WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE Edexcel Science revision. 00:00:04.125 --> 00:00:07.082 Unit S C 22: Hydrocarbons. 00:00:09.733 --> 00:00:12.889 Hydrocarbons contain only carbon and hydrogen. 00:00:13.039 --> 00:00:22.033 An alkane is saturated: it has only single carbon, carbon bonds and no carbon, carbon double bond; each carbon makes four covalent bonds. 00:00:24.700 --> 00:00:32.975 The first four alkanes are methane C H 4, ethane C 2 H 6, propane C 3 H 8 and butane C 4 H 10. 00:00:33.125 --> 00:00:39.044 For open-chain alkanes the general formula is C subscript n H subscript two n plus two. 00:00:39.194 --> 00:00:42.721 All carbon and hydrogen atoms and single bonds are shown. 00:00:47.400 --> 00:00:51.067 Displayed formulae show every atom and every covalent bond. 00:00:51.217 --> 00:00:59.899 Condensed formulae such as C H 3, C H 2, C H 3 group hydrogen atoms; they are useful but are not a complete displayed formula. 00:01:00.049 --> 00:01:04.666 All hydrogen atoms and bonds are shown; double bonds count as two bonds. 00:01:09.333 --> 00:01:15.061 Alkenes are unsaturated hydrocarbons containing the functional group C double bonded to C. 00:01:15.211 --> 00:01:26.207 Open-chain alkenes with one double bond have formula C subscript n H subscript two n; ethene is C 2 H 4, propene C 3 H 6 and butene C 4 H 8. 00:01:28.867 --> 00:01:38.455 But-1-ene has C H two double bonded to C H to C H 2, C H 3; but-2-ene has C H 3, C H double bonded to C H to C H 3. 00:01:38.605 --> 00:01:43.219 Their molecular formulae are the same but the double bond occupies a different position. 00:01:43.369 --> 00:01:47.541 Both have C 4 H 8, with the double bond in a different position. 00:01:52.200 --> 00:02:00.347 In a homologous series, members have the same functional group and similar chemical reactions, and successive members differ by C H 2. 00:02:00.497 --> 00:02:04.893 Physical properties such as boiling point change gradually with chain length. 00:02:07.567 --> 00:02:12.070 Check that each carbon atom forms four bonds: this is carbon’s valency. 00:02:12.220 --> 00:02:15.121 Count a double bond as two when checking the total. 00:02:15.271 --> 00:02:18.780 An alkane must not contain a C double bonded to C bond. 00:02:21.433 --> 00:02:27.721 Alkenes undergo addition reactions: the carbon, carbon double bond opens and atoms add across it. 00:02:27.871 --> 00:02:37.811 Ethene reacts with bromine to form one, two-dibromoethane: C H two double bonded to C H two plus B R 2 produces C H 2 B R, C H 2 B R. 00:02:37.961 --> 00:02:43.304 One bromine atom attaches to each of the two carbons formerly linked by the double bond. 00:02:47.967 --> 00:02:50.207 Bromine water is orange-brown. 00:02:50.357 --> 00:03:00.765 Shaking it with an alkene changes it to colourless as bromine is used in addition; an alkane does not decolourise it under the normal test conditions without ultraviolet light. 00:03:03.433 --> 00:03:08.595 Use small samples and teacher-approved dilute bromine water, wearing eye protection. 00:03:08.745 --> 00:03:17.864 A colour change supports unsaturation, but identify the unknown in context rather than assuming every decolourising substance must be an alkene. 00:03:20.533 --> 00:03:28.159 Complete combustion of both alkanes and alkenes requires sufficient oxygen and produces carbon dioxide and water. 00:03:28.309 --> 00:03:35.994 Balance carbon first, then hydrogen, then oxygen: C H 4 plus 2 O 2 produces C O 2 plus 2 H 2 O. 00:03:38.667 --> 00:03:44.425 Incomplete combustion can produce carbon monoxide and or carbon soot as well as water. 00:03:44.575 --> 00:03:51.422 Carbon monoxide is toxic; a smoky flame indicates incomplete combustion, not an absence of carbon. 00:03:54.100 --> 00:03:59.955 For a homologous series, use the same functional-group reaction to predict products of another member. 00:04:00.105 --> 00:04:06.791 An addition product keeps all the atoms of the two reacting molecules, unlike substitution or condensation. 00:04:09.467 --> 00:04:21.305 Yeast enzymes convert dissolved sugars to ethanol and carbon dioxide under anaerobic conditions: C 6 H 12 O 6 produces 2 C 2 H 5 O H plus 2 C O 2. 00:04:21.455 --> 00:04:24.451 The sugar feedstock can come from renewable crops. 00:04:24.601 --> 00:04:30.233 Distillation concentrates the ethanol after fermentation; it does not cause fermentation. 00:04:34.900 --> 00:04:43.891 Provide an aqueous sugar solution and a suitable warm temperature, commonly around 30 degrees Celsius, with oxygen excluded as far as practical. 00:04:44.041 --> 00:04:49.938 Low temperature slows enzyme reactions; high temperature can denature enzymes and kill yeast. 00:04:52.600 --> 00:04:56.987 An airlock allows carbon dioxide to escape while reducing air entry. 00:04:57.137 --> 00:05:02.789 Do not seal a fermenting vessel so tightly that gas pressure can build without a safe outlet. 00:05:05.467 --> 00:05:10.969 Fermentation makes a dilute ethanol solution containing water and other substances. 00:05:11.119 --> 00:05:16.557 Ethanol becomes toxic to yeast at high concentrations, limiting continued production. 00:05:19.233 --> 00:05:25.557 Fractional distillation concentrates ethanol because ethanol and water have different boiling behaviours. 00:05:25.707 --> 00:05:36.308 Ethanol boils near 78 degrees Celsius at atmospheric pressure; collect the ethanol-rich fraction using suitable apparatus, with no naked flame near flammable ethanol. 00:05:38.967 --> 00:05:44.427 Ordinary fractional distillation concentrates ethanol but cannot make it completely pure. 00:05:44.577 --> 00:05:52.193 At a particular composition, ethanol and water boil together without further separation: this mixture is called an azeotrope. 00:05:52.343 --> 00:05:57.249 For GCSE, describe the product as a concentrated ethanol solution. 00:05:59.900 --> 00:06:06.751 Compare production methods using renewability, land use, rate, energy for separation, purity and costs. 00:06:06.901 --> 00:06:11.848 Renewable feedstock alone does not make every stage environmentally harmless. 00:06:14.500 --> 00:06:16.569 That completes Hydrocarbons. 00:06:16.719 --> 00:06:20.065 Revisit the notes and test yourself on the revision website.