double exchange reactions in between salt and base is a chemical phenomenon in which an inorganic salt interacts with an inorganic base, originating obligatorily a new salt and a new base, as we can see in the equation that represents the process below:
Note: The inorganic base has a metal or ammonium cation linked to the hydroxyl group (OH), and the inorganic salt has any cation (different from hydronium-H+) and any anion (other than hydroxide-OH-).
YX + AOH → AX + YOH
As we can see in this equation, the cation of salt (Y+) interacts with the hydroxy group (OH-) of the base, forming the new base, while the cation (A+) of the base interacts with the anion (X-) of the salt, giving rise to the new salt.
We can only perceive visually, that is, with the naked eye, that a double exchange reaction between salt and base occurs when one or more products, among the following options, are formed:
Practically insoluble salt (see table of solubility for salts bellow);
Table showing the criteria for the solubility or not of salts
Practically insoluble bases, which are not formed by metals from the IA and IIA;
Ammonium hydroxide (NH4OH), base that is characterized by instability and consequent transformation into water and ammonia gas:
NH4OH → H2O + NH3
If none of the above products originate, what we will observe at the end of the reaction is a homogeneous mixture, that is, as if the reaction had not occurred. Below are some examples of double exchange reactions between salt and base.
1st Example: Reaction between calcium hydroxide [Ca(OH)2] it's the aluminum sulfate [Al2 (ONLY4)3].
In this double exchange reaction between salt and base:
Aluminum cation (Al+3) interacts with the hydroxide anion (OH-), forming aluminum hydroxide, which is a practically insoluble base, as aluminum does not belong to the IA family;
Calcium Cation (Ca+2) interacts with sulfate anion (SO4-2), forming the calcium sulfate salt (CaSO4), which is a practically insoluble salt, since sulfate only forms soluble salt with magnesium, among the elements of the IIA family.
1 Al2 (ONLY4)3 + 3 Ca (OH)2 → 3 Case4 + 2 Al(OH)3
As both the salt and the base formed in this reaction are practically insoluble, we can say that the reaction occurs visually.
2nd Example: Reaction between gold hydroxide III [Au (OH)3] and potassium phosphate [K3DUST4].
In this double exchange reaction between salt and base:
Potassium cation (K+) interacts with the hydroxide anion (OH-), forming potassium hydroxide, which is a soluble base, as potassium belongs to the IA family;
Gold cation III (Au+3) interacts with the phosphate anion (PO4-3), forming the gold phosphate salt III (AuPO4), which is a practically insoluble salt, as the phosphate only forms soluble salt with the element of the IA family or with ammonium-NH4+.
1K3DUST4 + 1 Au (OH)3 → 1 AuPO4 + 3 KOH
As in this reaction the salt is practically insoluble, we can say that the reaction occurs visually.
3rd Example: Reaction between the sodium hydroxide [NaOH] and ammonium cyanide [NH4CN].
In this double exchange reaction between salt and base:
Ammonium cation (NH4+) interacts with the hydroxide anion (OH-), forming ammonium hydroxide, which is a soluble base, but unstable because it transforms into H-water2O and ammonia-NH3;
Sodium cation (Na+) interacts with cyanide anion (CN-), forming the sodium cyanide salt (NaCN), which is a soluble salt, as the cyanide only forms a soluble salt with the element of the IA family or with the ammonium-NH4+.
1 NH4CN + 1 NaOH → 1 NaCN + 1 NH4oh
or
1 NH4CN + 1 NaOH → 1 NaCN + 1 NH3 + H2O
As the base formed in this reaction is ammonium hydroxide, which is unstable, we can say that the reaction occurs visually.
4th Example: Reaction between lithium hydroxide [LiOH] and sodium sulfide [Na2S].
In this double exchange reaction between salt and base:
Sodium cation (Na+) interacts with the hydroxide anion (OH-), forming sodium hydroxide, which is a soluble base, as sodium belongs to the IA family;
Lithium cation (Li+) interacts with the sulfide anion (S-2), forming the lithium sulphide salt (Li2S), which is a soluble salt, as the sulfide only forms soluble salt with the IA family element or with NH-ammonium4+.
1 in2S + 2 LiOH → 2 NaOH + 1 Li2s
As in this reaction both the salt and the base formed are soluble, we can say that the reaction, visually, does not occur.
By Me. Diogo Lopes Dias
Source: Brazil School - https://brasilescola.uol.com.br/quimica/reacoes-dupla-troca-entre-sal-base.htm