THEdouble exchange reaction in between salts is the name given to the chemical phenomenon that occurs whenever we mix two salts that do not have the same cation or the same anion. The result of this reaction is always the formation of two new salts.
a) Criteria for the occurrence of a double exchange reaction between salts
The general formula for a salt is XY, where X (the first component of the salt formula) is always the cation and Y (the second component of the salt formula) is the anion.
If we mix in a container, for example, a solution of sodium chloride (NaCl) and another solution of sodium iodide. sodium (NaI), the double exchange reaction will not occur because the cation (sodium - Na) present in the two salts is the same.
Now, if we mix a solution of sodium chloride (NaCl) and a solution of potassium iodide (KI) in the same container, the double exchange reaction because the cations (Sodium - Na and Potassium - K) and the anions (Chloride - Cl and Iodide - I) present in the salts are many different.
b) Determination of the cation and anion charge of a salt
- For salt without index in the formula:
When we don't have an index in the formula of a salt, cation and anion have the same charge value, but with opposite signs. Thus, knowing the charge of one of them, that of the other will only have the opposite sign.
Example: CaS
Since Ca is an alkaline earth metal, it has a +2 charge, so S will have a -2 charge.
- For salt with index in the formula:
When salt has an index in the formula (on the lower right side of an element's abbreviation), automatically this index is the charge of the opposite group.
Example: CrCl3
In the formula, we have index 1 in front of Cr and index 3 in front of Cl, so the charge of Cr will be +3 (positive because the first group is the cation) and the charge of Cl will be -1 (negative because the second group is always the anion).
- For salt with parentheses in the formula:
When the salt has an index in front of parentheses, automatically, this index is the charge of the opposite group.
Example: Al2(ONLY4)3
In the formula we have index 2 in front of Al and index 3 in front of SO4, so the charge on Al will be +3 (positive because the first group is the cation) and the charge on SO4 will be -2 (negative because the second group is always the anion).
c) Principle of a double exchange reaction
The reaction is called double exchange because we have the exchange of two components between salts (XY and BA). The cation (X) of one salt interacts with the anion (A) of the other salt, and the cation (B) of the other salt interacts with the anion (Y) of the first, resulting in the formation of two new salts (XA and BA). We can clearly visualize this double exchange in the general equation that represents this type of chemical reaction:
XY + BA → XA + BA
In the mixture between the solutions of sodium chloride (NaCl) and Potassium iodide (KI), sodium iodide (NaI) and potassium chloride (KCl) were formed, as shown in the equation:
NaCl + KI → NaI + KCI
d) Visual changes of a double exchange reaction
Not always when we perform a double exchange reaction, we visualized some modification in the experiment. In two colorless aqueous salt solutions, for example, when we mix the two together, we know that new salts have formed, but the result is a colorless material. The absence of visual alteration, therefore, does not mean that the double exchange reaction did not take place.
We will have a visual change if one or two practically insoluble salts are generated in the process. If only soluble salts are formed, we will have visual change only if one of the dissolved salts changes the color of the solution. The table below provides information on when a salt is soluble or practically insoluble:
Salt solubility table
e) Examples of assembling equations representing double exchange reactions between salts
Now follow some examples of assembling the equation of double exchange reactions between salts:
Example 1: Double exchange between potassium cyanide (KCN) and silver chloride (AgCl)
Initially, let's know what the cation and anion of each of the salts is:
1) For KCN: As there is no index written in the formula, we consider that there is index 1 in front of K and CN.
- the cation is K+1 (+1 because every alkali metal has NOX +1);
- the anion is CN-1 (-1 because, when the formula indices are equal, cation and anion have charges of the same value, but with opposite signs).
2) For AgCl: As we have no index written in the formula, we consider that there is index 1 in front of Ag and Cl.
- the cation is Ag+1 (+1 because Ag has fixed NOX +1);
- the anion is Cl-1 (-1 because, when the formula indices are equal, cation and anion have charges of the same value, but with opposite signs).
Knowing the ions, it is easy to understand that the double exchange between these salts occurs with the union of the following ions:
K+1 with Cl-1, resulting in the KCl salt after crossing the +1 and -1 charges of the ions. As the loads have the same number (1), it is not necessary to write it in the final formula.
Ag+1 with CN-1, resulting in the AgCN salt after crossing the +1 and -1 charges of the ions.
The balanced chemical equation that represents the double exchange reaction between these salts is:
1 KCN + 1 AgCl → 1 KCl + 1 AgCN
In this reaction, we have the formation of a KCl soluble salt (chloride with alkali metal) and another practically insoluble AgCN (cyanide, any anion, without alkali metal or NH4+). So, when looking at the experiment, we'll see a solid (AgCN) at the bottom of the container, as it doesn't dissolve in water.
