First, write the definitions of ion symbols and chemical formulas.
Middle school chemistry textbooks clearly tell us that substances that are easy to dissolve and ionize write ions. Write the chemical formula of insoluble, hard to ionize or volatile substances. Combined with the actual situation of chemistry teaching in middle schools, we can draw the conclusion that strong electrolytes (i.e. strong acids, strong bases and water-soluble salts) that are easily soluble in water write ions, while others (i.e. weak acids, weak bases, water-insoluble salts, simple substances and oxides) write chemical formulas.
1. Write the strong electrolyte dissolved in water in ionic form, and others (including the strong electrolyte insoluble in water) in chemical formula. For example, the ionic equation of the reaction between Na2CO3 and HCl is: CO32-+2h+= = CO2 ↑+H2O; The ionic equation of the reaction between CaCO3 and HCl is CaCO3+2h+= = CO2 =+H2O+Ca2+. Therefore, remembering which substances belong to strong electrolytes and which strong electrolytes can be dissolved in water is the basis and key to writing ionic equation.
2. Write the chemical formula except that the strong electrolyte is soluble in water. For example, the ionic equation of Na2O reacting with dilute HCl is Na2O+2h+= 2na+H2O; The ionic equation of H3PO4 reacting with enough NaOH solution is: H3PO4+3OH-= = PO43-+3H2O; The ionic equation of CuSO4 and Ba(OH)2 is: Cu2+SO42-+Ba2+2OH-= = baso4 ↓+Cu (OH) 2 ↓; The ionic equation of FeS reacting with dilute H2SO4 to generate H2S is: FeS+2H+==Fe2++H2S↑.
3, the ionic reaction between solid substances, related substances are written in the chemical formula, can't write ionic equation. For example, the reaction of mixing Ca(OH)2 solid and NH4Cl solid to generate NH3 in the laboratory.
4. Concentrated H2SO4 reacts with solid substances, and related substances are written in the chemical formula, so ionic equation cannot be written. For example: the reaction of Cu with concentrated H2SO4; Concentrated H2SO4 reacts with corresponding solid substances to generate HCl, HF and HNO3.
Second, ionic equation's writing
After the ion symbols and chemical formulas are clearly written, ionic equation's writing can generally be completed correctly. However, for some specific ion reactions, it is necessary to deal with some special situations according to specific conditions, so that the written chemical formula, ion symbol and ionic equation are consistent with the actual situation.
1. In the ionic reaction with acid salt, when writing ionic equation, the acid radical of weak acid cannot be decomposed into H+ and acid radical (except HSO4-). For example, the reaction between NaHCO3 and NaOH should be written as HCO3-+OH-==H2O+CO32- instead of H++OH-==H2O.
2. When ionic equation writes with a slightly soluble substance (or generates a slightly soluble substance), if the slightly soluble substance is a reactant and clear, he writes in the form of ions; If the slightly soluble matter is turbid, it should be written in the form of chemical formula; When a slightly soluble substance is a product, it is usually represented by a chemical formula. For example, the ionic equation of Ca(OH)2 solution clarified by proper amount of CO2 should be written as Ca2++2OH-+CO2 = = CaCO3 ↓+H2O; Ionic equation of lime milk (turbid solution of Ca(OH)2) and Na2CO3 solution should be written as Ca(OH)2+CO32-==CaCO3+2OH-.
3. Obey the law of conservation of mass and charge. Ionic equation should match not only the number of planes, but also the number of ion charges. For example, the ionic equation of the reaction of Fe with FeCl3 solution should be written as Fe+2fe3+= 3fe2+instead of Fe+Fe3+==2Fe2+.
4. The reduction factor cannot destroy the relational quantity. It is necessary to delete unreacted ions, but if only some ions are reduced, which violates the relationship between substances in the actual reaction, then it is wrong to reduce the coefficient. For example, the ionic equation of H2SO4 and Ba(OH)2 solution is Ba2++2OH-+2H++SO42-= = baso4 ↓+2H2O. If only the coefficients of OH- and H+ are written as Ba2++OH-+H++SO42-==BaSO4↓+H2O, it is not consistent with the actual generation.
Thirdly, the influence of external factors on ionic equation's writing.
After mastering the writing of chemical formula, ion symbol and ionic equation, we should also pay attention to the reaction conditions, because some reactions, external factors change, and the ion reaction formula will also change. When writing ionic equation, we should pay attention to the writing methods and strategies, so that the written ionic equation is consistent with the given external factors.
1, the effect of relative amount of reactants on ionic reaction
In some reactions, the relative amounts of reactants change anyway, and the reactions between substances remain unchanged; There are also many reactions. When the relative amounts between substances are different, the reactions between substances are different, and so is ionic equation.
