Energy in chemical reactions



Metals are extracted from the Earth’s crust. Most metals reacted in the past so are found as oxides, carbonates or sulphides etc. These are called ores. Only very un-reactive metals can be found as metals/in their native state e.g. Gold and Platinum.  The ores are of very little use to us and we need to reverse these reactions to free the metal within. We do this by reacting the ore with a metal that is MORE reactive than the metal trapped in the ore. In this reaction, called a reduction reaction, the more reactive element takes the non-metal from the ore leaving behind the desired metal. In most cases, the plentiful element carbon is used to extract metals less reactive than itself.

2PbO + C -> 2Pb + CO2

One of the most commonly used metals and the most important during the industrial revolutions was definitely Iron. In the reduction reaction (see above) carbon reacts with the iron ore to release the iron from its oxide. Charcoal used to be the sources of the carbon but it was later changed to coke (purest coal). In the blast furnace, the three raw materials are added and a series of three main reactions:

C + O2 -> CO2

CO2 + C -> 2CO

Fe2O 3 + CO -> 2Fe + 3CO2

Blast furnace

The released molten iron is taken away and either cast in moulds (cast iron) or kept molten for further purification and the addition of other metals to make steel. When all of the impurities are removed from pig iron, we get pure iron which is very limited in its usefulness. Cast Iron is very brittle. It can be strengthened by working it which makes wrought iron but to make it into the material that we need daily, it has a small amount of carbon added to it in the liquid state and this makes steel. Carbon atoms sit within the iron atoms.

This makes it stronger as the layers cannot easily slide over each other because the ions within have different sizes. Carbon steel contains 0.03% to 1.5% carbon: used for - cars, machines…etc. Mild steel is less than 0.1% carbon and is soft enough to be pressed into shape. Low alloy steel is expensive and contains up to 5% other metals such as Nickel, Chromium, Vanadium, Tungsten and Titanium. Each adds its own special feature such as stainless steel or high temperature steel for cutting tools. Very expensive high alloy steels contain similar metals but up to 15% - used in cooking equipment etc.

An alloy is a mixture of two or more metals and they have to have been mixed when molten to gain a homogenous mixture or alloy. Common examples: Bronze is Tin and Copper, Brass is Zinc and Copper. Think of brass fittings and items that you have seen before, they are easily shaped and when polished, shiny and attractive but a lot cheaper than gold. Alloys get more and more sophisticated such as those that, under heat, either return to a previously assigned shape or tighten like the attachment of a dental brace.

Gold is very soft so not good as a wedding ring or it will become misshapen easily. It is alloyed with silver to make it harder but keep its appearance, different purities are called carats, e.g. 9 carat gold. Aluminium is also hardened by alloying it with other metals to keep its low density, resistance to corrosion but increase its strength. If Aluminium is so high on the reactivity series, why does it not react with my sandwiches or the chicken in the over? Aluminium is so reactive that as soon as it is open to the air it forms an oxide layer that is so tough, it will not corrode.

Aluminium Oxide is basically Ruby which is very tough. Titanium has very similar properties and they are both low density with relatively high strength. Titanium has one more property and that is its ability to work at very high temperatures. These two metals are then ideal for use on aircraft where it is essential that they are low enough in density to make a craft able to take off, they are strong enough to resist the stresses of flight and they will not corrode leaving holes in the fuselage.

Titanium can also be used inside the jet engine where temperatures are very high but keeping the mass down by using a low density metal. Aluminium is reactive so it has to be taken from its ore, bauxite, using electrolysis. This is very expensive as it uses masses of electricity for the electrolysis process but also to melt the ore in cryolite. Titanium is not as reactive but as it goes brittle in the presence of carbon, cannot undergo reduction reactions so is purified by electrolysis also.



Key words and terms for this topic: ore, reactivity series, reduction, blast furnace, cast iron, steel, alloy, stainless steel, aluminium, titanium, smelting, phytomining, bioleaching, displace, transition metals.

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