Thermodynamics helps us understand energy changes in chemical reactions. Hess's... Show more
AlgebraSign up to see the contentIt's free!
Access to all documents
Improve your grades
Join milions of students
34
•
Feb 1, 2026
•
Understanding Thermodynamics: An Overview of Hess's Law
thatweebudontknow
@thatweebudontknow
Hess's Law and Energy Calculations
Ever wonder how scientists figure out the energy changes for reactions they can't measure directly? That's where Hess's Law comes in handy! This principle states that the overall enthalpy change of a reaction remains the same whether it occurs in one step or multiple steps.
For example, to find the enthalpy change when graphite transforms into diamond, we can break it down into two steps. First, graphite reacts with oxygen . Then, diamond is formed from carbon dioxide . Adding these together gives us the energy change for the direct transformation.
When working with reactions, remember these key rules: If you reverse a reaction, the enthalpy value keeps the same magnitude but changes sign. If a reaction shows CO₂(g) → C(diamond) + O₂(g) with ΔH = +396 kJ/mol, then the reverse reaction C(diamond) + O₂(g) → CO₂(g) has ΔH = -396 kJ/mol.
💡 Think of enthalpy like climbing a hill - going up requires energy (endothermic), coming down releases energy (exothermic), but the height of the hill stays the same either way!
When you multiply or divide a chemical reaction by a factor, you must do the same to its enthalpy value. Doubling a reaction doubles the enthalpy change; halving a reaction halves the enthalpy change. For instance, if C(diamond) + O₂(g) → CO₂(g) has ΔH = -394 kJ/mol, then 2C(diamond) + 2O₂(g) → 2CO₂(g) has ΔH = 2(-394) = -788 kJ/mol.
Applying Hess's Law to Complex Problems
Now it's time to put Hess's Law into action with a more challenging problem! Let's find the enthalpy change for the reaction: N₂(g) + 2O₂(g) → 2NO₂(g).
We're given two reactions with known enthalpy changes:
- N₂(g) + O₂(g) → 2NO(g) with ΔH₁ = 180 kJ/mol
- 2NO₂(g) → 2NO(g) + O₂(g) with ΔH₂ = 112 kJ/mol
To solve this problem, we need to manipulate these reactions so they add up to our target reaction. First, keep the first reaction as is. Then, we need to reverse the second reaction to get: 2NO(g) + O₂(g) → 2NO₂(g).
When we add these reactions together, the 2NO(g) appears on both sides and cancels out, leaving us with: N₂(g) + 2O₂(g) → 2NO₂(g). The overall enthalpy change is ΔH = ΔH₁ + = 180 + (-112) = 68 kJ/mol.
🔍 Look for "unique chemicals" (those that appear only once in one reaction) as guideposts for how to manipulate your equations!
This problem-solving approach works for any complex reaction. Break it down into steps with known enthalpy values, manipulate as needed , then add everything up to find your answer. You've got this!
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.
Most popular content in Chemistry
Most popular content
Can't find what you're looking for? Explore other subjects.
Students love us — and so will you.
4.6/5
App Store
4.7/5
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
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 Knowunity AI. 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 Knowunity AI. 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 Thermodynamics: An Overview of Hess's Law
thatweebudontknow
@thatweebudontknow
Thermodynamics helps us understand energy changes in chemical reactions. Hess's Law is a powerful tool that allows chemists to calculate energy changes indirectly by breaking reactions into smaller steps with known energy values.
Hess's Law and Energy Calculations
Ever wonder how scientists figure out the energy changes for reactions they can't measure directly? That's where Hess's Law comes in handy! This principle states that the overall enthalpy change of a reaction remains the same whether it occurs in one step or multiple steps.
For example, to find the enthalpy change when graphite transforms into diamond, we can break it down into two steps. First, graphite reacts with oxygen . Then, diamond is formed from carbon dioxide . Adding these together gives us the energy change for the direct transformation.
