盐的酸碱性（Acidity and basicity of salt）

盐的酸碱性（Acidity and basicity of salt） 盐的酸碱性(Acidity and basicity of salt) Strong alkali weak acid salt soluble in water is alkaline, for example: sodium carbonate, sodium phosphate, sodium fluoride, sodium sulfide, hypochlorite Sodium, acetic acid Sodium, etc weak acid potassium salt, sodium salt, calcium salt, barium salt and so on (if soluble) . Two Strong acid, weak base salt, soluble in water is acidic, for example, ammonium sulfate, iron sulfate, copper sulfate, aluminum sulfate, etc. weak base sulfate, Nitrates, chlorides, bromides, iodides, etc (if dissolved) Three Some acid salts dissolved in water is alkaline, such as sodium bicarbonate, sodium hydrogen phosphate, sodium hydrosulfide. Some acid salts dissolved in water is acidic, for example, sodium bisulfate, Sodium Bisulfite, sodium dihydrogen phosphate. Some weak acids, weak bases, salts are acidic, for example, ammonium oxalate, ammonium fluoride, ammonium sulfite Some weak acids, weak bases, salts are alkaline, for example, ammonium hypochlorite, ammonium carbonate, ammonium sulfide. Remember, the acid salt of the weak acid is weaker than vinegar, and the acid is more acidic than that of the acid weak acid of the vinegar. Common strong acids: sulfuric acid, nitric acid, hydrochloric acid, hydrogen bromide, hydrogen iodate, perchloric acid. Common strong alkali: sodium hydroxide, potassium hydroxide, calcium hydroxide, barium hydroxide. Common weak acids: sulfurous acid, hydrofluoric acid, phosphoric acid, oxalic acid, acetic acid, carbonic acid, perchloric acid, silicic acid, hydrogen sulfate. Common base: ammonia, aluminum hydroxide, ferric hydroxide, copper hydroxide, ferrous hydroxide, magnesium hydroxide. [knowledge explanation] First, the essence of hydrolysis of salt The ionization of salts in aqueous solutions with ionized water H+ or OH - generation of molecules or ions that are difficult to ionize, from The ionization balance of water is destroyed, and the ionization degree of water is increased. Determine the hydrolysis conditions of hydrolysis of salt: See if it dissolves, No hydrolysis, no hydrolysis; two see if there is any weak, no weak, no hydrolysis. Two. Types and regularities of hydrolysis of salts Determine whether the hydrolysis of salt and hydrolysis, the pH value of the solution depends on the salt ionization of the corresponding acid or alkali phase About strength. One , Types of hydrolysis of salts Types of salt Hydrolysis often see can water solution The leave son solution PH value Monovalent: CH3COO- , HS- , AlO2- , ClO- , HCO-3 P H > 7 Alkali weak acid salt hydrolysis Two valence: S2- , CO32- , SiO32 - , HPO42- Trivalent: PO43- , ASO43- Monovalent: NH4+ , Ag+ P H < 7 Strong acid weak base salt hydrolysis Two valence: Fe2+ , Cu2+ , Sn2+ , Zn2+ Trivalent: Al3+ , Fe3+ Strong acid and alkali salt Non hydrolytic P H = 7 Two Hydrolysis Law: weak hydrolysis, no weak hydrolysis; weak who hydrolysis, the weaker the more hydrolysis; trivalent cations are hydrolysis, Stepwise hydrolysis of polybasic weak acid radical ions. Three Solution acid-base judgment: who is strong who?, Acid and alkali salts of acidic, weak acid by acid alkali salt. Radical separation Ionization and hydrolysis determine the relative strength of the daughter. (1) The weaker the salt, the weaker the root ion, the weaker the acid, the greater the degree of hydrolysis, and the stronger the alkalinity of the solution. The amount of the same substance Concentration CH3COONa and Na2CO3 Because carbonic acid is weaker than acetic acid Na2CO3 solution PH Value greater than CH3COONa Solution. A strong base weak acid salt solution