WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE Edexcel Science revision. 00:00:04.125 --> 00:00:07.830 Unit S C 16: Chemical cells and fuel cells. 00:00:10.500 --> 00:00:14.545 A chemical cell transfers chemical energy to electrical energy. 00:00:14.695 --> 00:00:21.585 Reactions at its electrodes produce a potential difference that can drive current through a complete external circuit. 00:00:24.267 --> 00:00:29.194 The voltage of a simple cell depends on the electrode materials and electrolyte. 00:00:29.344 --> 00:00:37.662 Two suitable different metals in an electrolyte can make a cell; the electrolyte lets ions move while electrons travel through the external wire. 00:00:40.333 --> 00:00:45.251 A cell supplies a voltage while the required reacting chemicals remain available. 00:00:45.401 --> 00:00:54.903 When a reactant is exhausted, the original cell reaction can no longer sustain its output; a voltage reading alone does not measure its total stored energy. 00:00:57.567 --> 00:01:03.272 In a non-rechargeable cell the useful reaction is not practically reversed during normal use. 00:01:03.422 --> 00:01:12.562 In a rechargeable cell, an external electrical supply drives reactions that restore reactants; charging needs energy and is not perfectly efficient. 00:01:15.233 --> 00:01:19.995 Cells in series can add their voltages if connected in the same orientation. 00:01:20.145 --> 00:01:27.785 Cell capacity, output power, energy stored, lifetime and voltage are different measures; choose the measure relevant to the device. 00:01:30.467 --> 00:01:38.374 Compare cells using operating voltage, useful energy, mass, cost, ability to recharge, safe use and disposal impacts. 00:01:38.524 --> 00:01:43.548 A lighter cell with a high voltage is not automatically the longest-lasting option. 00:01:46.200 --> 00:01:55.895 Investigating a simple cell requires clean electrodes, a consistent electrolyte volume and concentration, controlled temperature and an appropriate voltmeter. 00:01:56.045 --> 00:01:59.071 Change one factor at a time and repeat readings. 00:02:01.733 --> 00:02:13.038 A hydrogen, oxygen fuel cell uses a chemical reaction to produce electrical output, with water as its only reaction product: 2 H 2 plus O 2 produces 2 H 2 O. 00:02:13.188 --> 00:02:17.933 The overall reaction forms water; the external wire carries electrons. 00:02:22.600 --> 00:02:31.269 Unlike a sealed cell with a fixed initial supply, a fuel cell can keep operating while hydrogen and oxygen are supplied and products are removed. 00:02:31.419 --> 00:02:36.593 It does not need recharging in the same way as a rechargeable battery; it needs fuel. 00:02:39.267 --> 00:02:44.503 Hydrogen is oxidised and oxygen is reduced in separate electrode reactions. 00:02:44.653 --> 00:02:51.275 Electrons pass through the external circuit, supplying electrical energy, while ions move through the electrolyte. 00:02:51.425 --> 00:02:55.357 Detailed electrode equations depend on the electrolyte used. 00:02:58.033 --> 00:03:02.087 Water production at the point of use can reduce local air pollution. 00:03:02.237 --> 00:03:12.545 There is no carbon dioxide in the hydrogen, oxygen reaction, but hydrogen manufacture, compression, transport and equipment manufacture can still create emissions. 00:03:15.200 --> 00:03:22.282 Producing hydrogen by electrolysis needs electricity; making it from fossil fuels can emit carbon dioxide. 00:03:22.432 --> 00:03:28.521 Compare the whole production pathway rather than describing all hydrogen as automatically carbon-free. 00:03:31.200 --> 00:03:36.372 Hydrogen has low density, so useful storage can require compression or cooling. 00:03:36.522 --> 00:03:42.805 It is flammable, requires suitable containment and infrastructure, and fuel-cell catalysts can be costly. 00:03:45.467 --> 00:03:52.900 Advantages can include low local emissions, efficient conversion in appropriate conditions, quiet operation and refuelling. 00:03:53.050 --> 00:03:59.906 Disadvantages can include fuel-production impacts, storage challenges, infrastructure costs and equipment cost. 00:04:02.567 --> 00:04:12.693 Evaluate a fuel cell for a specific use using supplied data: useful energy output, fuel mass, refuelling access, price, range and environmental impacts. 00:04:12.843 --> 00:04:17.983 A sensible decision can differ between a remote sensor, a bus and a portable phone. 00:04:20.633 --> 00:04:27.523 Efficiency ( percent) equals useful electrical energy output divided by energy supplied times 100. 00:04:27.673 --> 00:04:30.926 Be clear which stages you are including (the system boundary). 00:04:31.076 --> 00:04:36.663 A fuel cell’s efficiency alone does not include energy used to produce and transport its fuel. 00:04:39.333 --> 00:04:46.383 For a device supplied with a fixed useful energy, operating time equals available useful energy divided by power. 00:04:46.533 --> 00:04:51.545 Use joules with watts to obtain seconds, or watt-hours with watts to obtain hours. 00:04:54.200 --> 00:04:58.659 Calculate cost per useful unit of energy using comparable inputs. 00:04:58.809 --> 00:05:07.011 Include replacement or refuelling costs if the question provides them; avoid assuming purchase price alone represents lifetime cost. 00:05:09.667 --> 00:05:17.164 Compare emissions over the full pathway and distinguish local exhaust emissions from manufacturing and fuel-production emissions. 00:05:17.314 --> 00:05:21.521 Conclusions should follow the supplied evidence and acknowledge missing data. 00:05:24.200 --> 00:05:26.919 That completes Chemical cells and fuel cells. 00:05:27.069 --> 00:05:30.415 Revisit the notes and test yourself on the revision website.