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04/12/2025
Chemistry
Chapter 22.5 - Free energy
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Understanding Free Energy Change
Free energy change (ΔG) is your go-to measurement for predicting whether a chemical reaction will actually happen. Think of it as the energy bookkeeper that considers everything going on during a reaction.
The clever bit is that ΔG takes into account two crucial types of energy: enthalpy change (ΔH) - the heat energy exchanged with surroundings, and the entropy change (ΔS) - how much disorder changes within the system itself.
You'll use the Gibbs equation to calculate this: ΔG = ΔH - TΔS. Remember that temperature must be in Kelvin, and watch out for units - entropy is often given in J K⁻¹ mol⁻¹, so divide by 1000 to match enthalpy's kJ mol⁻¹.
Key Rule: If ΔG is negative, your reaction is energetically feasible. Positive ΔG means it won't happen spontaneously.
Working Through a Free Energy Calculation
Let's tackle a real example: 2NaHCO₃(s) → Na₂CO₃(s) + H₂O(l) + CO₂(g). You're given ΔH° = +91 kJ mol⁻¹ and various entropy values that need converting from J to kJ.
First, calculate ΔS° using products minus reactants: (0.135 + 0.07 + 0.214) - (2 × 0.102) = +0.215 kJ K⁻¹ mol⁻¹. Notice how the gas formation increases entropy significantly.
Now plug everything into the Gibbs equation at 298 K: ΔG = +91 - (298 × 0.215) = +26.93 kJ mol⁻¹. Since this is positive, the reaction isn't feasible at room temperature.
Pro Tip: Gas formation usually creates large positive entropy changes, which can make unfavourable reactions feasible at higher temperatures.
Finding the Minimum Temperature for Feasibility
When ΔG equals zero, you've found the minimum temperature where your reaction becomes feasible. Set the Gibbs equation to zero: ΔG = ΔH - TΔS = 0, which rearranges to T = ΔH/ΔS.
Using our previous example: T = 91/0.215 = 423 K (150°C). Above this temperature, the reaction becomes thermodynamically feasible because TΔS finally outweighs ΔH.
The pattern is clear: exothermic reactions (negative ΔH) with positive entropy changes are always feasible. Endothermic reactions need high temperatures when ΔS is positive, but they're never feasible if ΔS is negative.
Memory Trick: For feasibility, you need TΔS to be larger than ΔH - think of entropy fighting against unfavourable enthalpy changes.
Limitations You Need to Know
Here's the crucial limitation that might catch you out in exams: ΔG only tells you about thermodynamic feasibility, not reaction rate. A reaction might have a lovely negative ΔG but still appear not to happen at all.
The missing piece is activation energy (Ea). If this energy barrier is sky-high, your reaction crawls along so slowly it seems like nothing's happening, even though ΔG says it should work perfectly.
This is why understanding both thermodynamics and kinetics matters - ΔG predicts the destination, but kinetics determines how long the journey takes.
Exam Alert: Always remember that negative ΔG means thermodynamically feasible, but you might need a catalyst to make it happen at a reasonable rate.
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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
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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
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
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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.
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
Understanding Free Energy: Enthalpy and Entropy Explained
Free energy is the ultimate judge of whether chemical reactions can actually happen - it's like having a crystal ball that tells you if your reaction will work or not. Understanding how to calculate and interpret free energy changes will... Show more
Understanding Free Energy Change
Free energy change (ΔG) is your go-to measurement for predicting whether a chemical reaction will actually happen. Think of it as the energy bookkeeper that considers everything going on during a reaction.
The clever bit is that ΔG takes into account two crucial types of energy: enthalpy change (ΔH) - the heat energy exchanged with surroundings, and the entropy change (ΔS) - how much disorder changes within the system itself.
You'll use the Gibbs equation to calculate this: ΔG = ΔH - TΔS. Remember that temperature must be in Kelvin, and watch out for units - entropy is often given in J K⁻¹ mol⁻¹, so divide by 1000 to match enthalpy's kJ mol⁻¹.
Key Rule: If ΔG is negative, your reaction is energetically feasible. Positive ΔG means it won't happen spontaneously.
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Working Through a Free Energy Calculation
Let's tackle a real example: 2NaHCO₃(s) → Na₂CO₃(s) + H₂O(l) + CO₂(g). You're given ΔH° = +91 kJ mol⁻¹ and various entropy values that need converting from J to kJ.
First, calculate ΔS° using products minus reactants: (0.135 + 0.07 + 0.214) - (2 × 0.102) = +0.215 kJ K⁻¹ mol⁻¹. Notice how the gas formation increases entropy significantly.
Now plug everything into the Gibbs equation at 298 K: ΔG = +91 - (298 × 0.215) = +26.93 kJ mol⁻¹. Since this is positive, the reaction isn't feasible at room temperature.
Pro Tip: Gas formation usually creates large positive entropy changes, which can make unfavourable reactions feasible at higher temperatures.
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Finding the Minimum Temperature for Feasibility
When ΔG equals zero, you've found the minimum temperature where your reaction becomes feasible. Set the Gibbs equation to zero: ΔG = ΔH - TΔS = 0, which rearranges to T = ΔH/ΔS.
Using our previous example: T = 91/0.215 = 423 K (150°C). Above this temperature, the reaction becomes thermodynamically feasible because TΔS finally outweighs ΔH.
The pattern is clear: exothermic reactions (negative ΔH) with positive entropy changes are always feasible. Endothermic reactions need high temperatures when ΔS is positive, but they're never feasible if ΔS is negative.
Memory Trick: For feasibility, you need TΔS to be larger than ΔH - think of entropy fighting against unfavourable enthalpy changes.
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Limitations You Need to Know
Here's the crucial limitation that might catch you out in exams: ΔG only tells you about thermodynamic feasibility, not reaction rate. A reaction might have a lovely negative ΔG but still appear not to happen at all.
The missing piece is activation energy (Ea). If this energy barrier is sky-high, your reaction crawls along so slowly it seems like nothing's happening, even though ΔG says it should work perfectly.
This is why understanding both thermodynamics and kinetics matters - ΔG predicts the destination, but kinetics determines how long the journey takes.
Exam Alert: Always remember that negative ΔG means thermodynamically feasible, but you might need a catalyst to make it happen at a reasonable rate.
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
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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
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