in which the amount of the same substance can be determined PH Value, or according to Salt solution The PH The magnitude of the corresponding acid. (2) The hydrolysis of acid radical ion of polybasic acid is step by step, and the degree of hydrolysis in the first step is greater than that of second hydrolysis Amount of substance; concentration Na2CO3 and NaHCO3 Solution, Na2CO3 Solution basicity NaHCO3 Strong. (3) Weak acid salt solution pH value is determined by the relative size of the degree of ionization acid radical ion and hydrolysis degree: if the electric The degree of hydrolysis is greater than the degree of hydrolysis, the solution is acidic, such as NaHSO3 , NaH2PO4 Etc.. Ionization Four , The salinity hydrolysis ionic equation is less than the degree of hydrolysis, and the solution is alkaline, as NaHS , NaHCO3 , Na2HPO4 Etc.. (1) Generally speaking, hydrolysis of salts is of little degree and should be used" "The hydrolysis balance usually does not produce precipitation and," he said The gas, so do not sign "down" and "up". (2) The hydrolysis of multicomponent weak acid salts is stepwise and can be expressed in a multistep hydrolysis equation. as Na2CO3 The hydrolysis of the solution can be expressed as: CO32 - +H2O HCO3 - +OH -, HCO3? - +H2O H2CO3+OH - it can not be written: CO32 - +H2O H2CO3+OH - Four. Application of salt hydrolysis knowledge One , Judge the acidity and alkalinity of the salt solution and compare the strength of the acidity and alkalinity of the salt solution, Salt hydrolysis is usually required. Same conditions, The amount of the same substance in the following eight solutions: Na2CO3 , NaAc , Na2SO4 , NaHCO3 , NaOH , (NH4) 2SO4 , NaClO etc. dissolve liquid , PH value from large reach Small The along order by : NaOH>NaClO>Na2CO3>NaHCO3>NaAc>Na2SO4> (NH4) 2SO4>NaHSO4 . Two In comparison with the relatively large concentration of each ion in the salt solution, hydrolysis of salt should be taken into consideration when the salt contains easily hydrolyzable ions. Three Determine whether a large amount of ions coexist in the solution. When there is a double hydrolysis between weak base cations and weak acid anions, they do not Can coexist in a large amount of solution. Such as: Al3+ , NH4 + and HCO3- , CO32- , SiO32- And so on, not in solution Coexistence. Four When preparing the salt solution which is easy to hydrolyze, it is necessary to consider the hydrolysis of salt, for example, when adding a strong acid weak base salt solution, it is necessary to add a few drops The corresponding strong acid is used to inhibit the hydrolysis of salt. Five The hydrolysis of salt should be taken into consideration when selecting the pathway for the preparation of salt. For example, preparation Al2S3 It can be finished because it can not be prepared in solution Total hydrolysis, It can only be prepared by direct reaction with dry process. Steam dry AlCl3 , MgCl2 , FeCl3 Equivalent solution, Cannot get AlC3 , MgCl2 , FeCl3 Crystals must be constantly in the process of evaporation HCl Gas to suppress FeCl3 Hydrolysis in order to To its solid. Six For the rational use of chemical fertilizer, salt hydrolysis is sometimes considered. Such as ammonium nitrogen and plant ash can not be mixed use; phosphoric acid two hydrogen Calcium and ash can not be mixed. Due to plant ash (active ingredient) K2CO3 Hydrolysis is alkaline. Seven Preparation Fe (OH) 3 Colloid, Al (OH) 3 Colloids and uses FeCl3 , AlCl3 When the water is purified, it is used Fe3+ , Al3+ water A solution Fe (OH) 3 , Al (OH) 3 Colloids can adsorb small particles suspended in water and settle to play a role in water purification. Eight