The metal Reactivity Series can be obtained by reacting a metal with a compound of another metal. Quite simply, either there is a reaction or there isn't! For a reaction to occur, that is, for one metal to displace another metal from its compounds, the added metal must be higher in the series than the metal in the compound. You can also use the non-metals hydrogen and carbon (either as coke, charcoal or carbon monoxide). If it doesn't involve a salt solution you need to heat the mixture.
Here you will look at how the displacement rule and displacement reactions can be used in a practical way to extract metals.
- Open Yenka file Model 1. The metal aluminium is a good reducing agent and is capable of reducing iron(III) oxide to iron metal in a reaction called the Thermit reaction. The iron oxide and aluminium have already been added to the vessel. Turn on the Bunsen burner and watch what happens.
- (a) Write a word equation for the Thermit reaction.
(b) Is it very exothermic? Explain how this reaction can be used to weld steel railway lines together.
(c) Is it an 'iron extraction'? Comment.
(d) Is the Thermit reaction an economic way of making iron? Comment.Answer(a) Iron oxide + aluminium → iron + aluminium oxide
(b) Very exothermic! Lots of heat is released. It's so hot, the iron is formed in a molten state. It can channel between the ends two steel (mostly iron) rail tracks. When it cools and solidifies, it joins the two tracks together.
(c) Yes, you have chemically extracted iron from iron oxide (the ore most commonly used to obtain iron is haematite, which is mainly Fe2O3).
(d) Definitely not! You need to extract another metal first (in this case aluminium, which is expensive), even before you extract the iron. It would also be very dangerous on a large scale!
- The element chromium is commercially extracted using a Thermit type reaction.
(a) Write a word equation for the reaction between chromium oxide and aluminium.
(b) What does this tell you about the relative reactivity of chromium and aluminium?
(c) Unfortunately you can't use the cheaper metal iron rather than expensive aluminium. Suggest a reason for this.Answer(a) Chromium oxide + aluminium → chromium + aluminium oxide
(b) Aluminium is more reactive than chromium.
(c) Iron is not reactive enough to displace the chromium.
A more reactive metal can be used to extract a less reactive metal using a displacement reaction but it tends to be a costly process.
- Open Yenka file Model 2. Hydrogen gas is a common reducing agent, widely used both in the laboratory and in industry. Turn on the flow of hydrogen gas with the slider control, then use the Bunsen burner to heat the copper oxide.
- (a) What do you see happening in the reaction between copper oxide and hydrogen?
(b) Write a word equation for this reaction.
(c) What is dangerous about the reaction?Answer(a) Black copper oxide powder → red-brown copper, condensation of colourless liquid water on the cooler part of the test tube
(b) Copper oxide + hydrogen → copper + water
(c) You are passing explosive hydrogen gas over a hot surface. If the glass cracks, air will mix with the hydrogen!
- From your knowledge of the reaction of metals with water, do you think you could extract sodium using this method? Explain your answer.
AnswerNo. Since sodium reacts so vigorously with water, it is highly unlikely that sodium oxide would react with hydrogen to form sodium and water!
Non-metals such as hydrogen and carbon can be used to extract metals from ores containing a compound of the metal. The reaction mixture must be heated which adds to both the cost and danger of the operation.