Metals
Übersicht
At the state banquets of Napoleon III the guests he wished to flatter were handed cutlery made of aluminium, while everybody else had to make do with gold. That is not a joke about French taste. In the 1850s aluminium genuinely was the most expensive metal on the market, and yet it is the commonest metal in the rock beneath your feet. Gold is millions of times rarer. The whole of the puzzle, and the whole of its solution, sits inside this lesson.
What separates the two is not how much of each there is but how tightly each one clings to whatever it has combined with. Gold sits in the ground as gold, waiting to be picked up. Aluminium is locked into its oxide so firmly that nobody could prise it out cheaply until 1886. Order every metal by how strongly it holds on and you get a single list that quietly decides what fizzes in acid, what displaces what, how iron is smelted, why recycling a can is worth doing, and what a bag of scrap iron can pull out of a blue solution.
Ziele
- Metals are useful materials as they are good conductors of heat and electricity. They can also be bent or hammered into shape because the layers of atoms in metals are able to slide over each other.
- An alloy is a mixture of at least two elements, at least one of which is a metal. Alloys often have properties that are different from the metals they contain. This makes them more useful than the pure metals alone. Steels are a mixture of iron with carbon and sometimes other metals. Students may be given information on the composition of specific alloys so that they can evaluate their uses.
- Copper is useful for electrical wiring and plumbing because it has the following properties: it is a good conductor of heat and electricity; it can be bent but is hard enough to be used to make pipes or tanks; it does not react with water.
- Metals can be arranged in an order of their reactivity from their reactions with water and dilute acids. Students should be able to recall and describe the reactions, if any, of potassium, sodium, lithium, calcium, magnesium, zinc, iron and copper with water or dilute acids. Students should be able, where appropriate, to place these metals in order of reactivity.
- Displacement reactions involving metals and their compounds in aqueous solution establish positions within the reactivity series. Students should be able to describe displacement reactions in terms of oxidation and reduction, and to write the ionic equations.
- Unreactive metals such as gold are found in the Earth as the metal itself but most metals are found as compounds that require chemical reactions to extract the metal.
- Metals that are less reactive than carbon can be extracted from their oxides by reduction with carbon: for example, iron oxide is reduced in the blast furnace to make iron. Knowledge and understanding are limited to the reduction of oxides using carbon. Knowledge of reduction is limited to the removal of oxygen. Details of the blast furnace are not required, but students should know the raw materials used and explain the simple chemistry involved, including the use of equations. Knowledge of the details of the extraction of other metals is not required. Examination questions may provide further information about specific processes for students to interpret or evaluate.
- Metals that are more reactive than carbon, such as aluminium, are extracted by electrolysis of molten compounds. The use of large amounts of energy in the extraction of these metals makes them expensive. Knowledge of the details of industrial methods of electrolysis is not required, other than the detail required for aluminium (see Section 3.3.2).
- New ways of extracting copper from low-grade ores are being researched to limit the environmental impact of traditional mining. Copper can be extracted by phytomining, or by bioleaching. Students should know and understand that: phytomining uses plants to absorb metal compounds and that the plants are burned to produce ash that contains the metal compounds; bioleaching uses bacteria to produce leachate solutions that contain metal compounds. Further specific details of these processes are not required.
- Copper can be obtained from solutions of copper salts by electrolysis. Students should know the electrode material and be able to write the ionic half equations for the reactions occurring at both electrodes.
- Copper can be obtained from solutions of copper salts by displacement using scrap iron. Students should be able to describe this in terms of oxidation and reduction, and to write the ionic equation.
- We should recycle metals because extracting them uses limited resources, and is expensive in terms of energy and in terms of effects on the environment. Students are not required to know details of specific examples of recycling, but should understand the benefits of recycling in the general terms specified here.
- The carbonates of magnesium, copper, zinc, calcium and lithium decompose on heating (thermal decomposition) in a similar way. Students should be aware that not all carbonates of metals in Group 1 of the periodic table decompose at the temperatures reached by a Bunsen burner.
- Metal carbonates react with acids to produce carbon dioxide, a salt and water.
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- Which property of metals is explained by layers of atoms sliding over each other? A. They conduct electricity. B. They can be hammered into shape. C. They have high melting points. D. They have high densities. Answer: B
- Which metal can be extracted from its oxide by heating with carbon? A. potassium B. sodium C. iron D. aluminium Answer: C
- An alloy is best described as: A. a compound formed from two metals B. a mixture of at least two elements, at least one of which is a metal C. a pure metal that has been heated and then cooled quickly D. a metal from which all impurities have been removed Answer: B
- In the reaction Fe(s) + Cu2+(aq) -> Fe2+(aq) + Cu(s), which statement is correct? A. Iron is oxidised and copper ions are reduced. B. Iron is reduced and copper ions are oxidised. C. Both the iron and the copper ions are oxidised. D. Neither the iron nor the copper ions change. Answer: A
- A green solid is heated strongly and turns black, and a gas is given off that turns limewater milky. The green solid is most likely to be: A. copper(II) carbonate B. zinc carbonate C. calcium carbonate D. lithium carbonate Answer: A
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