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12/3/2025
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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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.
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I found this app a couple years ago and it has only gotten better since then. I really love it because it can help with written questions and photo questions. Also, it can find study guides that other people have made as well as flashcard sets and practice tests. The free version is also amazing for students who might not be able to afford it. Would 100% recommend
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Best app if you're in Highschool or Junior high. I have been using this app for 2 school years and it's the best, it's good if you don't have anyone to help you with school work.😋🩷🎀
Marco B
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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 😍😁😲🤑💗✨🎀😮
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This app is phenomenal down to the correct info and the various topics you can study! I greatly recommend it for people who struggle with procrastination and those who need homework help. It has been perfectly accurate for world 1 history as far as I’ve seen! Geometry too!
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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
I think it’s very much worth it and you’ll end up using it a lot once you get the hang of it and even after looking at others notes you can still ask your Artificial intelligence buddy the question and ask to simplify it if you still don’t get it!!! In the end I think it’s worth it 😊👍 ⚠️Also DID I MENTION ITS FREEE YOU DON’T HAVE TO PAY FOR ANYTHING AND STILL GET YOUR GRADES IN PERFECTLY❗️❗️⚠️
Thomas R
iOS user
Knowunity is the BEST app I’ve used in a minute. This is not an ai review or anything this is genuinely coming from a 7th grade student (I know 2011 im young) but dude this app is a 10/10 i have maintained a 3.8 gpa and have plenty of time for gaming. I love it and my mom is just happy I got good grades
Brad T
Android user
Not only did it help me find the answer but it also showed me alternative ways to solve it. I was horrible in math and science but now I have an a in both subjects. Thanks for the help🤍🤍
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
I found this app a couple years ago and it has only gotten better since then. I really love it because it can help with written questions and photo questions. Also, it can find study guides that other people have made as well as flashcard sets and practice tests. The free version is also amazing for students who might not be able to afford it. Would 100% recommend
Aubrey
iOS user
Best app if you're in Highschool or Junior high. I have been using this app for 2 school years and it's the best, it's good if you don't have anyone to help you with school work.😋🩷🎀
Marco B
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 app is phenomenal down to the correct info and the various topics you can study! I greatly recommend it for people who struggle with procrastination and those who need homework help. It has been perfectly accurate for world 1 history as far as I’ve seen! Geometry too!
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 specifically built for the needs of students. Based on the millions of content pieces we have on the platform we can provide truly meaningful and relevant answers to students. But its not only about answers, the companion is even more about guiding students through their daily learning challenges, with personalised study plans, quizzes or content pieces in the chat and 100% personalisation based on the students skills and developments.
Where can I download the Knowunity app?
You can download the app in the Google Play Store and in the Apple App Store.
Is Knowunity really free of charge?
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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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
I think it’s very much worth it and you’ll end up using it a lot once you get the hang of it and even after looking at others notes you can still ask your Artificial intelligence buddy the question and ask to simplify it if you still don’t get it!!! In the end I think it’s worth it 😊👍 ⚠️Also DID I MENTION ITS FREEE YOU DON’T HAVE TO PAY FOR ANYTHING AND STILL GET YOUR GRADES IN PERFECTLY❗️❗️⚠️
Thomas R
iOS user
Knowunity is the BEST app I’ve used in a minute. This is not an ai review or anything this is genuinely coming from a 7th grade student (I know 2011 im young) but dude this app is a 10/10 i have maintained a 3.8 gpa and have plenty of time for gaming. I love it and my mom is just happy I got good grades
Brad T
Android user
Not only did it help me find the answer but it also showed me alternative ways to solve it. I was horrible in math and science but now I have an a in both subjects. Thanks for the help🤍🤍
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
I found this app a couple years ago and it has only gotten better since then. I really love it because it can help with written questions and photo questions. Also, it can find study guides that other people have made as well as flashcard sets and practice tests. The free version is also amazing for students who might not be able to afford it. Would 100% recommend
Aubrey
iOS user
Best app if you're in Highschool or Junior high. I have been using this app for 2 school years and it's the best, it's good if you don't have anyone to help you with school work.😋🩷🎀
Marco B
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 app is phenomenal down to the correct info and the various topics you can study! I greatly recommend it for people who struggle with procrastination and those who need homework help. It has been perfectly accurate for world 1 history as far as I’ve seen! Geometry too!
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
I think it’s very much worth it and you’ll end up using it a lot once you get the hang of it and even after looking at others notes you can still ask your Artificial intelligence buddy the question and ask to simplify it if you still don’t get it!!! In the end I think it’s worth it 😊👍 ⚠️Also DID I MENTION ITS FREEE YOU DON’T HAVE TO PAY FOR ANYTHING AND STILL GET YOUR GRADES IN PERFECTLY❗️❗️⚠️
Thomas R
iOS user
Knowunity is the BEST app I’ve used in a minute. This is not an ai review or anything this is genuinely coming from a 7th grade student (I know 2011 im young) but dude this app is a 10/10 i have maintained a 3.8 gpa and have plenty of time for gaming. I love it and my mom is just happy I got good grades
Brad T
Android user
Not only did it help me find the answer but it also showed me alternative ways to solve it. I was horrible in math and science but now I have an a in both subjects. Thanks for the help🤍🤍
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
I found this app a couple years ago and it has only gotten better since then. I really love it because it can help with written questions and photo questions. Also, it can find study guides that other people have made as well as flashcard sets and practice tests. The free version is also amazing for students who might not be able to afford it. Would 100% recommend
Aubrey
iOS user
Best app if you're in Highschool or Junior high. I have been using this app for 2 school years and it's the best, it's good if you don't have anyone to help you with school work.😋🩷🎀
Marco B
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 app is phenomenal down to the correct info and the various topics you can study! I greatly recommend it for people who struggle with procrastination and those who need homework help. It has been perfectly accurate for world 1 history as far as I’ve seen! Geometry too!
Paul T
iOS user