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๐ ๐๐๐๐๐ ๐
02/12/2025
Chemistry
AQA CHEMISTRY ENERGY CHANGES
59
โข
2 Dec 2025
โข
๐ ๐๐๐๐๐ ๐
@grace.xxcharlotte08
Chemical reactions either release energy to the surroundings (exothermic) or... Show more
Ever wondered why some reactions make things hot whilst others make them cold? Exothermic reactions transfer energy from the reacting molecules to the surroundings, making the temperature increase. Think of burning wood - it releases loads of energy as heat and light.
You'll also see exothermic reactions in oxidation reactions and neutralisation (when acids and alkalis react). These reactions are dead useful in real life - hand warmers and self-heating food cans both use exothermic reactions to generate heat on demand.
Energy profile diagrams show how energy changes during a reaction. For exothermic reactions, the products have less energy than the reactants because energy has been transferred out to the surroundings. The difference between reactant and product energy levels tells you exactly how much energy was released.
๐ก Quick Tip: In exams, you might need to label energy profile diagrams showing the energy change - remember that exothermic reactions slope downwards from reactants to products.
Endothermic reactions do the opposite - they take in energy from the surroundings, making the temperature decrease. Thermal decomposition is a classic example where compounds break down when heated, absorbing energy in the process.
On energy profile diagrams for endothermic reactions, the products sit higher than the reactants because energy has been absorbed from the surroundings. The difference between these energy levels shows how much energy the reaction took in.
Both reaction types still need to get over the same hurdle though - they rise to a peak before dropping (or rising) to the products. This peak represents the activation energy, which we'll explore more in the next section.
๐ก Memory Trick: Endothermic = Energy IN (temperature drops), Exothermic = Energy EXits (temperature rises).
Here's something crucial: reactions can only happen when particles collide with enough energy. Activation energy is the minimum amount of energy particles need to react when they smash into each other.
Both exothermic and endothermic reactions have activation energy - it's the energy barrier they must overcome to get started. On energy profile diagrams, activation energy is measured from the reactants up to the peak of the curve.
Think of it like pushing a boulder over a hill - you need enough energy to get it to the top (activation energy) before it can roll down the other side (the reaction happens). Without sufficient activation energy, particles just bounce off each other without reacting.
๐ก Exam Alert: You might be asked to mark activation energy on energy profile diagrams - it's always from reactants to the peak, regardless of whether the reaction is exothermic or endothermic.
When chemists write about energy changes, they use positive and negative values to show the direction of energy transfer. Exothermic reactions show negative energy changes because energy transfers out from the chemicals to the surroundings.
Endothermic reactions show positive energy changes because energy has been gained from the surroundings. This sign convention helps you quickly identify reaction types from data.
In exam questions, you'll often see energy profile diagrams with labels like A (progress of reaction), B (activation energy), and C (products). Make sure you can identify these key features and explain what each part represents.
๐ก Sign Success: Negative = energy leaves (exothermic), Positive = energy enters (endothermic).
Every chemical reaction involves breaking existing bonds (which requires energy) and making new bonds (which releases energy). Breaking bonds is always endothermic, whilst making bonds is always exothermic.
Each type of chemical bond has a specific bond energy value - this tells you exactly how much energy is needed to break that bond. For example, breaking an H-H bond requires +436 kJ, whilst making an H-Cl bond releases -431 kJ.
To work out if a reaction is overall exothermic or endothermic, you calculate the total energy needed to break bonds, then subtract the total energy released when making new bonds. If the answer is negative, the reaction is exothermic; if positive, it's endothermic.
๐ก Calculation Tip: Energy required to break bonds - Energy released making bonds = Overall energy change. Negative result = exothermic reaction.
Required Practical 4 investigates temperature changes in the exothermic neutralisation reaction between hydrochloric acid and sodium hydroxide. You'll add increasing volumes of alkali to acid and measure the maximum temperature reached.
The method involves measuring 30cmยณ of hydrochloric acid in a polystyrene cup, then adding 5cmยณ of sodium hydroxide solution. Use a thermometer to stir and measure the highest temperature reached after the reaction.
