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Britain & the wider world: 1745 -1901
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0
0
G
11/12/2025
Chemistry
GCSE AQA The rate and extent of chemical change
25
•
11 Dec 2025
•
G
@gurneet
Ever wondered why some chemical reactions happen lightning-fast whilst others... Show more
You can work out how fast a reaction is happening by tracking either how quickly reactants disappear or how quickly products form. It's like measuring how fast you're eating your dinner by watching how much food disappears from your plate!
The formula is straightforward: mean rate of reaction = quantity used or formed ÷ time taken. You might measure this in grams per second or cubic centimetres per second , depending on whether you're tracking mass or volume changes.
When you plot these measurements on a graph, the slope of the tangent tells you the reaction rate at any specific moment. Think of it like checking your speed on a motorway - sometimes you're faster, sometimes slower, but the gradient shows your exact speed at that instant.
Quick Tip: Steeper slopes mean faster reactions - just like steeper hills make you work harder when cycling!
Several factors can speed up your reactions, and they all boil down to getting particles to collide more often and with more energy. Higher concentration means more particles crammed into the same space, leading to more crashes between molecules.
Pressure works similarly with gases - squeeze them together and they bump into each other more frequently. Breaking solids into smaller pieces increases their surface area, exposing more particles to potential collisions.
Temperature is particularly powerful because it doesn't just make particles move faster (more collisions), but also gives them more energy to actually react when they do collide. Catalysts are the clever ones - they provide a shortcut pathway that needs less energy, like finding a secret tunnel through a mountain instead of climbing over it.
Collision theory explains this perfectly: reactions only happen when particles crash together with enough energy to overcome the activation energy barrier.
Remember: Think of activation energy as the energy needed to push a boulder over a hill - catalysts make the hill shorter!
Some reactions are like a busy two-way street - products can turn back into reactants using the double arrow symbol (⇌). The classic example is heating blue copper sulfate to get white powder plus water, then adding water back to get blue crystals again.
Here's the key insight: if a reaction releases energy going forwards (exothermic), it must absorb the same amount going backwards (endothermic). It's like energy accounting - what goes out one way must come back the other.
Equilibrium happens when the forward and reverse reactions balance out perfectly, like two equally strong people pushing a door from opposite sides. The door doesn't move, but there's still action happening on both sides.
Le Chatelier's Principle predicts what happens when you disturb this balance. If you increase reactant concentration, the system fights back by making more products until balance is restored.
Think of it this way: Equilibrium is like a seesaw - tip it one way, and it automatically adjusts to find balance again!
Temperature changes can dramatically shift where your equilibrium sits. Increase temperature and endothermic reactions get a boost (more products form), whilst exothermic reactions get pushed backwards (fewer products). It's like the reaction "prefers" the direction that absorbs the extra heat energy.
Pressure changes only matter for gas reactions, and the rule is simple: higher pressure favours the side with fewer gas molecules. Think of it as the system trying to reduce pressure by making fewer particles overall.
Lower pressure does the opposite - the equilibrium shifts towards whichever side has more gas molecules. The system essentially "expands" to fill the available space by creating more particles.
These principles aren't just academic - they're used in industry to maximise product yields and efficiency. Understanding equilibrium gives you the power to control chemical reactions like a master chef adjusting a recipe.
Industrial insight: The Haber process for making ammonia uses high pressure and moderate temperature to maximise yield - it's equilibrium principles in action!
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 – all at your fingertips.
Quotes from every main character
English LiteratureApp Store
Google Play
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
G
@gurneet
Ever wondered why some chemical reactions happen lightning-fast whilst others take ages? Understanding reaction rates and chemical equilibrium is crucial for predicting how reactions behave and controlling them in real-world applications.
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You can work out how fast a reaction is happening by tracking either how quickly reactants disappear or how quickly products form. It's like measuring how fast you're eating your dinner by watching how much food disappears from your plate!
The formula is straightforward: mean rate of reaction = quantity used or formed ÷ time taken. You might measure this in grams per second or cubic centimetres per second , depending on whether you're tracking mass or volume changes.
When you plot these measurements on a graph, the slope of the tangent tells you the reaction rate at any specific moment. Think of it like checking your speed on a motorway - sometimes you're faster, sometimes slower, but the gradient shows your exact speed at that instant.
Quick Tip: Steeper slopes mean faster reactions - just like steeper hills make you work harder when cycling!
Access to all documents
Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Several factors can speed up your reactions, and they all boil down to getting particles to collide more often and with more energy. Higher concentration means more particles crammed into the same space, leading to more crashes between molecules.
Pressure works similarly with gases - squeeze them together and they bump into each other more frequently. Breaking solids into smaller pieces increases their surface area, exposing more particles to potential collisions.
Temperature is particularly powerful because it doesn't just make particles move faster (more collisions), but also gives them more energy to actually react when they do collide. Catalysts are the clever ones - they provide a shortcut pathway that needs less energy, like finding a secret tunnel through a mountain instead of climbing over it.
Collision theory explains this perfectly: reactions only happen when particles crash together with enough energy to overcome the activation energy barrier.
Remember: Think of activation energy as the energy needed to push a boulder over a hill - catalysts make the hill shorter!
Access to all documents
Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Some reactions are like a busy two-way street - products can turn back into reactants using the double arrow symbol (⇌). The classic example is heating blue copper sulfate to get white powder plus water, then adding water back to get blue crystals again.
Here's the key insight: if a reaction releases energy going forwards (exothermic), it must absorb the same amount going backwards (endothermic). It's like energy accounting - what goes out one way must come back the other.
Equilibrium happens when the forward and reverse reactions balance out perfectly, like two equally strong people pushing a door from opposite sides. The door doesn't move, but there's still action happening on both sides.
Le Chatelier's Principle predicts what happens when you disturb this balance. If you increase reactant concentration, the system fights back by making more products until balance is restored.
Think of it this way: Equilibrium is like a seesaw - tip it one way, and it automatically adjusts to find balance again!
Access to all documents
Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Temperature changes can dramatically shift where your equilibrium sits. Increase temperature and endothermic reactions get a boost (more products form), whilst exothermic reactions get pushed backwards (fewer products). It's like the reaction "prefers" the direction that absorbs the extra heat energy.
Pressure changes only matter for gas reactions, and the rule is simple: higher pressure favours the side with fewer gas molecules. Think of it as the system trying to reduce pressure by making fewer particles overall.
Lower pressure does the opposite - the equilibrium shifts towards whichever side has more gas molecules. The system essentially "expands" to fill the available space by creating more particles.
These principles aren't just academic - they're used in industry to maximise product yields and efficiency. Understanding equilibrium gives you the power to control chemical reactions like a master chef adjusting a recipe.
Industrial insight: The Haber process for making ammonia uses high pressure and moderate temperature to maximise yield - it's equilibrium principles in action!
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 – all at your fingertips.
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Quotes from every main character
English LiteratureApp Store
Google Play
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
Sociology