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In chemistry, metal displacement reactions refer to those chemical reactions where one more reactive metal displaces another less reactive metal present in a compound.

The general representation of displacement reactions is as follows

A - B + C → A - C + B

In this general equation, C is more reactive than B. Thus it replaces it and bonds with the metal A.

The thing which you should know is that this type of displacement reaction is not only limited to metals. Instead, various non-metals also undergo displacement.

However, in this article, we will mainly discuss the metal displacement reactions occurring in an aqueous solution.

Types of displacement reactions occurring in aqueous solutions

As we know that the elements exist either in cation or anion form. Always remember that the elements that tend to form cations will always displace cations and vice versa.

Based on this theory, we can classify displacement reaction as follows

  • Cationic displacement reactions
  • Anionic displacement reactions

Since metals are extremely electropositive and give off cations, metals always undergo cationic displacement reactions.

Quick tip

Which metal displaces which one in a reaction depends upon the position of that metal in the electrochemical series.

In simpler words, a metal listed at the higher positions in an electrochemical series will potentially displace all the metals listed below.

For example, the position of metals in electrochemical series is such as, K > Na > Ca > Mg > Al > Zn > Fe. Thus, potassium can displace sodium, and sodium can displace calcium, and so on.

Mechanism of metal displacement reactions

To understand the reaction mechanism more precisely, let us consider a general reaction.

Consider a container in which a strip of metal A is placed in an aqueous solution of metal B. In this aqueous solution, metal B is present in the forms of B+

If metal A is more reactive than metal B, it will oxidize and start to disintegrate into the solution in the form of A+ ions such as

A → A+ + e-

On the other hand, the metal B+ will be reduced and start deposit at the strip in the form of solid metal such as

B+ + e- → B-

Since oxidation and reduction are occurring side by side, these reactions also demonstrate the redox reactions.

Examples of metal displacement reactions

A very common example of metal displacement reaction in an aqueous medium is the reaction of magnesium metal with copper sulfate.

The chemical equation for this reaction is such as

Mg(s) + CuSO4(aq) → MgSO4(aq) + Cu(s)

In this reaction, a thin slice of magnesium metal is placed in a container having a blue-colored aqueous solution of copper sulfate in it.

Since the position of magnesium is higher than that of copper in the electrochemical series, it displaces the copper.

Resultantly, magnesium starts to deposit in the aqueous solution to form MgSO4(aq), and copper starts to deposit on electrodes to form Cu(s).

The progress of this reaction can be predicted from the color change of the reaction medium. At the start, the aqueous solution is blue due to the presence of copper. However, as the reaction proceeds, it becomes colorless.

Just like the example mentioned above, the Cu metals can be displaced by various metals such as

Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)

Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)

2Al(s) + 3CuSO4(aq) → Al2(SO4)3(aq) + 3Cu(s)

Pb(s) + CuCl2(aq) → PbCl2(aq) + Cu(s)

The other examples of metal displacement reactions in aqueous solution are given below

The reaction of iron with lead nitrate

Iron replaces lead from its aqueous solution of lead nitrate and forms iron nitrate and lead.

Fe(s) + Pb(NO3)2(aq) → Fe4(NO3)2(aq) + Pb(s)

Reaction of sodium with dilute HCl

Sodium metal reacts vigorously with the hydrogen(in the form of dilute acids) to form hydrogen gas and metal salts such as

2Na + 2HCl → 2NaCl + H2

The reaction of potassium with dilute HCl

Potassium metal reacts with the dilute acids in the same way as sodium does. The equation for this reaction is such as

2K + 2HCl → 2KCl + H2

The reaction of Zn with dilute H2SO4

Zn metal reacts with dilute H2SO4 to form hydrogen gas and zinc sulfate. This reaction is greatly used to manufacture hydrogen gas in labs. The equation for this reaction is such as

Zn + H2SO4 → ZnSO4 + H2

Final words

Metal displacement reactions are just like the other substitution reactions we studied in chemistry.

The only difference is that here we discuss only the metals and their behaviors. Moreover, the rules for balancing chemical equations for metal displacement reactions are also the same as for other types of chemical reactions.