that the total enthalpy change is the same regardless of the route taken. Think of it like climbing a mountain: whether you take one path or several, the height difference remains the same.
Thermochemical cycles let you calculate unknown enthalpy changes using known values. For formation enthalpies, use ΔH = products - reactants. Remember that elements already in their standard states have ΔfH° = 0.
For combustion enthalpies, flip it around: ΔH = reactants - products. Oxygen can't combust in oxygen, so its ΔcH° = 0. These cycles might look complex, but they're just organised ways of adding and subtracting known values.
Memory Aid: Formation goes "products minus reactants", combustion goes "reactants minus products" - opposite directions for opposite processes.
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Bond Enthalpies and Energy Calculations
Bond dissociation enthalpy tells you exactly how much energy you need to break one mole of a specific covalent bond into gaseous atoms. However, the same bond behaves differently in different molecules, which is where mean bond enthalpy comes in - it's an average across various compounds.
Your energy calculation formula becomes: ΔH = bonds broken - bonds formed. Breaking bonds is endothermic (positive), whilst forming bonds is exothermic (negative). The overall sign tells you whether your reaction gives out or takes in energy.
Common bond enthalpies you should memorise include C-H , C-F , and H-F . Notice how the H-F bond is particularly strong - fluorine really doesn't like letting go of hydrogen!
Exam Tip: Always draw out the structural formulae and count every single bond - missing just one C-H bond can throw off your entire calculation.
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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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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