WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE Edexcel Science revision. 00:00:04.125 --> 00:00:08.517 Unit S C 13: Transition metals, alloys and corrosion. 00:00:11.200 --> 00:00:15.322 Transition metals occupy the central block of the periodic table. 00:00:15.472 --> 00:00:22.904 Typical examples include iron, copper and nickel; their properties differ from the very reactive, low-density Group 1 metals. 00:00:25.567 --> 00:00:30.317 Many transition metals have high densities, high melting points and good strength. 00:00:30.467 --> 00:00:36.567 They conduct electricity and thermal energy; these are typical trends, not rules without exceptions. 00:00:39.233 --> 00:00:45.226 Transition-metal compounds are often coloured, and the metals or their compounds can act as catalysts. 00:00:45.376 --> 00:00:51.221 Iron catalyses the Haber process; a catalyst speeds a reaction without being used up overall. 00:00:53.900 --> 00:00:59.456 Copper is useful for wiring because it conducts electricity well and can be drawn into wires. 00:00:59.606 --> 00:01:04.097 Its good thermal conductivity also suits heat-transfer applications. 00:01:06.767 --> 00:01:10.907 Aluminium has low density and forms a protective oxide layer. 00:01:11.057 --> 00:01:18.875 Its alloys are useful where low mass and strength matter, such as aircraft parts; low density alone does not establish suitability. 00:01:21.533 --> 00:01:26.701 Gold is unreactive, attractive and malleable, but pure gold is relatively soft. 00:01:26.851 --> 00:01:32.867 Jewellery often uses gold alloys for greater hardness; cost and appearance also influence the choice. 00:01:35.533 --> 00:01:43.479 Choose a metal using data on density, conductivity, corrosion resistance, strength, price and the environment in which it will operate. 00:01:43.629 --> 00:01:46.146 No material is best for every use. 00:01:48.800 --> 00:01:56.286 Corrosion is gradual destruction of a metal by chemical reaction with its surroundings, involving oxidation of the metal. 00:01:56.436 --> 00:02:00.769 Rusting is the corrosion of iron, not a name for every metal’s corrosion. 00:02:03.433 --> 00:02:06.813 Iron rusts when both water and oxygen are available. 00:02:06.963 --> 00:02:13.969 Salt can speed rusting; a fair investigation compares identical iron samples while changing one condition. 00:02:16.633 --> 00:02:24.994 To show the need for both reactants, compare iron in moist air, dry air with a drying agent, and boiled water protected by an oil layer. 00:02:25.144 --> 00:02:29.529 Boiling removes dissolved oxygen; oil limits oxygen returning. 00:02:32.200 --> 00:02:38.052 Paint, oil, grease and plastic coatings act as barriers that exclude water and or oxygen. 00:02:38.202 --> 00:02:41.445 Damage to the barrier can expose iron and allow rusting. 00:02:44.100 --> 00:02:46.489 Galvanising coats iron with zinc. 00:02:46.639 --> 00:02:49.679 The zinc keeps water and oxygen away from the iron. 00:02:49.829 --> 00:02:58.383 It is also more reactive than iron, so it oxidises instead of the iron (sacrificial protection), even if a small scratch exposes the iron. 00:02:58.533 --> 00:03:04.120 Sacrificial protection can still work at exposed areas while electrical contact remains. 00:03:08.800 --> 00:03:14.441 Sacrificial protection connects iron to a more reactive metal such as zinc or magnesium. 00:03:14.591 --> 00:03:19.615 That metal loses electrons in preference to iron and eventually needs replacement. 00:03:22.267 --> 00:03:29.477 A less reactive coating such as tin protects intact iron as a barrier, but a scratch can permit faster iron corrosion. 00:03:29.627 --> 00:03:33.723 A coating and a sacrificial metal are not always interchangeable. 00:03:36.400 --> 00:03:40.555 Electroplating deposits a thin metal layer using electrolysis. 00:03:40.705 --> 00:03:48.501 The object to be plated is the negative cathode; positive metal ions gain electrons there and form metal atoms on its surface. 00:03:48.651 --> 00:03:52.509 The object is connected to the negative terminal of the power supply. 00:03:57.167 --> 00:04:02.047 For copper plating, use a copper-containing electrolyte and usually a copper anode. 00:04:02.197 --> 00:04:04.629 Copper dissolves at the positive anode, 00:04:04.779 --> 00:04:08.043 replacing ions removed at the cathode: C U, 00:04:08.193 --> 00:04:09.708 charge 2 plus, 00:04:09.858 --> 00:04:13.677 plus 2 electrons produces C U and C U produces C U, 00:04:13.827 --> 00:04:15.341 charge 2 plus, 00:04:15.491 --> 00:04:17.155 plus 2 electrons. 00:04:19.833 --> 00:04:24.974 Clean the object before plating and use controlled current and time for a suitable coating. 00:04:25.124 --> 00:04:31.693 Plating can improve appearance or corrosion resistance without making the entire object from an expensive metal. 00:04:34.367 --> 00:04:38.540 An alloy is a mixture containing a metal and one or more other elements. 00:04:38.690 --> 00:04:46.763 Different-sized atoms distort the regular layers of a pure metal, making it harder for layers to slide; alloys are often harder and stronger. 00:04:46.913 --> 00:04:52.448 Different-sized atoms make sliding harder; this model explains a common strengthening effect. 00:04:57.100 --> 00:05:00.915 Adding carbon and other elements makes steels with different properties. 00:05:01.065 --> 00:05:09.299 Stainless steels contain chromium, which helps form a protective surface layer; not every steel is stainless or equally corrosion-resistant. 00:05:11.967 --> 00:05:18.642 Brass is mainly copper and zinc; it is useful for items requiring a workable, corrosion-resistant material. 00:05:18.792 --> 00:05:25.489 Magnalium contains aluminium and magnesium, combining low density with improved mechanical properties. 00:05:28.167 --> 00:05:33.414 Gold alloys can improve hardness; copper alloys can trade some conductivity for strength. 00:05:33.564 --> 00:05:38.578 Evaluate property data rather than assuming every alloy improves every property. 00:05:41.233 --> 00:05:48.763 The proportion of an element in an alloy ( percent) equals mass of that element divided by total alloy mass times 100. 00:05:48.913 --> 00:05:54.289 Compare samples with the same total mass when investigating the effect of composition. 00:05:56.967 --> 00:06:00.312 That completes Transition metals, alloys and corrosion. 00:06:00.462 --> 00:06:03.807 Revisit the notes and test yourself on the revision website.