What is a displacement reaction?

The displacement reaction or substitution reaction is a type of chemical reaction in which an atom or a small group of atoms in a molecule is replaced or displaced by another atom or group of atoms.

Displacement reactions can be single or double, and are very common in nature. Many of the chemical reactions we see on a daily basis are displacement reactions, such as terminal sulfation in car batteries.

Types of displacement reactions

There are two types of displacement reactions, simple and double, which are described below along with some specific examples:

Simple displacement reaction

These are displacement reactions in which one element replaces another in a chemical compound . They are recognized because in both the reactants and the products there are only two substances involved, and in each case one of them is a pure element while the other is a compound.

These reactions can have one of the following general equations:

As can be seen, in this reaction element A reacts with compound BC and displaces or replaces element B to form compound AC.

In this case, element A also reacts with compound BC, but displaces or replaces element C to form compound BA, releasing C as a pure element.

One of the most important characteristics to recognize a simple displacement reaction is that the incoming element (A) must end up occupying the same place as the outgoing element (B or C, as the case may be) and this must end up as a pure element .

It should be noted that, in many cases, the C part of the molecule is not necessarily an individual element, but can be a group of atoms such as nitrate ions (NO  ) or sulfate (SO  ).

Examples of simple displacement reactions

– Displacement of silver by copper

In this case, copper (Cu) acts as element A, silver (Ag) corresponds to element B and nitrate ions (NO  ) represent C.

As can be seen, in this reaction copper replaces silver in silver nitrate, releasing silver in elemental form.

– Sulfation of the battery terminals

This simple displacement reaction is what occurs in the electrical terminals or contacts of many lead accumulators, that is, the batteries found in most gasoline cars.

The reaction is evidenced by the formation of a white solid that covers the lead contacts of these batteries. As you can see from the equation, this is a reaction in which lead displaces hydrogen in sulfuric acid.

– Displacement of bromine by fluorine

In this case, fluorine, which is a highly reactive element, replaces bromine in sodium bromide (NaBr) to form sodium fluoride (NaF). Note that here the element that is replaced is not the one that appears first in the formula of the compound, but rather it is the second, as in the second form of the general reaction shown above.

Double displacement reaction

Double displacement reactions, also called double replacement or double decomposition reactions, can be viewed as a pair swap. They are reactions in which two different chemical compounds exchange elements to form two different new compounds. These reactions have the following general equation:

As can be seen from this general reaction, element A of compound 1 (AB) replaces element C of compound 2 (CD) to thereby form compound 3 (AD). At the same time, element C of compound 2 (CD) replaces element A of compound 1 (AB) to form compound 4 (CB).

In double displacements it is essential that the elements that are substituted for each other occupy equivalent positions in the new compounds. This means that, in the general reaction, A, which is on the left and therefore probably a cation, can only replace C in compound CD, but cannot replace D.

This is similar to a dance partner swap. If all pairs are made up of a man and a woman, the new pairs formed after the reaction must also be made up of a man and a woman.

Examples of double displacement reactions

– Reaction between sodium chloride and silver nitrate

This is a clear example of a double displacement reaction. Here, sodium replaces silver in silver nitrate to form sodium nitrate (NaNO 3 ), while silver replaces sodium in sodium chloride to form silver chloride (AgCl).

– Reaction between calcium sulfate and sodium chloride

In this double displacement reaction, calcium replaces sodium in sodium chloride to form calcium chloride while sodium replaces calcium in calcium sulfate to form sodium sulfate.

Another way of looking at this reaction, which is equally valid, is that the sulfate is replacing the chloride in the sodium chloride to form the sodium sulfate while the chloride is replacing the sulfate.

Although both forms are equally acceptable, the former is more common than the latter.

– Reaction between sulfuric acid and potassium hydroxide

The reaction between sulfuric acid and potassium hydroxide is an acid-base neutralization reaction, but it is also a double-displacement reaction.

In this case, hydrogen is replacing potassium in potassium hydroxide to form water, while potassium is replacing hydrogen in sulfuric acid to form potassium sulfate.

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