Reactivity Series

How do we work out the order of the reactivity series?

To work out the order of the reactivity series, we obviously need to find out if one metal is more reactive than another metal. Reactivity is all about electrons. For metals, the reactivity of a metal depends on how easily it loses electrons. If it loses electrons really easily, it is really reactive, and so goes at the top of the reactivity series. Potasium and sodium lose electrons very easily. On the other hand, it is really hard for a metal like platinum to lose its electrons, so it is not reactive, so goes at the bottom.

In practice, we can work out reactivity by observing how a substance reacts with diifferent metals. We can see if the metal reacts with the substance instantly, if it reacts when more energy is added (e.g. heat), or if it does not react with the substance at all under any circumstance.

Let us see this in more detail, with two substances - water and acid - for better understanding.

Reacting with water

We can compare how vigorously metals react with water. For example, alkali metals react very vigorously with water, whilst most other metals e.g. copper, do not react at all. When metals react with water, they produce a salt and hydrogen gas. The hydrogen gas appears as fiz. The fizzing is how we know if a metal has reacted or not.

$$ Metal + Water \rightarrow Salt + H_{2(g)} $$

Fig. 1;: Calcium reacting with water | Mammoth Memory

We know that reactivity increases as you go down group 1, so we can already order the alkali metals in order of reactivity:

1. Potassium.
2. Sodium.
3. Lithium.

We can also put all the other metals below them, as they do not react with water at all, so they must be less reactive.

There are some elements that take a long time to react or do react with water spontaneously and need to be heated. One example is magnesium, which reacts with water only when left in a beaker filled with water for several days. Therefore, we know that we can place elements that react like this between alkali metals (which react vigorously), and most transition metals (which do not react at all).

Reacting with acid

Reacting with water is very useful, but for some metals, this reaction is very, very slow. In these cases, we can see how metals react with acid. Reacting with acid tends to be faster and more vigorous than reacting with water.

Like with water, when metals react with acid, they produce a salt and hydrogen gas. The hydrogen gas appears as fizzing. The fizzing is how we know if a metal has reacted or not.

$$ Metal + dil. Acid \rightarrow Salt + H_{2(g)} $$

The most reactive elements, such as potassium and sodium, will explode when they are reacted with dilute acid.

The fairly reactive elements, such as magnesium and aluminium, fizz and give off hydrogen gas when they react with dilute acid. The least reactive elements, such as silver and gold, still do not react even with acid.

Fig. 2: Potassium, magnesium, and zinc reacting with a dilute acid | The Science Hive

Here is a table that shows the observations of the reactions between each element and water, and each element with acid.

You can see that the more reactive the elements are, the more vigorously they react.As they decrease in reactivity, the reactions become less vigorous.

When do we use the reactivity series?

The reactivity series is really useful for a number of situations.

One way is we use it to predict the outcome of displacement reactions, where a more reactive metal will displace a less reactive metal in a compound. If you want to learn more, head over Displacement reactions!

We can also use the reactivity series with hydrogen and carbon, to extract these metals from their ores. You can read more about it in Extracting Metals.