Displacement Reactions

What are displacement reactions?

This might sound really confusing but it just means that if one metal, e.g. magnesium, is more reactive than another metal within a compound, e.g. copper (II) ions in copper sulfate, then the more reactive metal will replace and kick out the less reactive metal ions in the compound. These metal ions then form the metal element in the solution.

Importantly, all displacement reactions are also redox reactions (oxidation-reduction reactions) as they involve the exchange of electrons.

What do displacement reactions tell us?

Displacement reactions help us to order different metals by reactivity. So it is important in working out the reactivity series.

In the ‘Reactivity Series’ article, we covered two ways of figuring out which metals are more reactive than others by observing and comparing how vigorously a metal reacts with water and with acid. The more vigorously a metal reacts the more reactive it is.

However, you may notice that some of them react very similarly and are similarly reactive - e.g. magnesium, aluminium, zinc, and iron all have very slow reactions with water - so it is difficult to order those metals by reactivity accurately.

This is where displacement reactions come in really useful.

Because we know that a more reactive metal will always displace a less reactive metal from an aqueous solution of one of its salts, we can react [metal A] and [metal B compound] together to see if metal B is displaced (kicked out).

If it is, then metal A must be more reactive than metal B.

Displacement reactions do not only tell us information about the reactants, but they also have many important applications in chemistry, including the production of metals from their ores, the synthesis of new compounds, and the analysis of unknown compounds.

What is the equation for displacement reactions?

There are two types of displacement reactions: single and double replacement reactions. They both follow the same principle (a higher reactivity metal displaces or "kicks out" a lower reactivity metal from a compound), but look slightly different.

The formulas for single- and double-displacement reactions are as follows:

Single displacement reaction:

$$A + BC \rightarrow AC + B$$

A would be the higher reactivity metal that kicks out the lower reactivity metal, B.

Double displacement reaction:

$$AB + CD \rightarrow AD + BC$$

How do we predict the outcome of displacement reactions?

We can see if one metal will displace another metal in a displacement reaction by asking a couple of questions:

• Is the metal that isn’t part of a compound more reactive than the metal that is part of a compound?
• Is the metal compound (salt) in an aqueous solution (is the state symbol beside it (aq))?

If the answer to both questions is yes, then the two will react in a displacement reaction, and the metal element will displace the metal in the salt!

For instance, let’s take a look at the reaction between iron and lead (II) nitrate solution.

$$Fe(s) + Pb(NO_3)_2(aq) \rightarrow ?$$

Let’s answer the two questions together:

• Is the metal that isn’t part of a compound more reactive than the metal that is part of a compound?
  • Yes, looking at the reactivity series, iron is more reactive than lead which is in a salt (lead nitrate).
• Is the metal compound (salt) in an aqueous solution (is the state symbol beside it (aq))?
  • Yes, the lead nitrate is in an aqueous solution, as there is an (aq) beside it \(Pb(NO_3)_2(aq)\).

As a result, we know that the iron will displace the lead ions in the salt to form iron nitrate. The lead ions are displaced and so form lead metal, so it is simply in the solution as just lead.

The symbol equation for the reaction is:

$$Fe(s) + Pb(NO_3)_2(aq) \rightarrow Fe(NO_3)_2(aq) + Pb(s)$$

It might be helpful to think of it simply as the iron and lead swapping places.

Examples of displacement reactions

Single displacement reactions: when zinc metal is added to a solution of copper sulfate, the zinc displaces the copper from the compound, forming zinc sulfate and copper metal:

$$Zn + CuSO_4 \rightarrow ZnSO_4 + Cu$$

Double displacement reaction: when aqueous solutions of potassium iodide and lead nitrate are combined, the iodide ion and the nitrate ion switch places, forming solid lead iodide and aqueous potassium nitrate:

$$2KI (aq) + Pb(NO_3)_2 (aq) \rightarrow 2KNO_3 (aq) + PbI_2 (s)$$

Redox reaction: remember that all displacement reactions are redox reactions because there is an exchange of electrons. When magnesium metal is added to hydrochloric acid, the magnesium is oxidized to magnesium ions and hydrogen gas is produced, while the hydrogen ions are reduced to form hydrogen gas:

$$Mg + 2HCl \rightarrow MgCl_2 + H_2$$

Examples of halogen displacement reaction

Halogen displacement reactions are very similar to metal displacement reactions, but instead of one metal displacing another, it is the more reactive halogen that displaces another halogen in a compound. Here are some examples of halogen displacement reactions:

Chlorine displacement: chlorine gas can displace bromine or iodine from their compounds. For example, when chlorine gas is bubbled through a solution of potassium bromide, the chlorine displaces the bromine to form potassium chloride and bromine gas:

$$Cl_2 (g) + 2KBr (aq) \rightarrow 2KCl (aq) + Br_2 (l)$$

Bromine displacement: Bromine can displace iodine from its compounds. For example, when bromine water is added to a solution of potassium iodide, the bromine displaces the iodine to form potassium bromide and iodine:

$$Br_2 (aq) + 2KI (aq) \rightarrow 2KBr (aq) + I_2 (s)$$

Fluorine displacement: Fluorine is the most reactive of all the halogens and can displace any of the other halogens from their compounds. For example, when fluorine gas is passed over a sample of iodine, the fluorine displaces the iodine to form iodine pentafluoride:

$$5F_2 (g) + 2I_2 (s) \rightarrow 2IF_5 (l)$$

Fig. . Table of halogen displacement reaction with product information.