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Reuben Cowell
10/12/2025
Chemistry
1.6.2 reversible reactions
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10 Dec 2025
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Understanding Reversible Reactions - GCSE Chemistry Notes
Reuben Cowell
@reubencowell
1 / 4
Reversible Reactions and Energy Changes
Unlike regular reactions that just go in one direction until everything's used up, reversible reactions can flip back and forth between reactants and products. Think of it like a two-way street where traffic can flow in both directions.
You'll spot these reactions by their special ⇌ symbol, which shows both forward and backward reactions happening simultaneously. The direction depends entirely on the conditions - heat might push it forward, whilst cold pushes it backward.
Here's something brilliant about energy: if a reversible reaction gives out energy (exothermic) going forward, it takes in exactly the same amount going backward (endothermic). The copper sulfate example shows this perfectly - adding heat removes water (endothermic), whilst adding water releases heat (exothermic).
Quick Check: Blue hydrated copper sulfate turns white when heated, but goes blue again when you add water!
Dynamic Equilibrium
Dynamic equilibrium sounds fancy, but it's actually quite straightforward - it's when the forward and backward reactions happen at exactly the same speed. This only works in a closed system where nothing can escape.
Picture this: as reactants get used up, the forward reaction slows down. Meanwhile, as more products form, the backward reaction speeds up. Eventually, they meet in the middle at equal rates.
Once equilibrium is reached, the concentrations stay constant - but don't think the reactions have stopped! They're still happening frantically, just at equal speeds so nothing appears to change overall.
Remember: At equilibrium, the rates are equal, but the amounts of reactants and products don't have to be equal!
Le Chatelier's Principle
When you mess with a system at equilibrium, it fights back! Le Chatelier's principle explains that systems will always try to counteract any changes you make to restore balance.
The equilibrium position can shift left (favouring reactants) or right (favouring products) depending on what you do. Think of it as the system's way of being stubborn - change something, and it'll work against you.
Understanding this principle lets you predict exactly what happens when you change concentration, temperature, or pressure. It's like having a crystal ball for chemical reactions!
Exam Tip: Always think "the system opposes the change" - this simple rule will help you predict which direction the equilibrium shifts.
Changing Conditions at Equilibrium
Concentration changes are the easiest to understand. Add more reactants, and the system makes more products to use them up. Remove products, and the system makes more to replace what's missing.
Temperature changes depend on whether your reaction is exothermic or endothermic. Increase temperature, and the system favours the endothermic direction to absorb that extra heat. Decrease temperature, and it favours the exothermic direction to generate warmth.
Pressure changes only matter for gas reactions. Increase pressure, and equilibrium shifts towards the side with fewer gas molecules (less crowding). Decrease pressure, and it shifts towards more gas molecules to fill the space.
Memory Trick: For pressure changes, count the gas molecules on each side - equilibrium always moves to relieve pressure by going where there's less crowding!
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Xander S
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Elisha
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Understanding Reversible Reactions - GCSE Chemistry Notes
Reuben Cowell
@reubencowell
Ever wonder why some chemical reactions can go backwards? Reversible reactions are everywhere in chemistry, and understanding how they balance out is crucial for your GCSE exams. Once you grasp these concepts, you'll be able to predict what happens when... Show more
Reversible Reactions and Energy Changes
Unlike regular reactions that just go in one direction until everything's used up, reversible reactions can flip back and forth between reactants and products. Think of it like a two-way street where traffic can flow in both directions.
You'll spot these reactions by their special ⇌ symbol, which shows both forward and backward reactions happening simultaneously. The direction depends entirely on the conditions - heat might push it forward, whilst cold pushes it backward.
Here's something brilliant about energy: if a reversible reaction gives out energy (exothermic) going forward, it takes in exactly the same amount going backward (endothermic). The copper sulfate example shows this perfectly - adding heat removes water (endothermic), whilst adding water releases heat (exothermic).
Quick Check: Blue hydrated copper sulfate turns white when heated, but goes blue again when you add water!
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Dynamic Equilibrium
Dynamic equilibrium sounds fancy, but it's actually quite straightforward - it's when the forward and backward reactions happen at exactly the same speed. This only works in a closed system where nothing can escape.
Picture this: as reactants get used up, the forward reaction slows down. Meanwhile, as more products form, the backward reaction speeds up. Eventually, they meet in the middle at equal rates.
Once equilibrium is reached, the concentrations stay constant - but don't think the reactions have stopped! They're still happening frantically, just at equal speeds so nothing appears to change overall.
Remember: At equilibrium, the rates are equal, but the amounts of reactants and products don't have to be equal!
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Le Chatelier's Principle
When you mess with a system at equilibrium, it fights back! Le Chatelier's principle explains that systems will always try to counteract any changes you make to restore balance.
The equilibrium position can shift left (favouring reactants) or right (favouring products) depending on what you do. Think of it as the system's way of being stubborn - change something, and it'll work against you.
Understanding this principle lets you predict exactly what happens when you change concentration, temperature, or pressure. It's like having a crystal ball for chemical reactions!
Exam Tip: Always think "the system opposes the change" - this simple rule will help you predict which direction the equilibrium shifts.
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Changing Conditions at Equilibrium
Concentration changes are the easiest to understand. Add more reactants, and the system makes more products to use them up. Remove products, and the system makes more to replace what's missing.
Temperature changes depend on whether your reaction is exothermic or endothermic. Increase temperature, and the system favours the endothermic direction to absorb that extra heat. Decrease temperature, and it favours the exothermic direction to generate warmth.
Pressure changes only matter for gas reactions. Increase pressure, and equilibrium shifts towards the side with fewer gas molecules (less crowding). Decrease pressure, and it shifts towards more gas molecules to fill the space.
Memory Trick: For pressure changes, count the gas molecules on each side - equilibrium always moves to relieve pressure by going where there's less crowding!
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Stefan S
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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.
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
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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
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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