O Level Chemistry Chap 14: Metals

Transcription

1 O Level Chemistry Chap 14: Metals Metals and Alloys 1. Metals have a regular arrangement of closely packed positive ions surrounded by a sea of mobile valence electrons. Atoms in a metal are packed tightly in layers and held by strong metallic bonds. 2. Properties of metals and the relation to its structure Physical Property Relation to structure Malleable (easily beaten into thin Atoms (of the same size) are sheets) and ductile (can be drawn regularly arranged in layers. They can into wires without breaking) slide over each other easily when forced is applied. High density Metal atoms are closely packed. High melting and boiling points Metal atoms are packed tightly in layers by strong metallic bonds. A lot of energy is needed to break the strong metallic bonds. Good conductors of heat and electricity In metals, the valence electrons of the atoms are mobile and can move throughout the metal. They allow heat and electricity to be conducted. 3. An alloy is a mixture of a metal with one or a few other elements. Bronze = Copper + Tin Brass = Copper + Zinc (Musical Instruments, decorative ornaments) Stainless steel = Iron + Chromium + Nickel + Carbon (Cutlery, surgical instruments) 4. Why metals are often used in the form of alloys a) Alloys are harder and stronger than pure metals. Structures of Metals VS Alloys Metals Pure metals are weak and soft. The atoms (of the same size) are regularly arranged in layers which can slide over each other easily when force is applied. Pure metals are thus soft and malleable. Alloys - When a pure metal is alloyed, a different element is added to the pure metal. - This disrupts the regular arrangement of atoms in the pure metal by the addition of other atoms of a different size. - Therefore, it is difficult for the atoms in an alloy to slide over each other. - This makes alloys harder, stronger and less malleable than pure metals. 1

2 b) While pure metals corrode easily, alloys have a higher resistance to corrosion. E.g. Copper-nickel alloy is used to make coins instead of pure copper. c) Alloying is used to lower the melting points of metals. E.g. Solder is an alloy of tin and lead and has a lower melting point than its constituent metals, thus can be used to join metals. The Reactivity Series 4. Chemical Properties of Metals: Form positive ions (cations) by loss of electrons (metals are reducing agents) Form ionic compounds e.g. metal chlorides/oxides Usually (but not always) react with dilute HCl or H 2 SO 4 to produce salt + hydrogen React with oxygen to form basic oxides or amphoteric oxides 5. Reactivity Series The reactivity series is a measure of a metal s tendency to lose electrons and form a positive ion. Metals high up in the reactivity series have a greater tendency to form its positive ions. The order of reactivity can be deduced by reference to - the metal s reaction with water, steam and dilute HCl - the reduction of their oxides by carbon and/or hydrogen, or any other form of decomposition. Reactive metals are unstable and tend to react to form compounds (compounds are stable). Unreactive metals are more stable but their compounds tend to be less stable than those of the more reactive metals. 2

3 Heating or physical extraction Heat oxide with coke Steam Water Electrolysis Reactivity Series of 13 metals Reactivity Element Extraction Dilute HCl/H2SO4 Carbon Hydrogen Heat (oxide) (oxide) (carb) Potassium x x x Sodium x x x Calcium x x / Magnesium x x / Aluminium x x / Zinc / x / Iron / / / Lead / / / Hydrogen Copper / / / Manganese Heat Heat Silver Heat Heat /* Gold Heat Heat Platinum Heat Heat Extraction method is from the metal oxide for Carbon and Hydrogen, and the metal carbonate for Heating. Refer to TB for details on the reactions of metals. Note: - Metals above hydrogen will react with dilute acids to produce hydrogen gas (they are more reactive and hence can displace hydrogen from solutions of acids). - Lead appears not to react with dilute HCl/H 2 SO 4 which is an exception to the acid + metal reaction (refer to chap on acids and bases). However, just remember that metals above hydrogen will react with dilute acids. - Dilute H 2 SO 4 reacts with metals similarly to HCl, the salt formed is a metal sulfate. - K to Ca reacts with steam too, but is similar to reacting with water. - There is some ambiguity at the borderlines between the groups. Mg is considered not to react with water (although it can) as the reaction is too slow unless the surface layer of oxide protecting the metal is specially removed. - Al appears relatively unreactive despite its high position in the reactivity series. This is because Al is reactive and reacts with oxygen, forming a thin and protective layer of Al 2 O 3 which coats the metal and prevents further reaction. This can be used to prevent corrosion (see Alloys) Applications of the Reactivity Series 6. Reduction of Metal Oxides by Reducing Agents The reactivity of metals can be studied by how easily metal oxides decompose. The more reactive a metal is, the more difficult it is to decompose its oxides (reduce the oxide to the metal) Above zinc = not reduced by reducing agents (too stable, use passing electricity). Below copper = oxide will decompose simply by heating without needing a reducing agent. 2Ag 2 O (s) 4Ag (s) + O 2 (g) 3

