This unit is about ideas on the different reactivity of metals and issues related to corrosion. Basic chemical equations are explored and the consequences of the relative reactivity of the various metals considered.
A - Reactions with Oxygen and Water
Objectives:
- Explain how metals oxidise, tarnish and corrode
- Write word equations for chemical reactions of metals in air
- Use evidence to compare how quickly metals react
Almost every metal reacts with oxygen to create a surface layer of oxide. For example: iron reacts with oxygen to produce iron oxide (usually called rust). The precious metals are very unreactive and can be found in perfect condition, even after hundreds of years. The most famous are gold and platinum. Some oxides are called tarnish, copper is an example of this. The gradual deterioration of a copper coin is the formation of a layer of tarnish. It can be removed by polishing (abrading it) or dropping it in an acid (cola!).
The more REACTIVE a metal is, the faster it reacts with oxygen and the harder it will be to separate it from the oxygen again. This is why the highly reactive metals lithium, sodium and potassium are SO expensive. It is ridiculously difficult to isolate the pure metal from its tightly bound oxides. By far the most reactive metals are the so-called ALKALI METALS. They react violently with water to produce a hydroxide, which is alkaline. The hydrogen from the water is released, and if heated enough by the reaction can explode! Lithium is famous now as it is the metal that is found in laptop and phone batteries.
\[lithium+water\rightarrow lithium\,hydroxide+hydrogen\]
The more REACTIVE a metal is, the faster it reacts with oxygen and the harder it will be to separate it from the oxygen again. This is why the highly reactive metals lithium, sodium and potassium are SO expensive. It is ridiculously difficult to isolate the pure metal from its tightly bound oxides. By far the most reactive metals are the so-called ALKALI METALS. They react violently with water to produce a hydroxide, which is alkaline. The hydrogen from the water is released, and if heated enough by the reaction can explode! Lithium is famous now as it is the metal that is found in laptop and phone batteries.
\[lithium+water\rightarrow lithium\,hydroxide+hydrogen\]
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Alkali Metals video clip. Had this knocking around for years, can't remember the source! It is old, but good. Compare how the reactions change as the metals get further down the group.
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Brainiac - loud and entertaining. Fairly sure that the cesium is faked though! Found on Youtube.
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B - Reactions with Acids
Objectives:
- Write word equations for the reactions of reactive metals with water
- Describe how reactive metal are extracted from their compounds
- Use evidence to place the reactive metals in order of reactivity
You have seen that some metals react in a big way when they meet water. Most react in a very slow and boring manner - taking days if not weeks to corrode. You saw that when I left a clean strip of magnesium in a beaker of water for a weekend it turned black, but only at the surface. Heating the water up will speed up the reaction a bit - see animation below.
So, if we want to get some more exciting chemistry to be able to determine which metals are more reactive than the others, we need to use acid rather than water. The reaction is similar as hydrogen is released, but a salt is produced instead. These salts are usually soluble, so give a clear solution. Acids contain water. Dilute acids more so.
\[metal+acid \rightarrow salt+hydrogen\]
An example would be dropping magnesium into sulphuric acid:
\[magnesium+sulphuric\,acid \rightarrow magnesium\,sulphate+hydrogen\]
Recall from the previous topic the patterns/rules for making salts.
Obviously reacting lithium et al with even a dilute acid would be insanely dangerous - SO WE DON'T DO IT!
Hydrogen is highly flammable, so can be ignited by the heat of the reaction. The test for hydrogen is the famous 'squeaky pop' test!
So, if we want to get some more exciting chemistry to be able to determine which metals are more reactive than the others, we need to use acid rather than water. The reaction is similar as hydrogen is released, but a salt is produced instead. These salts are usually soluble, so give a clear solution. Acids contain water. Dilute acids more so.
\[metal+acid \rightarrow salt+hydrogen\]
An example would be dropping magnesium into sulphuric acid:
\[magnesium+sulphuric\,acid \rightarrow magnesium\,sulphate+hydrogen\]
Recall from the previous topic the patterns/rules for making salts.
- sulphuric acid (sulphates)
- hydrochloric acid (chlorides)
- nitric acid (nitrates)
- phosphoric acid (phosphates)
Obviously reacting lithium et al with even a dilute acid would be insanely dangerous - SO WE DON'T DO IT!
Hydrogen is highly flammable, so can be ignited by the heat of the reaction. The test for hydrogen is the famous 'squeaky pop' test!
C - Methods of Reducing Corrosion
Objectives:
- Describe ways which metals can be protected against corrosion and suggest some practical uses of this.
