The Reactivity Series

Metals are placed in a reactivity series — a list of metals from most reactive to least reactive. Displacement reactions are used to investigate whether one metal is more reactive than another.

Reactions of metals with acids

We can use the reactions of metals with acids to tell us how reactive that metal is. The more vigorously it reacts, the more reactive the metal. The slower the reaction (sometimes there is no reaction at all), the less reactive the metal.

Metals react with acids to form a salt and hydrogen, as described in the equation below:

You can use the following experimental set-up to determine the order of reactivity of different metals:

• Add equal volumes of dilute hydrochloric acid or dilute sulfuric acid into a series of test tubes then add a equal mass of metal to each test tube. It is important that each metal has the same surface area because this will affect the rate of reaction.

• Count the number of bubbles produced in a given time. The bubbles are hydrogen gas and can be confirmed using a lit splint, which will produce a ‘squeaky pop’ when the hydrogen burns.

• The faster the bubbles are given off, the faster the rate of reaction and the more reactive the metal.

Let’s say we carry out the experiment described above by adding potassium, magnesium and zinc into test tubes containing dilute sulfuric acid. We’d expect the potassium to react explosively (in reality you would never do this in a school laboratory as it is too dangerous), while the magnesium would bubble vigorously and the zinc would form bubbles very slowly. We can therefore confirm their order of reactivity (from most reactive to least) as: potassium, magnesium then zinc.

Reaction of metals with water

We can also use the reaction of metals with water to determine order of reactivity. The equation for this reaction is:

• Reactive metals (such as potassium, sodium, lithium and calcium) will react rapidly in cold water

• Less reactive metals (such as magnesium, zinc and iron) won’t react with cold water but will react with water vapour (steam)

• Unreactive metals (such as copper, silver and gold) won’t react with either cold water or steam

The reactivity series

The reactivity series lists the metals in order of reactivity, from the most reactive at the top (potassium) to the least reactive at the bottom (gold). You will need to learn this list for the GCSE exam and it’s much easier if you use a mnemonic to help you. For example:

A more reactive metal will replace a less reactive one from a compound. For example, let’s say we add calcium to aluminium oxide. Calcium is higher up the reactivity series compared to aluminium so it will displace aluminium, forming calcium oxide. This happens because calcium binds more strongly to the oxygen.

Displacement reactions like the one shown above are redox reactions where one element is oxidised (loses electrons or gains oxygen) and the other is reduced (gains electrons or loses oxygen). Remember OILRIG: oxidation is loss and reduction is gain. In our example above, we can see that calcium is oxidised since it gains oxygen and the aluminium is reduced.

Another example of a displacement reaction is the reaction between a metal and a salt. If we mix together sodium with magnesium sulfate, we will produce sodium sulfate (since sodium is more reactive and can displace magnesium) and pure magnesium. If you look at the symbol equation for this reaction, you can see that the sodium forms a positive ion when it forms the sulfate salt (remember that salts are ionic compounds made of positive and negative ions). Sodium loses electrons to form the positive ion, so we know that sodium has been oxidised, whereas magnesium gains electrons and is reduced.

Finally, let’s say we mix a less reactive metal, such as copper with a salt containing a more reactive metal, such as zinc chloride. Copper is below zinc in the reactivity series so will be unable to displace it in the compound and there will be no reaction.

Oxidising agents and reducing agents

We have already talked about how oxidation is the loss of electrons (or gain of oxygen) and reduction is the gain of electrons (or loss of oxygen). You also need to know that the substance which oxidises an element (so is reduced itself) is called an oxidising agent. The substance that reduces another element (so is oxidised itself) is called a reducing agent.

Rust

Rust is formed when iron reacts with oxygen (either in the air or in water) to form hydrated iron (III) oxide. Rusting is an oxidation reaction since iron gains oxygen and loses electrons. The equation for the reaction is:

We can use the experiment below to show that iron does not rust in the absence of air or water. In the second tube, the dissolved oxygen has been removed from the water by boiling and the layer of oil prevents any more air from dissolving. The calcium chloride in the third tube absorbs water vapour in the air. Rusting has not occurred in either of these two tubes, proving that we need both air and water for rusting to occur.

There are two ways to prevent rusting: barrier methods and galvanising.

• Barrier methods: the iron is coated with a barrier to prevent iron from coming into contact with water and oxygen. The barrier could be a layer of paint, plastic, oil or grease.

• Galvanising: the iron is coated with a layer of a more reactive metal, usually zinc. The layer of zinc acts as a barrier to prevent iron from coming into contact with air and water. It also acts as a sacrificial metal. This means that it will react with oxygen more readily than iron because it is higher up in the reactivity series. Zinc will be oxidised (lose its electrons) instead of the iron.