The hydrolysis of salt is necessary for laboratory storage of certain reagents. as Na2CO3 , Na2SiO3 The hydrolysis is alkaline, No. Reagent bottle that can be stored in the mouth glass stopper; NH4 F can not be stored in glass bottles, answer NH4 F hydrolysis should occur HF , Corrosive glass. Nine The hydrolysis of some ions should be considered in the solution. Ten In industrial and agricultural production and daily life, we often use the hydrolysis knowledge of salt. (1) Foam extinguisher makes use of foam Al2 (SO4) 3 and NaHCO3 Phase mixing occurs and double hydrolysis occurs CO2 , Al3++3HCO3 - =Al (OH) 3 Here +3CO2 Arrows. (2) Washing oil with hot lye in daily life is better than cold lye, which is due to heating Na2CO3 Hydrolysis make Alkaline solution enhancement. (3) The main component of scale is CaCO3 and Mg (OH) 2 , Basically not generated MgCO3, Because MgCO3 Slightly soluble in Water is hydrolyzed to produce more insoluble when heated Mg (OH) 2 . Five, difficulty and doubtful point analysis: One How to judge the type of reaction between salt solution and salt solution?: (1) When salt reacts with a salt solution, if the gas in the product is generated, the insoluble substance is produced or the matter is difficult to ionize The two kinds of hydrolysis methods are mixed with the same salt solution, and because of mutual inhibition, the metathesis reaction usually occurs. Such as: CuSO4+Na2S=Na2SO4+CuS Here, FeCl3+3AgNO3=Fe (NO3) 3+3AgCl Here (2) When a salt solution is mixed with a salt solution, both cations and anions can be hydrolyzed and precipitated or produced by gas, Mutual promotion, hydrolysis and hydrolysis , Double hydrolysis is common. as Al3+ and CO32 -, HCO3 -, SiO32 -, AlO2 -, ClO - wait. (3) If a salt can ionize a strong oxidizing ion, another salt can ionize a strong reductive ion, then Redox reactions occur in general. as 2FeCl3+2KI=2FeCl2+2KCl+I2 Two , How to determine whether a large amount of ions coexist in a solution?: To determine whether a large amount of ions coexist in a solution is actually to determine whether the ions in the solution react with each other Generally consider the following aspects: (1) See if precipitation occurs between ions. Common ion precipitation reactions are : H+ and SiO32 -, AlO2 -; A L3+ , Zn2+ , Fe3+ , Cu2+ , Fe2+ , Ag+ and OH -; Fe2+ , Zn2+ , Cu2+ , Ag+ , Hg2+ And, S2 -; Ag+ and Cl -, Br -, I -, CO32 -, PO43 -; Ba2+ , Mg2+ , Ca2+ and CO32 -, SO32 -, PO43 -, HPO42 - both can generate precipitation and can not coexist in a large amount of solution. (2) See if the gas is generated between the ions. as H+ and HS -, S2 -, HCO3 -, CO32 -, S2O32 -, SO32 -, HSO3 -; NH4+ and OH - such as gas generation, can not coexist in a large number of solutions. (3) See if ions can produce weak electrolytes. as H+ and F -, PO43 -, HPO42 -, ClO -, CH3COO - ; OH - and HCO3 -, HS -, HSO3 -, H2PO4 -, HPO42 -, NH4+ And so on, can not be in a large number of Solutions Deposit. (4) See if double hydrolysis occurs between ions: for example, Fe3+ , Al3+ , NH4+ and CO32 -, HCO3 -, ClO2 -, AlO2 -, SiO32 -; Al3+ and S2 -, HS - and so on; can not coexist in a large amount of solution. (5) See if redox reactions occur between the ions. as Fe3+ and S2 -, HS -, I - under acid conditions MnO4. - or NO3 - and Fe2+ , S2 -, I -, SO32 - and so on; can not coexist in a large amount of solution. In addition, we must pay attention to the problem of dry Request if it is a colorless solution, Fe2+ , Cu2+ , MnO4 -, Fe3+ Not existing in solution; as acidic liquid , So be able to H+ The reaction ion cannot exist; if it is an alkaline solution, then OH - The reaction ions do not exist. Etc..