Acid salts: formation mechanism
Acid salts are formed as a result of partial neutralization of polybasic acids. The main rule of this process is that neutralization proceeds stepwise, and the final product depends entirely on the molar ratio in which the acid and alkali were mixed.
If the alkali is in deficit, the reaction does not go to completion. As a result, an acid salt or a mixture of various salts is formed in the solution.
Let us examine this mechanism using the interaction of phosphoric acid ($H_3PO_4$) and sodium hydroxide ($NaOH$):
- First substitution. Sodium dihydrogen phosphate is formed:
$$H_3PO_4 + NaOH = NaH_2PO_4 + H_2O$$
- Second substitution. When another portion of alkali is added, sodium hydrogen phosphate is obtained:
$$NaH_2PO_4 + NaOH = Na_2HPO_4 + H_2O$$
- Third substitution. Complete neutralization leads to the formation of a normal salt — sodium phosphate:
$$Na_2HPO_4 + NaOH = Na_3PO_4 + H_2O$$
Properties and preparation methods of acid salts
In their chemical composition, acid salts occupy an intermediate position between a classical normal salt and an acid. This gives rise to chemical duality.
A striking example of such duality (a "chameleon salt") is sodium bicarbonate ($NaHCO_3$). Being a product of the incomplete neutralization of carbonic acid, it formally belongs to acid salts. However, in solution, this substance exhibits basic properties, creating an alkaline medium.
In addition to direct partial neutralization, acid salts can be obtained using displacement reactions:
- Interaction of a polybasic acid salt with a strong acid (provided the latter is in deficit):
$$Na_2CO_3 + HCl = NaHCO_3 + NaCl$$
- Interaction of a polybasic acid salt with the same acid:
$$Na_2CO_3 + H_2CO_3 = 2NaHCO_3$$
Basic salts: structure and isolation challenges
Basic salts are products of the partial neutralization of polyacidic bases by various acids.
The molecular structure of a basic salt must simultaneously contain two components:
- A hydroxide group ($-OH$).
- An acid residue.
A typical example of such a compound is aluminum hydroxyacetate: $Al(OH)(CH_3COO)_2$.
Unlike acid salts, basic salts precipitate from solutions as individual solids much less frequently, although they can be present in solutions in small quantities. The main problem in obtaining them lies in the insolubility of most metal hydroxides. Under conditions that are theoretically suitable for the crystallization of a basic salt, the metal hydroxide itself often precipitates, blocking the formation of the target product.
Double salts and their crystallization conditions
Double salts are chemical compounds containing two different metals bound to the same acid residue.
The main condition for the formation of such salts is a difference in solubilities. A double salt precipitates from solution only if it is less soluble than each of the simple salts involved in its formation.
A classic example is alum (potassium alum). If solutions of potassium sulfate ($K_2SO_4$) and aluminum sulfate ($Al_2(SO_4)_3$) are mixed, a white precipitate drops out as a result of chemical interaction.
The reaction equation is as follows: $$K_2SO_4 + Al_2(SO_4)_3 + 24H_2O = 2KAl(SO_4)_2 \cdot 12H_2O\downarrow$$
The precipitate forms precisely because the solubility of the resulting double salt is extremely low and does not exceed 6%.
Briefly: answers to topical questions
Why does an aqueous solution of sodium bicarbonate have an alkaline reaction?
An aqueous solution of sodium bicarbonate has an alkaline reaction because its basic properties are expressed more strongly than its acidic ones. In solution, the bicarbonate ion participates in two competing processes. As a base, it undergoes hydrolysis to form hydroxide ions ($HCO_3^- + H_2O \rightleftarrows H_2CO_3 + OH^-$). Simultaneously, it can behave as an acid, undergoing ionization ($HCO_3^- + H_2O \rightleftarrows CO_3^{2-} + H_3O^+$). The predominance of hydrolysis over dissociation leads to the accumulation of $OH^-$ ions, which is why a baking soda solution acquires a pH of 8.5.
How do acid salts dissociate in aqueous solutions?
Polybasic acids dissociate in aqueous solutions in a stepwise manner: first, a proton is split off from the neutral molecule, and then from the resulting negatively charged anion. The abstraction of a proton from a negatively charged anion is more difficult than from a neutral molecule due to electrostatic attraction. The following processes are given in sources for acid salt anions:
- Bicarbonate ion ($HCO_3^-$) — exhibits acidic properties: $HCO_3^- + H_2O \rightleftarrows CO_3^{2-} + H_3O^+$; simultaneously, it exhibits basic properties: $HCO_3^- + H_2O \rightleftarrows H_2CO_3 + OH^-$.
- Dihydrogen phosphate ion ($H_2PO_4^-$) — dissociates according to the equation $H_2PO_4^- \rightleftarrows H^+ + HPO_4^{2-}$.
Thus, for the bicarbonate ion, dissociation competes with hydrolysis.
Acid salts retain hydrogen (H) from the acid, basic salts retain the hydroxide group (OH) from the base, and double salts combine two different metals under a single acid residue.
Class and exam questions
What does the term "chameleon salt" mean?
This is an informal name for salts that exhibit chemical duality. For example, sodium bicarbonate is structurally an acid salt, but in aqueous solution it exhibits basic properties and creates an alkaline medium.
Why do basic salts rarely precipitate?
The crystallization of basic salts is hindered by the fact that most metal hydroxides are insoluble. Instead of forming a basic salt, the corresponding hydroxide often precipitates.
What else to study on this topic
- Reaction of potassium dihydrogen phosphate with sulfuric acid
- Interaction of potassium dihydrogen phosphate with potassium hydroxide
- Reaction of potassium phosphate with a double amount of phosphoric acid
- Searching for additional examples of double salts in chemical nomenclature
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