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What is a double substitution reaction?

The double substitution , double displacement or metathesis reaction is one in which a double ion exchange occurs between two compounds, without any of these being oxidized or reduced. It constitutes one of the most elementary chemical reactions.

The new bonds are formed by the large electrostatic attractive forces between the ions. Likewise, the reaction favors the formation of the most stable species, mainly the water molecule. The image below illustrates the general chemical equation for the double substitution reaction.

The initial compounds AX and BY react by exchanging “their partners” and thus form two new compounds: AY and BX. This reaction occurs if and only if A and Y are more related than A and B, or if the BX bonds are more stable than those of BY. Since the reaction is a simple ion exchange, neither ion gains or loses electrons (redox reaction).

Thus, if A is a cation with +1 charge in compound AX, it will have the same +1 charge in compound AY. The same goes for the rest of the “letters.” This type of reaction is the support of acid-base reactions and formation of precipitates.

Neutralization

A strong acid reacts with a strong base to produce soluble salts and water. When one of the two — the acid or the base — is weak, the salt produced is not completely ionized; that is, in an aqueous medium capable of being hydrolyzed. Similarly, the acid or base can be neutralized with a salt.

The above can again be represented by the chemical equation with the letters AXBY. However, since Brønsted acidity is indicated only by the H + and OH  ions , these then come to represent the letters A and Y:

HX + BOH => HOH + BX

This chemical equation corresponds to neutralization, which is simply the reaction between an HX acid and a BOH base to produce HOH (H 2 O) and the BX salt, which may or may not be soluble in water.

Its skeleton can vary according to the stoichiometric coefficients or the nature of the acid (if it is organic or inorganic).

Precipitation

In this type of reaction, one of the products is insoluble in the medium, generally aqueous, and precipitates (the solid settles from the rest of the solution).

AX + BY => AY (s) + BX

AX + BY => AY + BX (s)

In the event that both AY and BX are insoluble in water, the pair of ions that present the strongest electrostatic interactions will precipitate, which can be quantitatively reflected in their values ​​of solubility constants (Kps).

However, in most precipitation reactions, one salt is soluble and the other precipitates. Both reactions – neutralization and precipitation – can occur in the same mixture of substances.

Examples of double substitution reactions

Example 1

HCl (aq) + NaOH (aq) => H 2 O (l) + NaCl (aq)

What kind of reaction is this? Hydrochloric acid reacts with sodium hydroxide, generating as a consequence water and sodium chloride. Since NaCl is very soluble in aqueous medium, and a water molecule was also formed, the reaction of Example 1 is one of neutralization.

Example 2

Cu (NO 3 ) 2 (aq) + Na 2 S (aq) => CuS (s) + 2NaNO 3 (aq)

Neither the H +  nor the OH  ion are present in this reaction , and neither is the water molecule on the right hand side of the chemical equation.

Copper (II) nitrate, or cupric nitrate, exchanges ions with sodium sulfide. Copper sulfide is insoluble, precipitating unlike sodium nitrate, a soluble salt.

The Cu (NO 3 ) 2 solution is bluish, while the Na 2 S solution is yellowish. When both are mixed, the colors disappear and CuS, which is a blackish solid, precipitates.

Example 3

CH 3 COOH (aq) + NaOH (aq) => CH 3 COONa (aq) + H 2 O (l)

Again, this is another neutralization reaction. Acetic acid reacts with sodium hydroxide to form the sodium acetate salt and a water molecule.

Unlike example 1, sodium acetate is not a salt that is fully ionized, since the anion is hydrolyzed:

CH 3 COO  (aq) + H 2 O (l) <=> CH 3 COOH (aq) + OH  (aq)

Example 4

2HI (aq) + CaCO 3 (s) => H 2 CO 3 (aq) + CaI 2 (aq)

In this reaction, which although it does not appear to be neutralization, the hydroiodic acid reacts completely with the limestone to generate carbonic acid and calcium iodide. Furthermore, the evolution of heat (exothermic reaction) decomposes carbonic acid into carbon dioxide and water:

2 CO 3 (aq) => CO 2 (g) + H 2 O (l)

The overall reaction is as:

2HI (aq) + CaCO 3 (s) => CO 2 (g) + H 2 O (l) + CaI 2 (aq)

Also, calcium carbonate, the basic salt, neutralizes hydroiodic acid.

Example 5

AgNO 3 (aq) + NaCl (aq) => AgCl (s) + NaNO 3 (aq)

Silver nitrate exchanges ions with sodium chloride, thus forming the insoluble salt silver chloride (whitish precipitate) and sodium nitrate.

Example 6

2H 3 PO 4 (aq) + 3Ca (OH) 2 (aq) => 6H 2 O (l) + Ca 3 (PO 4 ) 2 (s)

Phosphoric acid is neutralized by calcium hydroxide, consequently forming the insoluble salt calcium phosphate and six moles of water molecules.

This is an example of a double substitution reaction of both types: the neutralization of the acid and the precipitation of an insoluble salt.

Example 7

2 S (aq) + MgSO 4 (aq) => K 2 SO 4 (aq) + MgS (s)

Potassium sulfide reacts with magnesium sulfate, bringing together the S 2– and Mg 2+ ions in solution to form the insoluble magnesium sulfide salt and the soluble potassium sulfate salt.

Example 8

Na 2 S (aq) + HCl (aq) → NaCl (aq) + H 2 S (g)

Sodium sulfide neutralizes hydrochloric acid, generating sodium chloride and hydrogen sulfide.

In this reaction, water is not formed (unlike the most common neutralizations) but the non-electrolytic molecule hydrogen sulfide, whose smell of rotten eggs is very unpleasant. The H 2 S escapes from the solution in gaseous form and the rest of the species remain dissolved.

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