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Updated Mar 25, 2026
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Comprehensive Chemistry Notes on Alkenes, Alkanes, and Polymers
Annabel
@annabel_xo
1 / 6
Alkenes and Their Properties
Alkenes are unsaturated hydrocarbons that contain a carbon-carbon double bond , which makes them much more reactive than alkanes. The first four alkenes you need to know are ethene, propene, butene, and pentene, and they all follow the general formula CₙH₂ₙ.
What makes alkenes special is their ability to undergo addition reactions. The double bond can "open up" to allow other atoms to attach, turning the unsaturated alkene into a saturated compound. This is something alkanes simply can't do because they only have single bonds.
When alkenes burn, they produce smoky flames due to incomplete combustion. This happens because the double bonds make it harder for complete combustion to occur, which is why alkenes aren't typically used as fuels despite being hydrocarbons.
Key Point: The C=C double bond is what gives alkenes their reactive nature - it's like a "hook" that other molecules can attach to!
Addition Reactions of Alkenes
Alkenes can undergo three main types of addition reactions, each producing different useful products. Hydrogenation involves adding hydrogen gas with a nickel catalyst to produce alkanes - this process is used in making margarine from vegetable oils.
Hydration adds steam (water) at about 300°C with a catalyst to create alcohols. This is actually how industrial ethanol is produced for use in chemicals and fuels.
Halogenation involves adding halogens like bromine or chlorine to create halogenoalkanes. This reaction is particularly important for testing - when you add bromine water to an alkene, it changes from orange-brown to colourless, proving an alkene is present.
Test Tip: Remember the bromine water test - it's colourless with alkenes but stays orange-brown with alkanes. This often comes up in exams!
Alcohols and Their Reactions
Alcohols contain the functional group -OH (hydroxyl group) and follow the general formula CₙH₂ₙ₊₁OH. The ones you'll encounter most are methanol, ethanol, propanol, and butanol - notice how the names end in "-ol".
Alcohols are surprisingly reactive and undergo several important reactions. When mixed with strong oxidising agents, they form carboxylic acids. They react with sodium metal to produce hydrogen gas, and they dissolve in water to give neutral solutions.
Fermentation is a crucial process for producing ethanol using yeast and sugar. The ideal conditions are 37°C (body temperature), slightly acidic conditions, and no oxygen present (anaerobic). This ancient process is still used today for making alcoholic beverages and biofuels.
Real-World Connection: Fermentation happens in your kitchen when you make bread - the yeast produces both alcohol (which evaporates) and CO₂ (which makes the bread rise)!
Carboxylic Acids
Carboxylic acids contain the -COOH (carboxyl group) and have the general formula CₙH₂ₙ₊₁COOH. Common examples include methanoic acid (HCOOH) and ethanoic acid (CH₃COOH) - you'll recognise ethanoic acid as the main component in vinegar.
These are weak acids because they only partially ionise in water, making them less dangerous than strong acids like hydrochloric acid. When dissolved in water, they create acidic solutions but won't burn through your skin like concentrated strong acids would.
Carboxylic acids react with alcohols (using an acid catalyst) to form esters and water. They also react with carbonates to produce a salt, carbon dioxide, and water - just like other acids do.
Memory Trick: Remember "COOH" sounds like "COO" - carboxylic acids are the ones that make vinegar smell and taste sharp!
Polymerisation
Polymerisation is the process of joining small molecules (monomers) together to create long chains called polymers - think of it like connecting LEGO bricks to build something massive. There are two main types you need to understand.
Addition polymerisation uses alkenes as monomers because their C=C double bonds can open up and link together. The polymer contains exactly the same atoms as the original monomers - nothing is lost. Poly(ethene) plastic bags are made this way from ethene monomers.
Condensation polymerisation involves monomers with two functional groups that join together whilst producing small molecules like water as by-products. Amino acids use this process to form proteins and polypeptides in your body.
Think About It: Your hair, muscles, and even the enzymes in your digestive system are all polymers made from amino acids through condensation polymerisation!
Key Alkene Reactions Summary
Understanding alkene reactions becomes much easier when you focus on the specific conditions and products for each reaction type. Hydrogenation requires a nickel catalyst and produces alkanes, whilst hydration needs high temperatures (around 300°C) and a catalyst to form alcohols.
The halogenation reaction is particularly useful for identification purposes. When alkenes react with halogens like bromine, the orange-brown bromine water becomes colourless, creating compounds like dibromoethane from ethene and bromine.
Remember that alkenes aren't used as fuels because their double bonds cause incomplete combustion, resulting in sooty, smoky flames. This makes them less efficient and more polluting than alkanes for energy production.
Exam Success: Practice drawing the structural formulas for these reactions - examiners love testing whether you can show the before and after structures correctly!
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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.
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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.
