Subject: Chemistry | Level: GCSE | Exam Board: Edexcel
Master the fundamentals of hydrocarbons with this essential guide to alkanes and alkenes. You'll learn to draw structural formulas, distinguish saturated from unsaturated molecules, and ace the classic bromine water test—key skills that frequently appear in 4-6 mark exam questions.
Revision Notes & Key Concepts
Revision Podcast Transcript
GCSE Chemistry Podcast – Alkanes and Alkenes\nEpisode: Separate Chemistry 2 – Hydrocarbons\nDuration: approximately 10 minutes\n\n---\n\n[INTRO – 1 minute]\n\nHello and welcome to your GCSE Chemistry revision podcast! I'm so glad you're here, because today we're diving into one of the most important topics in Separate Chemistry – alkanes and alkenes. These are hydrocarbons, which means molecules made only of hydrogen and carbon atoms, and they come up time and time again in exam papers. Whether you're sitting AQA, Edexcel, or OCR, you need to know this topic inside out.\n\nSo grab a pen, maybe a piece of paper to sketch along, and let's get started. By the end of this episode, you'll be able to draw structural formulas, explain the difference between saturated and unsaturated compounds, describe the bromine water test, and write balanced combustion equations. Let's go!\n\n---\n\n[CORE CONCEPTS – 5 minutes]\n\nLet's start with the basics. Hydrocarbons are compounds containing only carbon and hydrogen. Simple as that. Alkanes and alkenes are both homologous series – that means they're families of compounds that share the same general formula and similar chemical properties, with each member differing by one CH2 unit.\n\nFirst up: ALKANES. The general formula for alkanes is CnH2n+2. So if n equals 1, you get methane – CH4. If n equals 2, ethane – C2H6. Then propane, C3H8, and butane, C4H10. You need to know all four of these for the exam.\n\nHere's the key thing about alkanes: they are SATURATED hydrocarbons. Saturated means they contain only single covalent bonds between carbon atoms. Every carbon is bonded to as many hydrogen atoms as possible. Think of it like a fully loaded lorry – there's no room for anything else. The carbon atoms are completely \"saturated\" with hydrogen.\n\nWhen you draw the structural formula of methane, you draw the carbon atom in the centre with four single bonds going out to four hydrogen atoms. For ethane, you have two carbon atoms connected by a single bond, and each carbon has three hydrogen atoms attached. Make sure you show every single bond in your drawing – examiners will deduct marks if you miss any.\n\nNow let's talk about ALKENES. The general formula for alkenes is CnH2n. So ethene is C2H4, propene is C3H6, and butene is C4H8. Notice that alkenes have two fewer hydrogen atoms than the corresponding alkane – and that's because of the double bond.\n\nAlkenes are UNSATURATED hydrocarbons. They contain a carbon-to-carbon double bond, written as C equals C. This double bond is called the functional group of alkenes, and it's what makes alkenes so reactive. The double bond means the molecule isn't fully loaded with hydrogen – there's still \"room\" to add more atoms across that double bond.\n\nWhen you draw ethene, you have two carbon atoms connected by a double bond, and each carbon has two hydrogen atoms. The double bond must be clearly shown as two lines between the carbons. Again – every bond must be visible in your structural formula.\n\nNow, how do you tell an alkane from an alkene in the lab? This is where the bromine water test comes in, and it's a classic exam question.\n\nBromine water is an orange-brown solution. When you add it to an alkane, absolutely nothing happens – the solution stays orange. But when you add bromine water to an alkene, the solution decolourises – it turns from orange to colourless. This is because the C=C double bond in the alkene reacts with the bromine in an addition reaction. The bromine molecules add across the double bond, breaking it open and forming a new compound called a dibromoalkane.\n\nFor ethene specifically, the equation is: ethene plus bromine gives dibromoethane. Written as: CH2=CH2 plus Br2 gives CH2BrCH2Br. The double bond is broken, the bromine atoms add on, and the product is colourless – which is why the solution loses its colour.\n\nThis is called an addition reaction because two reactants add together to form a single product. No atoms are lost. The bromine just slots in across the double bond.\n\nFinally, let's cover combustion. Both alkanes and alkenes undergo complete combustion when there is a plentiful supply of oxygen. The products are always carbon dioxide and water. Always. If you see a question asking what the products of complete combustion are, the answer is CO2 and H2O – every single time.\n\nFor methane: CH4 plus 2O2 gives CO2 plus 2H2O.\nFor ethene: C2H4 plus 3O2 gives 2CO2 plus 2H2O.\n\nWhen balancing these equations, count your carbons first, then your hydrogens, and balance the oxygens last. That strategy works every time.\n\n---\n\n[EXAM TIPS AND COMMON MISTAKES – 2 minutes]\n\nRight, let's talk about where students lose marks – because I see the same mistakes again and again.\n\nMistake number one: not showing all covalent bonds in structural formulas. If you draw methane and you only show three bonds instead of four, you will lose a mark. Every single bond must be drawn as a line. Examiners are very strict about this.