SC13 Flashcards
SC13b
1) Describe corrosion of metals as the result of oxidation.
2) Describe how rusting of iron occurs.
1) Corrosion is the destruction of materials by chemical substances in their environment which act on them over a period of time. Most metals can corrode in the presence of oxygen to form the corresponding metal oxide. Corrosion is caused by redox reactions: the metal loses electrons and is oxidised while the oxygen gains electrons and is reduced.
2) Rusting is the name given specifically to the corrosion of iron in the presence of water and oxygen from the air. Rusting is an oxidation reaction. The iron reacts with water and oxygen to form hydrated iron(III) oxide, which we see as rust. Iron and steel rust when they come into contact with water and oxygen – both are needed for rusting to occur.
SC13b
1) Explain how rusting can be prevented by excluding oxygen and/or water, and what happens when the barriers are damaged.
2) Explain how sacrificial protection works.
1) In order for a metal to rust, both oxygen and water are needed. Rust can be prevented by coating iron with barriers that prevent the iron from coming into contact with water and oxygen. Common barrier methods include: paint, oil, grease and plastic.
However, if the coatings are washed away or scratched, the iron is once again exposed to water and oxygen and will rust. Unlike some other metals, once iron begins to rust it will continue to corrode internally as rust is porous and allows both air and water to come into contact with fresh metal underneath any barrier surfaces that have been broken or scratched.
2) Sacrificial protection involves attaching a metal that is more reactive than the object’s metal. The more reactive metal will lose electrons and be oxidised more easily than the object, so it will corrode instead of the object.
SC13c
1) Recall what electroplating is.
2) Recall some common examples of electroplating.
1) Electroplating is a process where the surface of one metal is coated with a layer of a different metal. The metal being used to coat is a less reactive metal than the one it is covering.
2) - Silver or gold can be electroplated onto cheaper ‘base metals’ such as copper to create attractive jewellery that is cheaper than solid silver or gold.
- Electroplating may be use to improve a metal object’s ability to resist corrosion.
SC13c
1) Explain why metal objects may be electroplated.
2) Explain how electroplating is carried out.
3) If the sacrificial layer becomes scratched, exposing the metal underneath, will the metal underneath corrode and why?
1) Electroplating an object protects it from corrosion, and improves the appearance of the object.
2) Electroplating uses electrolysis to put a thin layer of a metal onto a metal object. The electrodes used in electroplating are non-inert - they take part in the electrolysis reactions.
3) The metal underneath will not corrode because the coated metal layer acts as sacrificial protection as well as a barrier, because the coated layer is more reactive than the metal underneath. Therefore the coated metal will lose electrons instead of the metal underneath, even if it is scratched.
SC13e
1) Recall common uses for aluminium, copper and gold.
2) Recall the names and compositions of 3 common alloys containing aluminium, copper and gold.
1) Gold and copper resist corrosion. They are also malleable, ductile and very good conductors of electricity. Copper is chosen for most electrical wiring because gold is extremely expensive, while gold is used in tiny amounts to connect microprocessors and memory chips. Aluminium resists corrosion, but it does not conduct electricity as well as copper. However, it is stronger, cheaper and less dense, so it is used for overhead electrical cables.
2) Magnalium is an alloy containing aluminium and magnesium. Brass is an alloy containing copper and zinc. Jewellery gold is an alloy containing gold and copper.
SC13a
1) Describe the position of the transition metals in the periodic table.
2) Describe some general physical properties of transition metals (state 7)
1) The transition metals are placed in the centre block of the periodic table, between groups 2 and 3.
2) Physical properties of transition metals include include:
- Malleable (they can be hammered or rolled into shape without shattering)
- Ductile (they can be stretched out to make this wires)
- Good conductors of electricity
- Shiny when polished
- High melting points
- High densities (for example, they can be used for weights)
- Transition metals have colourful compounds
SC13a
1) Describe some general chemical properties of transition metals (state 3)
2) Explain why iron has the typical properties of a transition metal.
1) The transition metals have the following chemical properties in common:
- They are less reactive than alkali metals such as sodium. They form coloured ions of different charges.
- Transition metals and their compounds typically show catalytic activity. Catalysts increase the rate of a reaction without being changed either chemically or in mass at the end of the reaction.
- Some transition metals are very unreactive and are resistant to corrosion (such as silver and gold).
2) Iron, a transition metal, has a higher melting point and a higher density than the non-transition metals. Like other transition metals, iron forms coloured compounds. Iron is the catalyst used to make ammonia in the Haber process.
SC13d
1) Recall the name of a common alloy (beginning with ‘s’), and explain what it’s made from
2) Describe what alloys are.
1) Steel is an alloy of iron, containing a small amount of carbon, and other elements. Steel is an alloy of iron, so it can rust. To prevent this, we can add chromium and nickel to form stainless steel.
2) An alloy is a mixture of two or more elements, where at least one element is a metal.
SC13d
1) Explain why iron is alloyed with other metals.
2) Explain why alloys are often stronger than the metals they contain.
1) Many pure metals are too soft for many uses. They can be made harder by adding another element to the pure metal, so forming an alloy. This explains why an alloy often has more uses than the pure elements it is made from.
Pure iron, for example, is very soft. Adding a small amount of tungsten to iron makes tool steel, which is harder than pure iron.
2) Atoms of different elements are different sizes. Metals can be bent and shaped because metallic structures have layers of atoms or ions that are able to slide over each other. Adding atoms of different elements to a pure metal distorts the layer, making it harder for them to slide over one another.
SC13e
Explain why different metals and their alloys have different uses (copper, brass, gold, magnalium and aluminium)
- Copper and brass resist corrosion and are good electrical conductors. Copper is a better conductor than brass, and it is used in electrical wiring. Brass is stronger than copper, so it is used for the pins in electrical plugs.
- Gold is a very soft and malleable metal. It is also very unreactive, so it resists corrosion and stays shiny. The gold used for jewellery is gold alloyed with other metals, often copper. This makes the jewellery much stronger while keeping its ability to stay shiny.
- Aluminium does not react with water. Its surface is protected by a natural layer of aluminium oxide that allows the metal to resist corrosion. Aluminium foil is used in the home for wrapping and storing food because it does not react to substances in food. It is malleable, so it is easily folded into shape around the food.
- Aluminium has a low density, so pieces of aluminium are relatively lightweight. Magnalium is stronger than aluminium alone but still has a low density. It is used to make aircraft parts.