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Elisha
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Comprehensive Chemistry Notes on Alkenes, Alkanes, and Polymers
Annabel
@annabel_xo
Organic chemistry might seem complex, but it's actually about understanding how different groups of carbon-based molecules behave and react with each other. You'll discover how alkenes, alcohols, carboxylic acids, and polymers each have their own unique properties and reactions that... Show more
Alkenes and Their Properties
Alkenes are unsaturated hydrocarbons that contain a carbon-carbon double bond , which makes them much more reactive than alkanes. The first four alkenes you need to know are ethene, propene, butene, and pentene, and they all follow the general formula CₙH₂ₙ.
What makes alkenes special is their ability to undergo addition reactions. The double bond can "open up" to allow other atoms to attach, turning the unsaturated alkene into a saturated compound. This is something alkanes simply can't do because they only have single bonds.
When alkenes burn, they produce smoky flames due to incomplete combustion. This happens because the double bonds make it harder for complete combustion to occur, which is why alkenes aren't typically used as fuels despite being hydrocarbons.
Key Point: The C=C double bond is what gives alkenes their reactive nature - it's like a "hook" that other molecules can attach to!
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Addition Reactions of Alkenes
Alkenes can undergo three main types of addition reactions, each producing different useful products. Hydrogenation involves adding hydrogen gas with a nickel catalyst to produce alkanes - this process is used in making margarine from vegetable oils.
Hydration adds steam (water) at about 300°C with a catalyst to create alcohols. This is actually how industrial ethanol is produced for use in chemicals and fuels.
Halogenation involves adding halogens like bromine or chlorine to create halogenoalkanes. This reaction is particularly important for testing - when you add bromine water to an alkene, it changes from orange-brown to colourless, proving an alkene is present.
Test Tip: Remember the bromine water test - it's colourless with alkenes but stays orange-brown with alkanes. This often comes up in exams!
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Alcohols and Their Reactions
Alcohols contain the functional group -OH (hydroxyl group) and follow the general formula CₙH₂ₙ₊₁OH. The ones you'll encounter most are methanol, ethanol, propanol, and butanol - notice how the names end in "-ol".
Alcohols are surprisingly reactive and undergo several important reactions. When mixed with strong oxidising agents, they form carboxylic acids. They react with sodium metal to produce hydrogen gas, and they dissolve in water to give neutral solutions.
Fermentation is a crucial process for producing ethanol using yeast and sugar. The ideal conditions are 37°C (body temperature), slightly acidic conditions, and no oxygen present (anaerobic). This ancient process is still used today for making alcoholic beverages and biofuels.
Real-World Connection: Fermentation happens in your kitchen when you make bread - the yeast produces both alcohol (which evaporates) and CO₂ (which makes the bread rise)!
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Carboxylic Acids
Carboxylic acids contain the -COOH (carboxyl group) and have the general formula CₙH₂ₙ₊₁COOH. Common examples include methanoic acid (HCOOH) and ethanoic acid (CH₃COOH) - you'll recognise ethanoic acid as the main component in vinegar.
These are weak acids because they only partially ionise in water, making them less dangerous than strong acids like hydrochloric acid. When dissolved in water, they create acidic solutions but won't burn through your skin like concentrated strong acids would.
Carboxylic acids react with alcohols (using an acid catalyst) to form esters and water. They also react with carbonates to produce a salt, carbon dioxide, and water - just like other acids do.
Memory Trick: Remember "COOH" sounds like "COO" - carboxylic acids are the ones that make vinegar smell and taste sharp!
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Polymerisation
Polymerisation is the process of joining small molecules (monomers) together to create long chains called polymers - think of it like connecting LEGO bricks to build something massive. There are two main types you need to understand.
Addition polymerisation uses alkenes as monomers because their C=C double bonds can open up and link together. The polymer contains exactly the same atoms as the original monomers - nothing is lost. Poly(ethene) plastic bags are made this way from ethene monomers.
Condensation polymerisation involves monomers with two functional groups that join together whilst producing small molecules like water as by-products. Amino acids use this process to form proteins and polypeptides in your body.
Think About It: Your hair, muscles, and even the enzymes in your digestive system are all polymers made from amino acids through condensation polymerisation!
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Key Alkene Reactions Summary
Understanding alkene reactions becomes much easier when you focus on the specific conditions and products for each reaction type. Hydrogenation requires a nickel catalyst and produces alkanes, whilst hydration needs high temperatures (around 300°C) and a catalyst to form alcohols.
The halogenation reaction is particularly useful for identification purposes. When alkenes react with halogens like bromine, the orange-brown bromine water becomes colourless, creating compounds like dibromoethane from ethene and bromine.
Remember that alkenes aren't used as fuels because their double bonds cause incomplete combustion, resulting in sooty, smoky flames. This makes them less efficient and more polluting than alkanes for energy production.
Exam Success: Practice drawing the structural formulas for these reactions - examiners love testing whether you can show the before and after structures correctly!
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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
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 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 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 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 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