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5
0
J
Jessica Plane
12/12/2025
Chemistry
Chemistry: topic 5
255
•
12 Dec 2025
•
Chemistry: Energy Changes and Their Importance
J
Jessica Plane
@essicalane_jeu6oyvga
1 / 10
Endothermic Reactions
Think about when you feel cold after sweating - that's because endothermic reactions are happening on your skin! These reactions absorb heat from their surroundings, making the temperature drop.
In endothermic reactions, breaking the old bonds requires more energy than what's released when new bonds form. It's like needing to spend more money than you earn - you end up with less energy overall.
Photosynthesis is a perfect example - plants absorb energy from sunlight to convert carbon dioxide and water into glucose. The reaction literally takes in energy from the environment.
Quick Tip: Remember "endo" means "in" - energy goes INTO the reaction!
Exothermic Reactions
Exothermic reactions are the complete opposite - they release heat into the surroundings, making things warmer. Think about a crackling fire or hand warmers on a cold day.
These reactions give out more energy when making new bonds than was needed to break the old ones. It's like earning more money than you spend - you end up with extra energy that gets released as heat.
Fire is the classic example everyone knows. When wood burns, it releases loads of energy as heat and light, warming everything around it.
Memory Trick: "Exo" means "exit" - energy EXITS the reaction and heats up the surroundings!
Endothermic Energy Diagrams
Energy diagrams show you exactly what's happening during reactions. For endothermic reactions, the products end up with more energy than the reactants started with.
The ΔH (delta H) value is always positive because energy has been absorbed from the surroundings. Think of it as the reaction's energy bill - positive means you owe energy.
You'll see the products sitting higher on the energy diagram than the reactants. The difference between them shows how much energy was taken in during the reaction.
Exam Tip: If ΔH is positive, the reaction is endothermic and the surroundings get colder!
Exothermic Energy Diagrams
In exothermic reactions, the energy diagram looks flipped compared to endothermic ones. The reactants start with more energy than the products end up with.
The ΔH value is negative because energy has been released to the surroundings. Think of it as the reaction paying you back in energy - that's why it's negative.
Activation energy (Ea) is still needed to get the reaction started, like pushing a boulder off a cliff. Once it starts, the reaction releases more energy than it took to begin.
Remember: Negative ΔH means energy flows OUT to the surroundings, making them warmer!
Calculating Energy Changes
You can work out whether a reaction is endothermic or exothermic by calculating bond energies. It's like doing the reaction's accounts - energy in versus energy out.
Use this formula: ΔH = Bonds broken - Bonds made. Count up all the energy needed to break bonds in reactants, then subtract the energy released when making new bonds in products.
In the methane example, breaking bonds needs 2660 kJ but making new bonds releases 3466 kJ. Since more energy comes out than goes in, ΔH is negative and the reaction is exothermic.
Pro Tip: Always count the actual bonds (lines in diagrams), not just the atoms!
Counting Bonds Correctly
Getting bond calculations right means counting carefully. Look at the actual bonds (the lines) connecting atoms, not just the number of atoms present.
For propane (CH₃CH₂CH₃), you've got 8 C-H bonds and 2 C-C bonds. Each line you can draw between atoms represents one bond that needs energy to break.
This might seem obvious, but it's dead easy to miscount in exam pressure. Take your time and literally count each connection.
Exam Saver: Draw out the structure if you need to - it's worth the extra seconds to get it right!
Non-Rechargeable vs Rechargeable Batteries
Non-rechargeable batteries are one-time deals. Once the chemical reactions inside are done, that's it - the battery's dead and you chuck it away.
As the reactions happen, the ion concentration in the electrolyte drops, so the voltage gradually decreases. Eventually there are no ions left to react and the voltage hits zero.
Rechargeable batteries are clever because their reactions can be reversed. Connect them to external power and you can force the reaction backwards, restoring the original chemicals.
Environmental Note: Rechargeable batteries create much less waste, but they do contain toxic chemicals!
Rechargeable Battery Pros and Cons
Rechargeable batteries are brilliant for the environment because you're not constantly throwing them away. They're portable, relatively cheap, and you don't need to replace them often.
However, they're not perfect. If your device can't recharge the battery, it becomes useless when it dies. You need access to charging facilities regularly.
They also have a finite lifespan - even rechargeable batteries eventually wear out after hundreds of charge cycles. Plus, they often contain toxic chemicals that need proper disposal.
Real Talk: For frequently used devices, rechargeable batteries save you loads of money in the long run!
Electrochemical Cells Basics
Electrochemical cells are the building blocks of all batteries. They contain two different electrodes (usually metals) and an electrolyte that allows ions to move between them.
The more reactive metal gets oxidised (loses electrons) whilst the less reactive metal gets reduced (gains electrons). In a zinc-copper cell, zinc loses electrons and copper gains them.
This electron flow through the external wire is what we call electric current. The chemical reactions literally push electrons around the circuit, powering your devices.
Key Point: The bigger the difference in reactivity between metals, the higher the voltage!
Factors Affecting Cell Potential
The voltage your electrochemical cell produces depends on several key factors. The type of electrodes matters most - metals that are very different in reactivity give higher voltages.
Electrolyte concentration also affects the voltage. Higher concentrations generally mean more ions available to react, which can increase the cell's potential difference.
You can predict and control cell voltage by choosing appropriate electrode materials and electrolyte concentrations. This is how battery manufacturers design cells for specific applications.
Practical Tip: Want higher voltage? Use metals that are far apart on the reactivity series!
