Double decomposition reaction?

The metathesis reaction is as follows:

The reaction in which two compounds exchange components to form two other compounds is called metathesis reaction. The equation of metathesis reaction can be simply written as AB+CD=AD+CB.

1, conditions of metathesis reaction:

① The acid in the reactant must be soluble, and at least one substance in the product is gas or precipitation or water.

② At least one reactant is soluble.

(3) Two salts in junior high school reactants are soluble, and at least one of the two salts obtained by the reaction is insoluble.

(4) The reactants are generally soluble, and at least one of the products is precipitation or gas. Ammonium salt reacts with alkali to produce gas.

2, the reaction principle:

In the metathesis reaction, reactants and products are called exchange salt pairs, and the general reaction formula is Ax+By=Bx+Ay. In the reaction formula, a and b represent cations; X and y represent anions. This system is called quaternary system in the phase diagram, and can be represented by AX-BY-H2O or A, B‖X, Y-H2O. For example: KCl+ sodium nitrite = potassium nitrate+sodium chloride, which can be written as: K, Na‖Cl, NO――H2O.

3. Reaction conditions:

(1) acid+salt → new acid+new salt The acid in the reactant must be soluble, and at least one substance in the product is gas, precipitation or water. Such as: 2hcl+CaCO3 = CaCl2+H2O+CO2 =

(2) At least one of the reactants of acid+alkali → salt+water is soluble. Such as: h2so 4+Cu(OH)2 = cuso 4+2h2o.

(3) Salt+Salt → Two new salts Both reactants in the early stage are soluble, and at least one of the two salts obtained by the reaction is insoluble. For example, Na2SO4+bacl2 = 2 NaCl+baso4 attachment: two salts (one insoluble or insoluble and unstable, the other soluble) decompose to generate more stable (or tend to be stable) salts (both reactants and products have insoluble salts), and salt (insoluble or insoluble)+salt = two new salts BAS+Na2SO4 = BAS.

(4) The reactants of salt+alkali → new salt+new alkali are generally soluble, and at least one of the products is precipitation or gas (only ammonia salt can react with alkali to generate gas). Such as: 2 NaOH+CuSO4 = Na2SO4+Cu (OH) 2 ↓; NaOH+NH4Cl = NaCl+NH3 ↑+H2ofe (OH) 3 does not react with NaCl.

4. Application of double decomposition reaction

To prepare the required products by metathesis reaction, it is generally required that the two salts as reactants have relatively high solubility in water, and the salts as products have relatively low solubility and can be precipitated in solid form within a certain temperature range. According to the product status, there are three situations:

① The precipitated solid crystal is the desired product. Such as potassium chloride and sodium nitrate, solid crystals of potassium nitrate can be precipitated at low temperature.

② The solution obtained by double decomposition needs further treatment to get the product. For example, when ammonium carbonate and calcium sulfate are decomposed again, the solution after calcium carbonate precipitation and separation needs to be evaporated to obtain ammonium sulfate solid product.

(3) Both salts of the product are precipitates, and their mixture is the product. For example, when zinc sulfate and barium sulfide double decompose, a precipitate mixture of zinc sulfide and barium sulfate is generated, which is filtered, washed and burned at high temperature to become white pigment lithopone.

The saturated concentration of salt in aqueous solution is related to temperature. Except that the product is a kind of salt with little solubility, the primary utilization rate (or conversion rate) of the raw salt in the process of double decomposition reaction is usually not very high, so it is necessary to recycle the mother liquor after the precipitation of solid crystals to improve the total utilization rate. Inorganic salts are produced by metathesis reaction, usually in a tank reactor with mechanical stirring, and the operation can be intermittent or continuous. In the process of producing inorganic salts by double decomposition reaction, chemical unit operations such as filtration and crystallization are often used.