Example 2: double exchange between Calcium carbonate (CaCO3) and Magnesium Sulfate (MgSO4)
Initially, let's know what the cation and anion of each of the salts is:
1) For CaCO3:As we don't have any index written in the formula, the charge present on the cation always has the same number as the charge on the anion.
- the cation is Ca+2 (+2 because every alkaline earth metal has this NOX);
- the anion is the CO3-2 (-2 because, since we don't have any index written in front of Ca, the anion charge will have the same value as the cation charge, but with the opposite sign).
2) For MgSO4: Since we don't have any index written in the formula, the charge present on the cation always has the same number as the charge on the anion.
- the cation is Mg+2 (+2 because every alkaline earth metal has this NOX);
- the anion is the OS4-2 (-2 because, as we don't have any index written in front of Mg, the anion charge will have the same value as the cation charge, but with the opposite sign).
Knowing the ions, it is easy to understand that the double exchange between these salts occurs with the union of the following ions:
Here+2 with OS4-2, resulting in the CaSO salt, after crossing the +2 and -2 charges of the ions.
mg+2 with CO3-2, resulting in the MgCO salt3 after crossing the +2 and -2 charges of the ions.
The balanced chemical equation that represents the double exchange reaction between these salts is:
1 CaCO3 + 1 MgSO4 → 1 Case4 + 1 MgCO3
In this reaction, we have the formation of two practically insoluble salts: the CaSO4 (alkaline earth metal sulfate) and MgCO3 (carbonate, without alkali metal or NH4+). So, when looking at the experiment, we will see two solids (CaSO4 and MgCO3) at the bottom of the container, as they do not dissolve in water.
Example 3: Double switch between sodium nitrate (NaNO3) and potassium dichromate (K2Cr2O7)
Initially, let's know what the cation and anion of each of the salts is:
1) For NaNO3: Since we don't have any index written in the formula, we consider that there is index 1 in front of Na and NO in it.3.
- the cation is Na+1 (+1 because every alkali metal has NOX +1);
- the anion is NO3-1 (-1 because, when the formula indices are equal, cation and anion have charges of the same value, but with opposite signs).
2) To K2Cr2O7
- the cation is K+1 (+1 because every alkali metal has NOX +1);
- the anion is the Cr2O7 -2 (-2 for having index 2 in K).
Knowing the ions, it is easy to understand that the double exchange between these salts occurs with the union of the following ions:
At+1 with Cr2O7 -2, resulting in salt Na2Cr2O7 after crossing the +1 and -2 charges of the ions.
K+1 with NO3-1, resulting in the KNO salt3 after crossing the +1 and -1 charges of the ions.
THE balanced chemical equation representing the double exchange reaction between these salts é:
2 NaNO3 + 1K2Cr2O7 → 1 In2Cr2O7 + 2 KNO3
In this reaction, we have the formation of two soluble salts: Na2Cr2O7 (dichromate, any anion, with alkali metal) and KNO3 (Nitrate, which is always soluble). So, when looking at the experiment, we won't see any solids at the bottom, but depending on the salt that dissolves, there may be a change in the color of the solution (not the case in the example).
Example 4: Double exchange between gold nitrite III [Au (NO2)3] and zinc acetate [Zn(H3Ç2O2)2]
Initially, let's know what the cation and anion of each of the salts is:
1) For Au (NO2)3
- the cation is Au+3 (+3 because of index 3 after NO parentheses2);
- the anion is NO2-1 (-1 because of index 1 in Au).
2) For Zn(H3Ç2O2)2
- the cation is Zn+2 (+2 because of the 2 after the anion parentheses);
- the anion is the H3Ç2O2-1 (-1 because of index 1 in Zn).
Knowing the ions, it is easy to understand that the double exchange between these salts occurs with the union of the following ions:
Au+3 with H3Ç2O2-1, resulting in the Au salt (H3Ç2O2)3 after crossing the +2 and -1 charges of the ions;
Zn+2 with NO2-1, resulting in the Zn salt (NO2)2 after crossing the +2 and -1 charges of the ions.
The balanced chemical equation that represents the double exchange reaction between these salts is:
2 Au (NO2)3 + 3 Zn (H3Ç2O2)2 → 2 Au (H3Ç2O2)3 + 3 Zn (NO2)2
In this reaction, we have a practically insoluble salt, Au (H3Ç2O2)3 (Acetate, any anion, without alkali metal or NH4+), and another soluble, Zn (NO2)2 (Nitrite, which is always soluble). So, when looking at the experiment, we'll see a solid at the bottom of the container.
By Me. Diogo Lopes Dias
Source: Brazil School - https://brasilescola.uol.com.br/quimica/reacao-dupla-troca-entre-sais.htm
