Salts in chemistry are divided into single salts and complex salts. Single salts are divided into normal salts, acid salts, and basic salts. Combined salts are divided into compound salts and complex salts. An acid salt is defined as a salt in which the cations generated during ionization include hydrogen ions in addition to metal ions (or ammonium radicals), and the anions are acid ions. Because the cation contains hydrogen atoms that can be ionized in water, it is called an "acid" salt. But in fact, only the acid salts of strong acids (such as sulfuric acid) and a small number of medium-strong acids (such as phosphoric acid) are acidic. Most of the acid salts of weak acids are alkaline due to the hydrolysis of anions.
It should be noted that when the acid salt exists in the form of ionic crystals, the anions do not ionize to produce hydrogen ions (but sodium bisulfate (NaHSO4) is an exception, which can ionize to produce H+ in aqueous solution. Not in the molten state), hydrogen ions are part of the acid ion. In the molten state, acid ions do not ionize either. (Sodium bisulfate does not ionize in the molten state)
Definition
In addition to metal ions or NH4+ (having metal ion properties), the cations generated during ionization also have hydrogen ions, and the anions are A salt of an acid ion.
Judgement of acidity and alkalinity
In layman's terms, the acid salts of strong acids are all acidic, the monosodium salts (or monopotassium salts) of medium-strong acids are all acidic, and the other sodium salts are acidic. Salts or potassium salts (including the acid salts of all weak acids) are alkaline.
Specific classification:
① Acidic salts whose aqueous solution is acidic
Acidic salts of strong bases and strong acids: such as NaHSO4, which undergo complete ionization in their aqueous solutions , so the solution is acidic;
Acidic salts of strong acids and weak bases: salts generated by strong acids and weak bases: such as NH4HSO4, the solution is acidic. The acidity of the solution is due to the ionization of HSO4-. The degree of HSO4- ionization is much greater than that of NH4+ hydrolysis, and it inhibits NH4+ hydrolysis to a certain extent. Therefore, do not think that the acidity comes from the hydrolysis of ammonium roots.
Acidic salts of strong bases and weak acids: The acid radicals are both ionized and hydrolyzed. You need to check the Ka value calculation. However, the acidic salts of medium and strong acids with the first hydrogen ion substituted are basically acidic and contain phenolic hydroxyl and carboxyl groups. Polyacids may not comply.
② Acid salts whose aqueous solution is alkaline:
Generally acid salts of strong base and weak acid, such as: HCO3-- salt, HS-- salt, due to "ionization" The trend is less than the "hydrolysis" trend, so the solution is alkaline.
Thermal stability of acid salts
Generally, the thermal stability of normal salts is greater than that of acid salts.
Na2CO3 is not easy to decompose when heated, 2NaHCO3
Na2CO3+CO2↑+H2O
CaCO3?=CaO+CO2↑ (high temperature conditions), Ca(HCO3)2
CaCO3+CO2↑+H2O
Reaction with acids and bases
The acid salt of a weak acid can react with both alkali and acid.
NaHCO3: NaHCO3+NaOH=Na2CO3+H2O
NaHCO3+HCl=NaCl+CO2↑+H2O
2NaHCO3+H2SO4=Na2SO4+2CO2↑+2H2O
Comparison of hydrolysis degrees
The hydrolysis degree of acidic salt of strong base and weak acid is less than the hydrolysis degree of normal salt of strong base and weak acid.
For example, NaHCO3 and Na2CO3
There is only one step in the hydrolysis of NaHCO3: HCO3-+H2O =H2CO3+OH-, [OH-]>[H+]
Obviously, the difference between two solutions of the same concentration is Alkaline: NaHCO3 Reference: Baidu Encyclopedia acid salt