4 a) with Carbon (Zn & below) [smelting] E.g. 2CuO (s) + C (s) Cu (s) + CO 2 (g) Cu 2+ ions from copper (II) oxide are reduced to copper; carbon is oxidised to carbon dioxide. Metals below magnesium are often extracted from their ores (industrially) by reduction with carbon. [?!] b) with Hydrogen (Fe & below) metal oxide + hydrogen metal + steam CuO (s) + H 2 (g) Cu (s) + H 2 O (g) 7. Decomposition of Metal Carbonates Compounds of reactive metals are stable and not easily reduced and decomposed by heat (more stable to heat/thermally stable). The more reactive a metal is, the more difficult it is to decompose its compounds (e.g. it takes a lot of heat and time to decompose CaCO 3, but CuCO 3 decomposes quickly and easily). Carbonates- Ca & below: metal carbonate metal oxide + carbon dioxide Silver carbonate: the silver oxide produced further decomposes to form silver + oxygen (unstable) 2Ag 2 CO 3 4Ag + 2CO 2 + O 2 8. Displacement of Metals More reactive metals can displace a less reactive metal from its salt solution or oxide. This is because more reactive metals have a greater tendency to form its positive ions. E.g. Mg can displace ZnSO 4 but Cu, Pb and Fe cannot. The more reactive a metal is, the more readily it forms compounds; unreactive metals tend to stay uncombined. a) from solution E.g. iron displaces copper ions in CuSO 4 to form copper metal and FeSO 4 Fe (s) + CuSO 4 (aq) FeSo 4 (aq) + Cu (s) Metal displacement reactions are redox reactions. The more reactive metal is oxidised (by becoming ions and forming compounds) while the less reactive metal is reduced (by changing back to atoms). E.g. Fe (s) + Cu 2+ (aq) Fe 2+ (aq) + Cu (s) Copper ions are reduced to copper atoms; iron is oxidised to iron (II) ions. b) from oxide More reactive metals can reduce the oxide of a less reactive metal. E.g. Thermite reaction- Al displaces Fe from Fe 2 O 3 Fe 2 O 3 (s) + 2Al (s) Al 2 O 3 (s) + 2Fe (l) Al atoms react with O 2- ions from Fe 2 O 3 to form Al 2 O 3, Fe 2 O 3 is reduced to molten Fe. Note: Highly exothermic reaction, molten iron (which melted due to heat) is formed. This reaction is used to weld railway lines together. 9. Using the Reactivity Series Refer to TB/242 4

5 Extraction of Metals 10. Metals are found in the ground in rocks called ores. E.g. iron is found in the ore haematite. An ore is a compound of the metal (oxides, sulphides, chlorides or carbonates) mixed with large amounts of earth and rock. 11. Metal Method of Extraction K to Al From ore: Using electricity to decompose the molten metal compounds (electrolysis) Zn to Pb From metal oxide: Reduce the metal oxide by heating with coke (smelting) Cu to Pt Found free in the ground naturally as uncombined metals / physical extraction The more reactive the metal is, the harder it is to extract the metal from its ore. E.g. compounds of reactive metals are stable and difficult to be reduced. Whenever possible, reduction with coke is used. Electrolysis is only used for reactive metals because carbon cannot take oxygen away from the metal oxide, as the bonds in the metal oxide are too strong. Smelting is cheap while electrolysis is expensive. Extraction of Iron from Haematite 12. Refer to TB/ The iron that is extracted from the blast furnace is known as cast iron, and is mainly used to produce steel. Steel 13. Steel is an alloy of iron with carbon and/or other metals. Different types of steel are made by varying the amount of carbon and by adding different metals to iron. Each type has different amounts of carbon and other metals added, hence its own unique properties and uses. 14. Carbon steels: Iron + Carbon Alloy steels: Iron + Carbon + One or more of: Mn, Ni, Cr, W, V. These metals are added to change the properties of the steel. Category Type of Steel Uses Special Properties Carbon Steels Mild Steel (low carbon) 0.25% Carbon Car bodies and machinery Hard, strong and malleable High Carbon Steel % Carbon Cutting and boring tools, e.g. knives, hammers Strong but brittle (more carbon atoms to prevent sliding) Alloy Steels Stainless Steel Alloy of iron, chromium, nickel & carbon. Equipments in chemical plants, cutlery, surgical instruments Extremely durable, resistant to rust and corrosion even when heated Qn: Explain how the properties of low carbon and high carbon steel differ. Low carbon steel is softer as it is more malleable. High carbon steel contains more carbon atoms which prevent sliding of the iron atoms. Hence, high carbon steel is harder but brittle. 5

