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.
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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 the salt is practically insoluble in this reaction, 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
Would you like to reference this text in a school or academic work? Look:
DAYS, Diogo Lopes. "Dual exchange reactions between salt and base"; Brazil School. Available in: https://brasilescola.uol.com.br/quimica/reacoes-dupla-troca-entre-sal-base.htm. Accessed on June 28, 2021.