Metals corrode when they react with oxygen, acids and water. The key to preventing corrosion is to provide a barrier between the metal and the oxygen etc. The most common methods are painting, plating and coating in plastic.
Painting is quick cheap and easy, but not as effective.
Painting is quick cheap and easy, but not as effective.
Another method used on aluminium is called anodising. The metal is put in an acid and connected to a battery. The result is that the aluminium oxidises to produce a hard-wearing protective layer. Not all metals will do this. For example iron oxides (rust) and magnesium oxides are porous and provide zero protection. Curious fact: \(1\, \text{mm}\) of steel will swell to \(8 \,\text{mm}\) when it rusts.
Plating can either be done with a less reactive metal such as tin, chromium, silver or gold(!), or, and this seems odd, a more reactive metal like zinc. Zinc plating of steel is very common and found everywhere. It results in a mottled dull grey look. The zinc forms an oxide, and this oxide is hard and acts as a barrier to water and oxygen. Coating in plastic is fairly common too. The metal is sprayed with a powdered plastic and heated to melt it. Once cooled, the metal in encased in the plastic.
D - The Reactivity Series
Objectives:
- Describe and compare the reactivity of different metals
- Use a table to interpret data.
After performing various reactions with water and acid, it is possible to create a "league table" of reactivity. You should learn this. Most students find the top and bottom of the reactivity series easy to remember. It is the central ones - from aluminium to lead - that are tricky.
The consequence of these different reactivities is that not only can we get an idea of which metals react in the most exciting ways in chemistry lessons, but also that some metals can 'steal' elements and compounds off another metal! This is know as a displacement reaction. We will do a number of these in the lab. An example of a displacement reaction is that between copper sulphate and magnesium. As the magnesium is more reactive than the copper, it pushes the copper away and takes the sulphate for itself.
\[magnesium+copper\,sulphate\rightarrow magnesium\,sulphate+copper\]
Note that the reaction is only one way. Adding copper to magnesium sulphate will not do anything as the magnesium is giving up its sulphate to the 'feeble' copper.
\[magnesium+copper\,sulphate\rightarrow magnesium\,sulphate+copper\]
Note that the reaction is only one way. Adding copper to magnesium sulphate will not do anything as the magnesium is giving up its sulphate to the 'feeble' copper.
Another valuable insight given by the reactivity series is how metals are made in the first place. Only the non-reactive metals such as gold and platinum are found in their elemental form. The rest are found in the Earth's crust as mineral compounds in the rocks. The earliest metals that were discovered were the less reactive ones, which must have been discovered by accident. The process of extracting the metal from a metal compound is called 'reduction'. A common form of reducing metals is called 'smelting' and can be done for copper, tin, silver, zinc, iron and lead and involves reacting the compound with carbon (soot).
\[copper\,oxide+carbon\rightarrow copper+carbon\,dioxide\]
One can imagine the prehistoric men surrounding a camp fire with rocks and noticing the formation of copper, and perhaps the other metals. Of course, as we found out in the previous topic, once they realised that if they melting and mixed them a stronger metal alloy was formed, which lead to the Bronze Age.
It took much longer to figure out how to extract the more reactive elements like magnesium, aluminum and lithium. These are extracted by melting the oxide at a very high temperature and running an electric current through the molten stuff. This is very expensive and unpleasant. Which is why we should always recycle aluminium - it is very easy and cheap to melt and purify.
\[copper\,oxide+carbon\rightarrow copper+carbon\,dioxide\]
One can imagine the prehistoric men surrounding a camp fire with rocks and noticing the formation of copper, and perhaps the other metals. Of course, as we found out in the previous topic, once they realised that if they melting and mixed them a stronger metal alloy was formed, which lead to the Bronze Age.
It took much longer to figure out how to extract the more reactive elements like magnesium, aluminum and lithium. These are extracted by melting the oxide at a very high temperature and running an electric current through the molten stuff. This is very expensive and unpleasant. Which is why we should always recycle aluminium - it is very easy and cheap to melt and purify.
Batteries work as the electrodes are dissimilar metals (or carbon) with different reactivities. The greater the different in reactivity, the greater the voltage that can be created. This is one of the reasons that lithium makes such powerful batteries. The other reason is that lithium is a very light metal so the phones and laptops are not so heavy. The downside is that if they overheat or get wet - interesting things happen!
Other Resources
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Where do the elements come from!?
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Periodic Table - Article from the Guardian celebrating its 150th year.