Energy changes are at the heart of chemical reactions. When... Show more
Chemistry169 views·Updated Jun 8, 2026·2 pagesUnderstanding Energy Changes
Evelyn Ridley@ev_alice
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Exothermic and Endothermic Reactions
Ever noticed how some chemical reactions get hot? These are exothermic reactions which release thermal energy into their surroundings, causing temperature increases. Common examples include combustion (burning fuels), respiration in your cells, neutralisation reactions, and when metals react with acids.
In contrast, endothermic reactions absorb energy from their surroundings, making them feel cold. These include thermal decomposition (like limestone in a blast furnace), photosynthesis in plants, and certain types of electrolysis. The fizzing sensation in sherbet occurs due to an endothermic reaction!
The energy changes in reactions can be understood through bond energies - the energy needed to break bonds between atoms. When bonds break, energy is absorbed (endothermic process), and when new bonds form, energy is released (exothermic process). The overall energy change depends on which process requires more energy.
Remember this! In an exothermic reaction, more energy is released when forming new bonds than is absorbed when breaking existing bonds. The opposite is true for endothermic reactions.
Chemists visualize these energy changes using reaction profiles, which show the energy pathway from reactants to products. All reactions require some initial energy to get started - the activation energy - even if they ultimately release energy overall. This explains why even highly exothermic reactions like combustion need a spark to begin.
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Chemical Cells and Fuel Cells
Have you ever wondered how batteries work? They're based on redox reactions where one metal loses electrons (oxidation) and another gains them (reduction). When two different metals are placed in a salt solution and connected by a wire, the more reactive metal donates electrons to the less reactive one, creating a flow of electric current.
In a simple cell, the voltage produced depends on the difference in reactivity between the metals. The greater this difference, the higher the voltage. This is why combining reactive magnesium with less reactive copper produces a higher voltage than pairing magnesium with zinc, which is closer to it in reactivity.
Fuel cells represent an exciting application of energy changes, particularly hydrogen fuel cells. Unlike traditional batteries, fuel cells can generate electricity continuously as long as fuel (hydrogen) is supplied. They work by oxidising hydrogen at the negative electrode while oxygen is reduced at the positive electrode, with water as the only waste product.
Think about this: Hydrogen fuel cells could revolutionize transportation with zero harmful emissions, but challenges remain in hydrogen production and storage.
Hydrogen fuel cells offer significant advantages - they don't need electrical recharging, produce no pollutants, and can be sized for different applications. However, they also present challenges: hydrogen is highly flammable, often produced using non-renewable sources, and difficult to store safely. Despite these challenges, fuel cell technology continues to advance as we seek cleaner energy solutions.
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Chemistry169 views·Updated Jun 8, 2026·2 pagesUnderstanding Energy Changes
Evelyn Ridley@ev_alice
Energy changes are at the heart of chemical reactions. When substances react, bonds break and form, leading to either the release or absorption of energy. Understanding these energy transfers helps explain why some reactions feel hot, others cold, and how... Show more
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Exothermic and Endothermic Reactions
Ever noticed how some chemical reactions get hot? These are exothermic reactions which release thermal energy into their surroundings, causing temperature increases. Common examples include combustion (burning fuels), respiration in your cells, neutralisation reactions, and when metals react with acids.
In contrast, endothermic reactions absorb energy from their surroundings, making them feel cold. These include thermal decomposition (like limestone in a blast furnace), photosynthesis in plants, and certain types of electrolysis. The fizzing sensation in sherbet occurs due to an endothermic reaction!
The energy changes in reactions can be understood through bond energies - the energy needed to break bonds between atoms. When bonds break, energy is absorbed (endothermic process), and when new bonds form, energy is released (exothermic process). The overall energy change depends on which process requires more energy.
Remember this! In an exothermic reaction, more energy is released when forming new bonds than is absorbed when breaking existing bonds. The opposite is true for endothermic reactions.
Chemists visualize these energy changes using reaction profiles, which show the energy pathway from reactants to products. All reactions require some initial energy to get started - the activation energy - even if they ultimately release energy overall. This explains why even highly exothermic reactions like combustion need a spark to begin.
2
of 2Sign up to see the content. It's free!
- Access to all documents
- Improve your grades
- Join milions of students
Chemical Cells and Fuel Cells
Have you ever wondered how batteries work? They're based on redox reactions where one metal loses electrons (oxidation) and another gains them (reduction). When two different metals are placed in a salt solution and connected by a wire, the more reactive metal donates electrons to the less reactive one, creating a flow of electric current.
In a simple cell, the voltage produced depends on the difference in reactivity between the metals. The greater this difference, the higher the voltage. This is why combining reactive magnesium with less reactive copper produces a higher voltage than pairing magnesium with zinc, which is closer to it in reactivity.
Fuel cells represent an exciting application of energy changes, particularly hydrogen fuel cells. Unlike traditional batteries, fuel cells can generate electricity continuously as long as fuel (hydrogen) is supplied. They work by oxidising hydrogen at the negative electrode while oxygen is reduced at the positive electrode, with water as the only waste product.
Think about this: Hydrogen fuel cells could revolutionize transportation with zero harmful emissions, but challenges remain in hydrogen production and storage.
Hydrogen fuel cells offer significant advantages - they don't need electrical recharging, produce no pollutants, and can be sized for different applications. However, they also present challenges: hydrogen is highly flammable, often produced using non-renewable sources, and difficult to store safely. Despite these challenges, fuel cell technology continues to advance as we seek cleaner energy solutions.
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?
You can download the app from Google Play Store and 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: Energy Profile
2Most popular content in Chemistry
9Most popular content
9Can't find what you're looking for? Explore other subjects.
Students love us — and so will you.
4.6/5App Store
4.7/5Google 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 SiOS 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 KlichAndroid 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.
AnnaiOS user