You'll repeat this experiment increasing the sodium hydroxide volume by 5cmยณ each time up to 40cmยณ. The polystyrene cup and plastic lid are crucial because polystyrene is a good thermal insulator that reduces heat loss, giving more accurate temperature measurements.
๐ก Safety First: Always wear safety goggles when working with acids and alkalis, and handle glassware carefully to avoid spills.
When you plot your results, you'll see the maximum temperature initially increases as you add more sodium hydroxide. This happens because more particles are available to react with the hydrochloric acid, releasing more energy in this exothermic reaction.
However, the temperature reaches a peak then starts decreasing. This occurs because there's now so much sodium hydroxide that there isn't enough hydrochloric acid left to react with it all - you've reached the limiting reactant situation.
The temperature actually drops after this point because you're adding greater volumes of solution, so the same amount of energy released gets spread out over a larger volume. This dilution effect reduces the temperature rise you can measure.
๐ก Graph Pattern: Temperature rises to a peak (complete neutralisation), then falls due to dilution - this creates a characteristic mountain shape.
The polystyrene cup with plastic lid isn't just random equipment choice - it's specifically designed to minimise heat loss during your temperature measurements. Polystyrene acts as a thermal insulator, reducing heat loss through the sides and bottom of the container.
The plastic lid prevents heat escaping to the air above the reaction mixture. Without these precautions, you'd lose heat to the surroundings and get inaccurate, lower temperature readings.
This practical demonstrates how important it is to consider heat transfer when measuring energy changes in reactions. Any heat lost to the surroundings won't be detected by your thermometer, leading to underestimated energy changes.
๐ก Exam Favourite: Questions often ask why we use polystyrene cups and lids - always mention thermal insulation and reducing heat loss for accurate measurements.
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 โ all at your fingertips.
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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 itโs 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 didnโt even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as Iโm 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 ๐๐๐ฒ๐ค๐โจ๐๐ฎ
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 itโs 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 didnโt even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as Iโm 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 ๐๐๐ฒ๐ค๐โจ๐๐ฎ
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
๐ ๐๐๐๐๐ ๐
@grace.xxcharlotte08
Chemical reactions either release energy to the surroundings (exothermic) or absorb energy from them (endothermic). Understanding these energy changes is crucial for predicting how reactions behave and helps explain everything from hand warmers to why certain reactions need heating to... Show more
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Ever wondered why some reactions make things hot whilst others make them cold? Exothermic reactions transfer energy from the reacting molecules to the surroundings, making the temperature increase. Think of burning wood - it releases loads of energy as heat and light.
You'll also see exothermic reactions in oxidation reactions and neutralisation (when acids and alkalis react). These reactions are dead useful in real life - hand warmers and self-heating food cans both use exothermic reactions to generate heat on demand.
Energy profile diagrams show how energy changes during a reaction. For exothermic reactions, the products have less energy than the reactants because energy has been transferred out to the surroundings. The difference between reactant and product energy levels tells you exactly how much energy was released.
๐ก Quick Tip: In exams, you might need to label energy profile diagrams showing the energy change - remember that exothermic reactions slope downwards from reactants to products.
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Join milions of students
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Endothermic reactions do the opposite - they take in energy from the surroundings, making the temperature decrease. Thermal decomposition is a classic example where compounds break down when heated, absorbing energy in the process.
On energy profile diagrams for endothermic reactions, the products sit higher than the reactants because energy has been absorbed from the surroundings. The difference between these energy levels shows how much energy the reaction took in.
Both reaction types still need to get over the same hurdle though - they rise to a peak before dropping (or rising) to the products. This peak represents the activation energy, which we'll explore more in the next section.
๐ก Memory Trick: Endothermic = Energy IN (temperature drops), Exothermic = Energy EXits (temperature rises).
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Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Here's something crucial: reactions can only happen when particles collide with enough energy. Activation energy is the minimum amount of energy particles need to react when they smash into each other.
Both exothermic and endothermic reactions have activation energy - it's the energy barrier they must overcome to get started. On energy profile diagrams, activation energy is measured from the reactants up to the peak of the curve.
