Ch 5 · Alkenes · IGCSE Chemistry 0620
Topic 5 of 8 in Organic Chemistry — IGCSE Chemistry 0620 — 48 lessons.
Introduction
Alkenes are the reactive relatives of the alkanes. Their carbon–carbon double bond lets them take part in addition reactions, which is why ethene is the starting point for ethanol, poly(ethene) and many other products. Most alkenes are made by cracking, the process that also balances supply and demand for petroleum fractions. Learn the conditions, the bromine water test and the equations; all three are examined every year.
Key ideas — Unsaturated hydrocarbons
Alkenes have the general formula mathrmC_nH_2n. The bonding is covalent and includes a double carbon–carbon covalent bond, mathrmC=C, in which two pairs of electrons are shared. Because the double bond can open up to let more atoms join the molecule, alkenes are unsaturated hydrocarbons. Ethene, mathrmCH_2=CH_2, is the simplest; propene is mathrmCH_2=CHCH_3.
🧑🏫 Teacher's explanation
Imagine a pair of hands held together by two strong rubber bands instead of just one. This is like the double bond in an alkene. Because there are two 'connections' between the carbon atoms, one of those connections can be 'broken' or opened up, allowing new atoms to snap into place. This ability to accept new atoms is why we call them 'unsaturated'—they haven't reached their maximum capacity of hydrogen atoms yet.
📖 Definitions
🔍 Worked example — Easy
Write the balanced chemical equation for the addition reaction of ethene (C_2H_4) with bromine (Br_2) to form bromoethane (C_2H_4Br_2). Include state symbols.
🔍 Worked example — Standard
A large alkane molecule is cracked to produce an alkene and an alkane. If the alkane is octane (C_8H_18) and the alkene produced is ethene (C_2H_4), write the balanced equation for the reaction.
🔍 Worked example — Exam-style
Calculate the molar mass (M_r) of propene (C_3H_6). Use A_r: C = 12, H = 1.
⚠️ Common errors
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🎯 Exam question
Key ideas — Cracking
Cracking is the thermal decomposition of a large alkane molecule into smaller molecules. The alkane vapour is passed over a catalyst of silica or alumina at a high temperature (about 600 to 700 °C). The products are always a shorter alkane and an alkene, or an alkene and hydrogen:
🧑🏫 Teacher's explanation
Imagine you have a long, heavy chain of paperclips. This chain is like a long-chain alkane; it is useful for making thick lubricating oils, but it is too heavy and 'clunky' to be used in a car engine. Cracking is like taking a pair of scissors and snipping that long chain into smaller, more manageable pieces.
📖 Definitions
🔍 Worked example — Easy
A long-chain alkane, octane (C_8H_18), undergoes cracking to produce an alkene and an alkane. Write a balanced equation for the production of ethene (C_2H_4) and hexane (C_6H_14).
🔍 Worked example — Standard
Calculate the mass of an alkene produced when 100g of an alkane undergoes cracking to produce 40g of hydrogen (H_2). (Assume the reaction is C_nH_2n+2 arrow C_nH_2n + H_2).
🔍 Worked example — Exam-style
Complete the following cracking equation for the decomposition of decane (C_10H_22) into propene (C_3H_6) and another hydrocarbon: C_10H_22 arrow C_3H_6 + ?
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
Key ideas — Testing for unsaturation
Aqueous bromine (bromine water) is orange. When shaken with an unsaturated hydrocarbon such as ethene it is rapidly decolourised: the orange colour disappears and the mixture becomes colourless. A saturated hydrocarbon such as ethane produces no change; the bromine stays orange. This is the standard test to distinguish alkanes from alkenes.
🧑🏫 Teacher's explanation
Imagine you are at a party where everyone is holding hands in pairs (saturated alkanes). Suddenly, a group of energetic dancers (alkenes) arrives. The dancers have 'spare' hands because they have a double bond, which is like a pair of hands that can easily open up to grab something new. When you add bromine to the room, the dancers immediately grab the bromine molecules to form new bonds, causing the orange 'glow' of the bromine to vanish as it gets locked into the molecules.