When working with reactions, remember these key rules: If you reverse a reaction, the enthalpy value keeps the same magnitude but changes sign. If a reaction shows CO₂(g) → C(diamond) + O₂(g) with ΔH = +396 kJ/mol, then the reverse reaction C(diamond) + O₂(g) → CO₂(g) has ΔH = -396 kJ/mol.
💡 Think of enthalpy like climbing a hill - going up requires energy (endothermic), coming down releases energy (exothermic), but the height of the hill stays the same either way!
When you multiply or divide a chemical reaction by a factor, you must do the same to its enthalpy value. Doubling a reaction doubles the enthalpy change; halving a reaction halves the enthalpy change. For instance, if C(diamond) + O₂(g) → CO₂(g) has ΔH = -394 kJ/mol, then 2C(diamond) + 2O₂(g) → 2CO₂(g) has ΔH = 2(-394) = -788 kJ/mol.
Sign up to see the contentIt's free!
Access to all documents
Improve your grades
Join milions of students
Applying Hess's Law to Complex Problems
Now it's time to put Hess's Law into action with a more challenging problem! Let's find the enthalpy change for the reaction: N₂(g) + 2O₂(g) → 2NO₂(g).
We're given two reactions with known enthalpy changes:
- N₂(g) + O₂(g) → 2NO(g) with ΔH₁ = 180 kJ/mol
- 2NO₂(g) → 2NO(g) + O₂(g) with ΔH₂ = 112 kJ/mol
To solve this problem, we need to manipulate these reactions so they add up to our target reaction. First, keep the first reaction as is. Then, we need to reverse the second reaction to get: 2NO(g) + O₂(g) → 2NO₂(g).
When we add these reactions together, the 2NO(g) appears on both sides and cancels out, leaving us with: N₂(g) + 2O₂(g) → 2NO₂(g). The overall enthalpy change is ΔH = ΔH₁ + = 180 + (-112) = 68 kJ/mol.
🔍 Look for "unique chemicals" (those that appear only once in one reaction) as guideposts for how to manipulate your equations!
This problem-solving approach works for any complex reaction. Break it down into steps with known enthalpy values, manipulate as needed , then add everything up to find your answer. You've got this!
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.
0
Smart Tools NEW
Transform this note into: ✓ 50+ Practice Questions ✓ Interactive Flashcards ✓ Full Practice Test ✓ Essay Outlines
Practice Test
Quiz
Flashcards
Essay
Similar Content
Enthalpy Change Calculations
Explore the principles of measuring enthalpy changes through calorimetry. This summary covers key concepts such as specific heat capacity, energy transfer calculations, and the distinction between exothermic and endothermic reactions. Ideal for students preparing for exams or needing a quick reference on enthalpy changes in chemical reactions.
Chemistry12
honors chem 1 unit 9 study guide (thermodynamics)
temperature vs heat, endothermic & exothermic, thermodynamic calculations (phase change including heat of fusion, vaporization; temperature change using q=mc/\T; enthalpy of reaction), entropy & gibbs free energy
AP Chemistry10th
Haber Process & NPK Fertilisers
Explore the Haber Process for ammonia synthesis, including reaction conditions, catalyst use, and the production of NPK fertilisers. Understand the impact of temperature and pressure on yield, and the environmental implications of fertiliser use. This summary provides essential insights for chemistry students.
Chemistry10
Gibbs Free Energy Insights
Explore the principles of Gibbs Free Energy, including enthalpy and entropy changes, thermodynamic favorability, and spontaneity. This summary covers key concepts such as standard entropy and enthalpy changes, providing essential insights for understanding chemical reactions and their feasibility. Ideal for students studying thermodynamics.
Chemistry12
AP Chemistry 6.5
Notes relating to phase change: triple point, critical points, and heating curves.
AP Chemistry10th
Phase Diagram and Emission Spectra
Notes on the topic
ChemistryMost popular content in Chemistry
Most popular content
Can't find what you're looking for? Explore other subjects.
Students love us — and so will you.
4.6/5
App Store
4.7/5
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
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 Knowunity AI. 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 Knowunity AI. 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