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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
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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
Chemistry: Energy Changes and Their Importance
J
Jessica Plane
@essicalane_jeu6oyvga
Ever wondered why some reactions feel hot whilst others make things cold? Chemical reactions either absorb or release energy, and understanding this helps explain everything from why batteries work to how your body gets energy from food.
Endothermic Reactions
Think about when you feel cold after sweating - that's because endothermic reactions are happening on your skin! These reactions absorb heat from their surroundings, making the temperature drop.
In endothermic reactions, breaking the old bonds requires more energy than what's released when new bonds form. It's like needing to spend more money than you earn - you end up with less energy overall.
Photosynthesis is a perfect example - plants absorb energy from sunlight to convert carbon dioxide and water into glucose. The reaction literally takes in energy from the environment.
Quick Tip: Remember "endo" means "in" - energy goes INTO the reaction!
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Exothermic Reactions
Exothermic reactions are the complete opposite - they release heat into the surroundings, making things warmer. Think about a crackling fire or hand warmers on a cold day.
These reactions give out more energy when making new bonds than was needed to break the old ones. It's like earning more money than you spend - you end up with extra energy that gets released as heat.
Fire is the classic example everyone knows. When wood burns, it releases loads of energy as heat and light, warming everything around it.
Memory Trick: "Exo" means "exit" - energy EXITS the reaction and heats up the surroundings!
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Endothermic Energy Diagrams
Energy diagrams show you exactly what's happening during reactions. For endothermic reactions, the products end up with more energy than the reactants started with.
The ΔH (delta H) value is always positive because energy has been absorbed from the surroundings. Think of it as the reaction's energy bill - positive means you owe energy.
You'll see the products sitting higher on the energy diagram than the reactants. The difference between them shows how much energy was taken in during the reaction.
Exam Tip: If ΔH is positive, the reaction is endothermic and the surroundings get colder!
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Exothermic Energy Diagrams
In exothermic reactions, the energy diagram looks flipped compared to endothermic ones. The reactants start with more energy than the products end up with.
The ΔH value is negative because energy has been released to the surroundings. Think of it as the reaction paying you back in energy - that's why it's negative.
Activation energy (Ea) is still needed to get the reaction started, like pushing a boulder off a cliff. Once it starts, the reaction releases more energy than it took to begin.
Remember: Negative ΔH means energy flows OUT to the surroundings, making them warmer!
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Calculating Energy Changes
You can work out whether a reaction is endothermic or exothermic by calculating bond energies. It's like doing the reaction's accounts - energy in versus energy out.
Use this formula: ΔH = Bonds broken - Bonds made. Count up all the energy needed to break bonds in reactants, then subtract the energy released when making new bonds in products.
In the methane example, breaking bonds needs 2660 kJ but making new bonds releases 3466 kJ. Since more energy comes out than goes in, ΔH is negative and the reaction is exothermic.
Pro Tip: Always count the actual bonds (lines in diagrams), not just the atoms!
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Counting Bonds Correctly
Getting bond calculations right means counting carefully. Look at the actual bonds (the lines) connecting atoms, not just the number of atoms present.
For propane (CH₃CH₂CH₃), you've got 8 C-H bonds and 2 C-C bonds. Each line you can draw between atoms represents one bond that needs energy to break.
This might seem obvious, but it's dead easy to miscount in exam pressure. Take your time and literally count each connection.
Exam Saver: Draw out the structure if you need to - it's worth the extra seconds to get it right!
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Non-Rechargeable vs Rechargeable Batteries
Non-rechargeable batteries are one-time deals. Once the chemical reactions inside are done, that's it - the battery's dead and you chuck it away.
As the reactions happen, the ion concentration in the electrolyte drops, so the voltage gradually decreases. Eventually there are no ions left to react and the voltage hits zero.
Rechargeable batteries are clever because their reactions can be reversed. Connect them to external power and you can force the reaction backwards, restoring the original chemicals.
Environmental Note: Rechargeable batteries create much less waste, but they do contain toxic chemicals!
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Rechargeable Battery Pros and Cons
Rechargeable batteries are brilliant for the environment because you're not constantly throwing them away. They're portable, relatively cheap, and you don't need to replace them often.
However, they're not perfect. If your device can't recharge the battery, it becomes useless when it dies. You need access to charging facilities regularly.
They also have a finite lifespan - even rechargeable batteries eventually wear out after hundreds of charge cycles. Plus, they often contain toxic chemicals that need proper disposal.
Real Talk: For frequently used devices, rechargeable batteries save you loads of money in the long run!
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Electrochemical Cells Basics
Electrochemical cells are the building blocks of all batteries. They contain two different electrodes (usually metals) and an electrolyte that allows ions to move between them.
The more reactive metal gets oxidised (loses electrons) whilst the less reactive metal gets reduced (gains electrons). In a zinc-copper cell, zinc loses electrons and copper gains them.
This electron flow through the external wire is what we call electric current. The chemical reactions literally push electrons around the circuit, powering your devices.
Key Point: The bigger the difference in reactivity between metals, the higher the voltage!
Sign up to see the contentIt's free!
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By signing up you accept Terms of Service and Privacy Policy
Factors Affecting Cell Potential
The voltage your electrochemical cell produces depends on several key factors. The type of electrodes matters most - metals that are very different in reactivity give higher voltages.
Electrolyte concentration also affects the voltage. Higher concentrations generally mean more ions available to react, which can increase the cell's potential difference.
You can predict and control cell voltage by choosing appropriate electrode materials and electrolyte concentrations. This is how battery manufacturers design cells for specific applications.
Practical Tip: Want higher voltage? Use metals that are far apart on the reactivity series!
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
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