The desiccants at secondary school level can be classified into three kinds according to their acidity and alkalinity:
1. Acidic desiccants: concentrated sulfuric acid, P2O5, silica gel
1) concentrated sulfuric acid (strong oxidizing acid)
2) P2O5 (acidic white powder),
3) silica gel (it is semi-transparent, with a large internal surface area) porous solid with good adsorption properties and strong adsorption of water. Silica gel containing cobalt salts is called color-changing silica gel, which is blue when it has not absorbed water, and pink when it is saturated with water).
2, alkaline desiccant: soda-lime, CaO, solid NaOH
1) soda-lime (it is white solid, the main components CaO and NaOH),
2) CaO (it is white solid, alkaline oxides),
3) solid NaOH (strong alkali)
3, neutral desiccant: anhydrous CaCl2, anhydrous CuSO4
1) anhydrous CaCl2 (white porous solid),
2) anhydrous CuSO4 (white powder, absorb water to a lesser extent, generally used to test the presence of water, after absorbing water, it turns into a blue color CuSO4-5H20)
Classification of common gases
Commonly seen gases in the secondary school level, according to the acidity and alkalinity can also be divided into three kinds:
①Acidic gases: CO2, SO2, NO2, Cl2, HCl, H2S, HBr, HI, etc.
②Alkaline gases: NH3
③Neutral gases: N2, O2, H2, CH4, etc.
Selection of the desiccant
1, the general principle: (acidity and alkalinity should be the same) And can not react)
a, acidic desiccant can not dry alkaline gas, can dry acidic gas and neutral gas
b, alkaline desiccant can not dry acidic gas, can dry alkaline gas and neutral gas
c, neutral desiccant can dry all kinds of gas. But this is only from the perspective of acid-base reaction to consider,
2, at the same time should also take into account the law outside of some of the specificity.
Such as the gas and the desiccant if the redox reaction occurs, or the generation of complexes, adducts, etc., you can not use this desiccant to dry the gas.
Special:
①Concentrated sulfuric acid can not be used to dry H2S, HBr, HI and other reducing gases, because of the redox reaction will occur.
such as: H2S + H2SO4 = 2H2O + SO2 + S ↓, H2SO4 + HBr = Br2 + SO2 + 2H2O, H2SO4 + HI = I2 + SO2 + 2H2O
② can not be used to dry the H2S gas anhydrous copper sulfate, the two will react: CuSO4 + H2S =
H2SO4 + CuS↓
③ anhydrous copper sulfate can not be used to dry NH3, the two can react to generate complexes: CuSO4 + NH3 = {Cu (NH3)4}SO4
④ anhydrous CaCl2 can not be used to dry NH3, the two will react to generate a number of adducts: CaCl2 + 8NH3 = CaCl2 - 8NH3