📖 Definitions
🔍 Worked example — Easy
Identify the observation when ethene (C_2H_4) is added to aqueous bromine (Br_2(aq)).
🔍 Worked example — Standard
Write the balanced chemical equation for the reaction between ethene and bromine (Br_2) to form 1,2-dibromoethane.
🔍 Worked example — Exam-style
A student adds bromine water to two test tubes. Tube A contains hexane (C_6H_14) and Tube B contains hexene (C_6H_12). State the observation for each tube.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(a) State what this observation tells you about the structure of X. [1] (b) Write a balanced chemical equation for the reaction between ethene (C_2H_4) and bromine (Br_2) to form 1,2-dibromoethane. [2] [3 marks]
Key ideas — Addition reactions of alkenes
🧑🏫 Teacher's explanation
Imagine a double bond as a pair of hands held tightly together by two people. These hands are strong, but they can be 'unlocked' to grab onto something else. When an alkene reacts, that double bond breaks (the 'unsaturated' part), and the two carbon atoms use their newly freed hands to grab onto atoms from a new molecule.
📖 Definitions
🔍 Worked example — Easy
Write a balanced chemical equation for the addition of bromine (Br_2) to ethene (C_2H_4).
🔍 Worked example — Standard
Calculate the molar mass of the product formed when propene (C_3H_6) reacts with hydrogen (H_2) in the presence of a nickel catalyst.
🔍 Worked example — Exam-style
A sample of ethene (C_2H_4) is reacted with steam (H_2O) using an acid catalyst. If 2.0 mol of ethene reacts completely, calculate the mass of ethanol (CH_3CH_2OH) produced.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Identify the type of hydrocarbon X is. [1] (b) Describe a chemical test to distinguish X from an alkane of the same carbon number. [2] (c) Write a balanced chemical equation for the reaction of X with steam to produce an alcohol. [2] [5 marks]
Worked example
Question. Dodecane, mathrmC_12H_26, is cracked to give octane, mathrmC_8H_18, and ethene only. (a) Write the equation for this reaction. [2] (b) Describe how you could show that one product is unsaturated. [2] (c) Calculate the maximum mass of ethene that can be obtained from 340 g of dodecane. (A_r: C = 12, H = 1) [3]
🔍 Worked example — Easy
Write the balanced chemical equation for the addition reaction of ethene (C_2H_4) with bromine (Br_2) to form 1,2-dibromoethane.
🔍 Worked example — Standard
Calculate the mass of bromine (Br_2) required to react completely with 5.6 g of ethene (C_2H_4). (A_r: C=12, H=1, Br=56)
🔍 Worked example — Exam-style
Dodecane, C_12H_26, is cracked to give octane, C_8H_18, and ethene only. Calculate the maximum mass of ethene that can be obtained from 340 g of dodecane. (A_r: C=12, H=1)
⚠️ Common errors
Common mistakes
Practice
Answers Alkanes contain only single C–C bonds and are saturated [B1]; alkenes contain a C=C double bond and are unsaturated [B1]. High temperature (600 to 700 °C) and a silica or alumina catalyst [B1]; any two of: to make short-chain fuels such as petrol, which are in higher demand; to make alkenes for polymers and ethanol; to make hydrogen [B1 each, max 2]. mathrmC_16H_34 arrow mathrmC_10H_22 + 3C_2H_4 or mathrmC_16H_34 arrow mathrmC_10H_22 + mathrmC_6H_12: correct formulae [B1], balanced [B1]. Nickel [B1]; C_3H_6 + H_2 arrow C_3H_8 [B1]; propane [B1]. Ethanol, CH_3CH_2OH: two carbon atoms joined by a single bond [B1], one carbon bonded to three H, the other to two H and an O–H group [B1].