Think of it like pushing a boulder over a hill - you need enough energy to get it to the top (activation energy) before it can roll down the other side (the reaction happens). Without sufficient activation energy, particles just bounce off each other without reacting.
๐ก Exam Alert: You might be asked to mark activation energy on energy profile diagrams - it's always from reactants to the peak, regardless of whether the reaction is exothermic or endothermic.
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Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
When chemists write about energy changes, they use positive and negative values to show the direction of energy transfer. Exothermic reactions show negative energy changes because energy transfers out from the chemicals to the surroundings.
Endothermic reactions show positive energy changes because energy has been gained from the surroundings. This sign convention helps you quickly identify reaction types from data.
In exam questions, you'll often see energy profile diagrams with labels like A (progress of reaction), B (activation energy), and C (products). Make sure you can identify these key features and explain what each part represents.
๐ก Sign Success: Negative = energy leaves (exothermic), Positive = energy enters (endothermic).
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Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Every chemical reaction involves breaking existing bonds (which requires energy) and making new bonds (which releases energy). Breaking bonds is always endothermic, whilst making bonds is always exothermic.
Each type of chemical bond has a specific bond energy value - this tells you exactly how much energy is needed to break that bond. For example, breaking an H-H bond requires +436 kJ, whilst making an H-Cl bond releases -431 kJ.
To work out if a reaction is overall exothermic or endothermic, you calculate the total energy needed to break bonds, then subtract the total energy released when making new bonds. If the answer is negative, the reaction is exothermic; if positive, it's endothermic.
๐ก Calculation Tip: Energy required to break bonds - Energy released making bonds = Overall energy change. Negative result = exothermic reaction.
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Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Required Practical 4 investigates temperature changes in the exothermic neutralisation reaction between hydrochloric acid and sodium hydroxide. You'll add increasing volumes of alkali to acid and measure the maximum temperature reached.
The method involves measuring 30cmยณ of hydrochloric acid in a polystyrene cup, then adding 5cmยณ of sodium hydroxide solution. Use a thermometer to stir and measure the highest temperature reached after the reaction.
You'll repeat this experiment increasing the sodium hydroxide volume by 5cmยณ each time up to 40cmยณ. The polystyrene cup and plastic lid are crucial because polystyrene is a good thermal insulator that reduces heat loss, giving more accurate temperature measurements.
๐ก Safety First: Always wear safety goggles when working with acids and alkalis, and handle glassware carefully to avoid spills.
Access to all documents
Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
When you plot your results, you'll see the maximum temperature initially increases as you add more sodium hydroxide. This happens because more particles are available to react with the hydrochloric acid, releasing more energy in this exothermic reaction.
However, the temperature reaches a peak then starts decreasing. This occurs because there's now so much sodium hydroxide that there isn't enough hydrochloric acid left to react with it all - you've reached the limiting reactant situation.
The temperature actually drops after this point because you're adding greater volumes of solution, so the same amount of energy released gets spread out over a larger volume. This dilution effect reduces the temperature rise you can measure.
๐ก Graph Pattern: Temperature rises to a peak (complete neutralisation), then falls due to dilution - this creates a characteristic mountain shape.
Access to all documents
Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
The polystyrene cup with plastic lid isn't just random equipment choice - it's specifically designed to minimise heat loss during your temperature measurements. Polystyrene acts as a thermal insulator, reducing heat loss through the sides and bottom of the container.
The plastic lid prevents heat escaping to the air above the reaction mixture. Without these precautions, you'd lose heat to the surroundings and get inaccurate, lower temperature readings.
This practical demonstrates how important it is to consider heat transfer when measuring energy changes in reactions. Any heat lost to the surroundings won't be detected by your thermometer, leading to underestimated energy changes.
๐ก Exam Favourite: Questions often ask why we use polystyrene cups and lids - always mention thermal insulation and reducing heat loss for accurate measurements.
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 โ all at your fingertips.
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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 itโs 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 didnโt even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as Iโm 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 ๐๐๐ฒ๐ค๐โจ๐๐ฎ
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 itโs 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 didnโt even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as Iโm 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 ๐๐๐ฒ๐ค๐โจ๐๐ฎ
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