Ch 4 · Alkanes · IGCSE Chemistry 0620
Topic 4 of 8 in Organic Chemistry — IGCSE Chemistry 0620 — 38 lessons.
Introduction
Alkanes are the simplest hydrocarbons and the main components of natural gas and petroleum. They are prized as fuels precisely because they are otherwise so unreactive. This lesson covers their bonding, why they are called saturated, how to write balanced equations for their complete combustion, and (for the Extended course) their one other important reaction, substitution by chlorine in ultraviolet light.
Key ideas — Saturated hydrocarbons
Alkanes have the general formula mathrmC_nH_2n+2. Every bond in an alkane is a single covalent bond: carbon atoms are joined to each other and to hydrogen atoms by shared pairs of electrons, and each carbon forms four bonds. Because there are no double bonds, alkanes are saturated hydrocarbons: each carbon atom is already bonded to the maximum number of other atoms, so no more atoms can be added to the molecule.
🧑🏫 Teacher's explanation
Think of an alkane molecule like a fully packed suitcase. Every single slot in the suitcase is filled with a piece of clothing (an atom). Because there is no empty space left, you cannot add anything else into the suitcase without removing something first. This is what we mean by 'saturated'.
📖 Definitions
🔍 Worked example — Easy
Calculate the relative molecular mass (M_r) of ethane, C_2H_6. (Given A_r: C = 12, H = 1)
🔍 Worked example — Standard
Write the balanced chemical equation for the complete combustion of propane, C_3H_8.
🔍 Worked example — Exam-style
Methane (CH_4) reacts with chlorine (Cl_2) in the presence of UV light to form chloromethane (CH_3Cl) and hydrogen chloride (HCl). Write the balanced equation for this substitution reaction.
⚠️ Common errors
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🎯 Exam question
Key ideas — Complete combustion
When an alkane, or any hydrocarbon, burns in a plentiful supply of oxygen it undergoes complete combustion to give carbon dioxide and water only. The reaction is strongly exothermic, which is why alkanes are used as fuels.
🧑🏫 Teacher's explanation
Think of combustion as a high-energy dance where carbon and hydrogen atoms are ripped away from each other and paired up with oxygen atoms. In 'complete' combustion, there is plenty of oxygen available, so every single carbon atom gets a full set of oxygen atoms to form CO_2, and every hydrogen atom finds two oxygens to form H_2 extO.
📖 Definitions
🔍 Worked example — Easy
Write the balanced chemical equation for the complete combustion of ethane, C_2H_6.
🔍 Worked example — Standard
Calculate the mass of CO_2 produced from the complete combustion of 32g of methane (CH_4).
🔍 Worked example — Exam-style
A sample of propane (C_3H_8) is burned completely. If 1.0 mole of propane is burned, what volume of CO_2 is produced at RTP (Room Temperature and Pressure)? (Assume molar volume of gas at RTP is 24 dm^3mol^-1)
⚠️ Common errors
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🎯 Exam question
(a) Name the two main products formed if the combustion is incomplete. [2] (b) Write a balanced chemical equation for the complete combustion of butane. [3] [5 marks]
Key ideas — Substitution by chlorine
🧑🏫 Teacher's explanation
Imagine a crowded dance floor where everyone is holding hands in pairs (the single covalent bonds in an alkane). Suddenly, a bright flash of light (ultraviolet radiation) acts like a sudden burst of energy that breaks a pair of dancers apart, creating 'free radicals' that are looking for someone to grab. A chlorine molecule breaks apart, and one chlorine atom 'swaps' places with a hydrogen atom. This is like a game of musical chairs where one person leaves and another enters.
📖 Definitions
🔍 Worked example — Easy
Write the balanced chemical equation for the reaction between methane and chlorine in the presence of UV light.
🔍 Worked example — Standard
Calculate the mass of chloromethane (CH_3Cl) produced when 1.6 g of methane (CH_4) reacts completely with excess chlorine.
🔍 Worked example — Exam-style
A sample of ethane (C_2H_6) undergoes photochemical substitution with chlorine. Draw the displayed formula for the monochlorinated product.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
Worked example
Question. Butane, mathrmC_4H_10, is used as a fuel in camping stoves. (a) Write the equation for the complete combustion of butane. [2] (b) Calculate the mass of carbon dioxide produced when 2.9 g of butane burns completely. (A_r: C = 12, H = 1, O = 16) [3] (c) Explain why butane is described as saturated. [1]
🔍 Worked example — Easy
Write the balanced equation for the complete combustion of methane, CH_4.
🔍 Worked example — Standard
Calculate the mass of oxygen required for the complete combustion of 32 g of methane, CH_4. (A_r: C=12, H=1, O=16)
🔍 Worked example — Exam-style
Ethane (C_2H_6) reacts with chlorine (Cl_2) under UV light to form chloroethane (C_2H_5Cl) and hydrogen chloride (HCl). Write the balanced equation for this substitution reaction.
⚠️ Common errors
Common mistakes
Practice
Answers Single covalent bonds [B1]; a hydrocarbon in which all carbon–carbon bonds are single, so no more atoms can be added [B1]. 2C_2H_6 + 7O_2 arrow 4CO_2 + 6H_2O: formulae [B1], balancing [B1]. Carbon monoxide [B1]; incomplete combustion because there is insufficient oxygen [B1]. Substitution (photochemical) [B1]; C_2H_6 + Cl_2 arrow C_2H_5Cl + HCl [B1]; it provides the activation energy [B1]. Without ultraviolet light there is no source of the activation energy needed to break the Cl–Cl bond, so no reaction starts. [B1]