6 Rusting 15. Rusting is the oxidation of iron to form hydrated iron (III) oxide. When an object made of iron (e.g. iron, mild steel) is exposed to moist air for some time, a reddish-brown substance (rust) slowly forms on the surface, through rusting/ corrosion of iron. Rusting occurs only in the presence of both oxygen and water. The presence of sodium chloride and acidic substances (e.g. sulphur dioxide, CO 2 ) speeds up the rusting process. A simplified equation is iron + oxygen + water hydrated iron (III) oxide [rust] 4Fe (s) + 3O 2 (g) + 2xH 2 O 2Fe 2 O 3.xH 2 O (s) Note: - Mild steel will rust faster than high carbon steel due to higher iron composition - Only iron (and alloys containing iron) will rust. Magnesium corrodes. 16. Rust Prevention Method Using a protective layer Using a sacrificial metal (sacrificial protection) Description - The metal may be coated with a layer of substance that stops water and air from reaching the metal. - This is done by painting, greasing, and coating with another metal. - E.g. In galvanising (or zinc-plating), a thin film of zinc is used to cover the iron. Note: even if the zinc layer is damaged, the iron will not rust because zinc is more reactive than iron and it will corrode in place of iron (the more reactive metal corrodes preferentially). - Involves attaching a more reactive metal (e.g. zinc) to iron. Since zinc is more reactive than iron, it gets corroded in place of the iron because zinc loses its valence electrons more readily. As long as a more reactive metal is present, iron will not rust. - E.g. fixing bars of zinc to a ship s hull prevents the ship s steel body from rusting, attaching magnesium blocks to underground pipes Using alloys - Stainless steel, which is a rust resistant alloy, can be used. On exposure to air and moisture, a very hard coating of chromium (III) oxide forms on the surface of stainless steel, preventing it from further corrosion. 6

7 Recycling of metals 17. Metals are finite resources and need to be conserved. The amount of metal ores in the Earth is limited. If metal extraction continues at present rates, the supplies of many metals will run out. Hence, there is a need to recycle metals. Advantages Economic - It saves the costs of extracting new metals from their ores. - Fewer landfills to dispose used metal objects will need to be built. This saves the cost of building landfill sites. Social - It helps to conserve the limited amount of metals in the Earth. - With the increasing world population, more land will be available if ore mining is reduced. Environmental - Reduces the environmental problems related to extracting metals from ores (e.g. waste gases including carbon monoxide which are formed in the blast furnace through iron extraction will be reduced) Disadvantages - It is very expensive, considering other processing costs like collecting, transporting and separating the scrap metals. - It takes time and effort for the world to practise recycling as a way of life. - The recycling process may cause pollution if not done properly (e.g. metal fumes from the recycling process) Sample Questions Q1 Q2 Q3 Some alloys are coated with a layer of aluminium. Explain how this prevents the alloy from corroding. Al, being a reactive metal, reacts with oxygen, forming a thin and protective layer of aluminium oxide which coats the alloy and protects it from further corrosion. Explain why magnesium blocks are often attached to underground steel pipes. Magnesium, being more reactive than iron, acts as a sacrificial metal to corrode in place of iron. It provides sacrificial protection against rusting for the steel pipes. A block of iron is coated with copper. The block was accidentally scratched and the iron beneath was exposed. a) Explain what would happen to the iron. b) Will the copper in contact with the iron corrode faster or slower than normal? Explain your answer. a) The iron will undergo rusting because it is now exposed to oxygen and water (moisture in air). In addition, the iron will rust faster than normal. This is because iron, being more reactive than copper, acts as a sacrificial metal to corrode in place of iron (when itself already undergoes rusting). b) The copper will corrode slower than normal, because iron provides sacrificial protection for the copper against rusting by corroding in place of copper. 7

8 The Reactivity Series Metal Reactivity Extraction K Potassium Na Sodium React Li Lithium with Sr Strontium water Electrolysis Ca Calcium Mg Magnesium Al Aluminum Carbon included for comparison Zn Zinc Cr Chromium Fe Iron Cd Cadmium React with acids Smelting with Co Cobalt coke Ni Nickel Sn Tin Pb Lead Hydrogen included for comparison Cu Copper Found naturally Ag Silver Highly unreactive uncombined Hg Mercury Au Gold Heat or physical Pt Platinum extraction Going from bottom to top, the metals: increase in reactivity; lose electrons more readily to form positive ions; corrode or tarnish more readily; require more energy (and different methods) to be separated from their ores; become stronger reducing agents. Notes: Carbon is able to reduce metal oxides Zn to Pb because it is higher up in the reactivity series than them