\n\nMistake number two: confusing the general formulas. Alkanes are CnH2n+2. Alkenes are CnH2n. A really common error is using the alkene formula for an alkane or vice versa. Remember: alkanes have two EXTRA hydrogens because they have no double bond.\n\nMistake number three: getting the bromine water test backwards. Some students write that alkanes decolourise bromine water. They don't! It's the ALKENE that decolourises it. Alkanes have no double bond, so they can't react with bromine water under normal conditions.\n\nMistake number four: not identifying the functional group correctly. The functional group of alkenes is the C=C double bond. If a question asks you to name the functional group, you must say \"carbon-carbon double bond\" or write C=C. Saying \"double bond\" alone is not enough for full marks.\n\nMistake number five: forgetting to balance combustion equations. Always check that your atoms balance on both sides. A 6-mark question on combustion will expect a fully balanced equation with state symbols.\n\nOne more tip: when a question says \"Explain why alkenes decolourise bromine water but alkanes do not,\" you need to say TWO things: first, that alkenes contain a C=C double bond; and second, that this double bond reacts with bromine in an addition reaction. One point alone won't get you full marks.\n\n---\n\n[QUICK-FIRE RECALL QUIZ – 1 minute]\n\nTime for a quick quiz! Pause after each question and try to answer before I give you the answer.\n\nQuestion 1: What is the general formula for alkanes? … Answer: CnH2n+2.\n\nQuestion 2: What type of bond makes alkenes unsaturated? … Answer: A carbon-carbon double bond, C=C.\n\nQuestion 3: What colour change occurs when an alkene is added to bromine water? … Answer: Orange to colourless.\n\nQuestion 4: What are the products of complete combustion of any hydrocarbon? … Answer: Carbon dioxide and water.\n\nQuestion 5: What type of reaction occurs when ethene reacts with bromine? … Answer: An addition reaction.\n\nHow did you do? If you got all five, brilliant – you're in great shape. If you missed any, go back and review that section.\n\n---\n\n[SUMMARY AND SIGN-OFF – 1 minute]\n\nLet's wrap up with the key points to take away from today.\n\nAlkanes are saturated hydrocarbons with the general formula CnH2n+2. They contain only single bonds. Alkenes are unsaturated hydrocarbons with the general formula CnH2n. They contain a C=C double bond.\n\nThe bromine water test distinguishes between them: alkenes decolourise bromine water through an addition reaction; alkanes do not react.\n\nComplete combustion of any hydrocarbon always produces carbon dioxide and water.\n\nAnd remember: in the exam, always show every covalent bond in structural formulas, always identify the C=C as the functional group of alkenes, and always balance your combustion equations.\n\nYou've got this! Keep revising, keep practising past paper questions, and I'll see you in the next episode. Good luck!\n\n---\nEND OF SCRIPT
Key Terms & Definitions
- Hydrocarbon
- A compound containing only carbon and hydrogen atoms.
- Homologous series
- A family of organic compounds with the same general formula, similar chemical properties, and a trend in physical properties.
- Saturated
- A molecule containing only single covalent bonds between carbon atoms.
- Unsaturated
- A molecule containing at least one carbon-carbon double bond.
- Functional group
- The part of a molecule that determines its chemical reactions (e.g., the C=C double bond in alkenes).
- Addition reaction
- A reaction in which two or more molecules combine to form a single larger molecule.
Worked Examples
Worked Example
Question: Describe a chemical test that can be used to distinguish between propane and propene. Give the expected result for both substances. (3 marks)
Solution: Step 1: Identify the test reagent. Add bromine water to both substances. Step 2: State the result for the alkane. With propane, the bromine water remains orange (no change). Step 3: State the result for the alkene. With propene, the bromine water decolourises (turns from orange to colourless).
Worked Example
Question: Write a balanced symbol equation for the complete combustion of propane (C3H8). (3 marks)
Solution: Step 1: Write the reactants and products. C3H8 + O2 -> CO2 + H2O Step 2: Balance the carbon atoms. Put a 3 in front of CO2. C3H8 + O2 -> 3CO2 + H2O Step 3: Balance the hydrogen atoms. Put a 4 in front of H2O. C3H8 + O2 -> 3CO2 + 4H2O Step 4: Balance the oxygen atoms. (3x2) + (4x1) = 10 oxygen atoms on the right. Put a 5 in front of O2. Final answer: C3H8 + 5O2 -> 3CO2 + 4H2O
Worked Example
Question: Explain why ethene is described as an unsaturated hydrocarbon. (2 marks)
Solution: Step 1: Define hydrocarbon. It contains only hydrogen and carbon atoms. Step 2: Define unsaturated. It contains a carbon-carbon double bond (C=C).
Practice Questions
Question: What is the general formula for the alkane homologous series?
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Question: Draw the displayed structure of propene.
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Question: A student tests an unknown hydrocarbon gas with bromine water. The bromine water turns from orange to colourless. Identify the type of hydrocarbon and explain this reaction.
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Question: Write the balanced symbol equation for the complete combustion of butane (C4H10).
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Question: Explain why alkanes are described as saturated hydrocarbons.
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