(1) can continue to react with excess substances. This kind of reaction needs to pay attention to the conditions given in the title to judge whether the product continues to react with excessive substances. As long as the final product is determined correctly, the corresponding ionic equation can be written. For example, the ionic equation of dropping a small amount of ammonia water into Ag(NO3)2 solution is: Ag ++ NH3·H2O = = agoh↓+NH4+ NH4+. Ionic equation of dropping excessive ammonia into Ag(NO3)2 solution is: Ag++NH3 H2O = = Agoh ↓+NH4+,Agoh+2nh3H2O = = [Ag (NH3) 2]+OH-+2H2O; NH4+ generated by the two reactions reacts with OH-: NH4 ++ OH-= = NH3 H2O；; Therefore, the total ionic equation is ag++2nh3H2O = [ag (NH3) 2]++2h2o.
(2) The reaction related to the order of dropping substances. There are many chemical reactions, and the chemical reactions are different because the dropping order of the two substances is different. For such a chemical reaction, special attention should be paid to the dropping order and the amount of the two substances. For example, the ionic equation of adding a small amount or excessive NaOH solution to AlCl3 solution is: Al3++3OH- ==3Al(OH)3↓, Al3++4OH-= = Al2-+2H2O; The ionic equation of adding a small amount or an excessive amount of AlCl3 solution to NaOH solution is: 4OH-+Al3+==AlO2-+2H2O, Al3++3OH- ==3Al(OH)3↓ respectively.
(3) The reaction is continuous. This reaction is that several particles in one substance will react with another substance. As long as we pay attention to the sequence of reactions, we can write the equation about ions correctly. For example, Cl2 is introduced into FeBr2 solution and FeI2 solution respectively, and the ionic equation is 2I-+Cl2==I2+2Cl-, 2Fe2++Cl2==2Fe3++2Cl-, 2Br-+Cl2==Br2+2Cl- respectively. This is because reducibility I->; Fe2+>； Br- reason.
(4) Reactions involving acid salts. This reaction should pay attention to the relationship between the amount of acid salt and the amount of another reactant. Different amounts lead to different chemical reactions. As long as we pay attention to the given quantity, we can get the correct ionic equation. For example, the ionic equation for the reaction of sodium bicarbonate solution with a small amount of clarified limewater is: 2hco3-+Ca2++2oh-= CaCO3 ↓+H2O+CO32-,and the ionic equation for the reaction of sodium bicarbonate solution with excessive clarified limewater is: HCO3-+Ca2++OH- ==CaCO3↓+H2O.
2. Effect of reaction conditions on ion reaction
In some reactions, external conditions (concentration and temperature) have little effect on the reaction; There are also some reactions, and the external conditions change, and the reactions between them are different.
Effect of (1) temperature on ionic reaction. Many chemical reactions in middle school chemistry are related to reaction temperature. When writing this kind of ionic equation, we should pay special attention to the reaction temperature. For example, at room temperature, the ionic equation of the reaction between NH4Cl and NaOH is NH4 ++ OH-= = NH3 ↑+ H2O； instead of NH4++OH-= = NH3 ↑+H2O; Another example is that the ionic equation of Cl2 reacting with cold NaOH solution is: Cl2+2OH- ==ClO-+Cl-+H2O, while the ionic equation of Cl2 reacting with hot NaOH solution is: 5Cl2+6OH- ==ClO3-+5Cl-+3H2O.
(2) Effect of concentration on ion reaction. Some chemical reactions are related to the concentration of substances. When writing this kind of ionic equation, we should pay attention to the concentration of substances, especially the change of concentration will cause the change of chemical reaction. For example, the ionic equation of the reaction of a sufficient amount of copper with a certain amount of concentrated nitric acid is Cu+4h+2no3-= Cu2+NO2 ↑+2h2o, 3cu+8h+2no3-= 3cu2+2no ↑+4h2o, because the concentrated nitric acid becomes dilute nitric acid with the progress of the reaction. Another example is that the ionic equation of saturated brine with sufficient NH3 and CO2 is Na++NH3+CO2+H2O==NaHCO3↓+NH4+, which is because the solubility of NaHCO3 is small and most of the generated NaHCO3 precipitates.
3. The influence of acidity and alkalinity on ionic reaction
The difference of acidity and alkalinity of reactants sometimes affects the degree of reaction. For example, the ionic equation of introducing a little or too much CO2 into sodium phenolate solution is: C6H5O-+CO2+H2O = = C6H5OH+HCO3-; The ionic equation of sodium phenolate solution containing little or excessive SO2 is 2c6h5o-+SO2+H2O = = 2c6h5oh+SO32-; C6H5O- +SO2+H2O==C6H5OH+HSO3-. This is because the relative strength of acidity is: H2SO4 3 >; HSO 3-& gt； c 6 H5 oh； H2CO3 & gtc 6 H5 oh & gt； Because bicarbonate-
4. Influence of solubility on ionic reaction
Ions in solution always tend to combine with each other and become more insoluble. For example, one ion in a solution can combine with several other ions to form an insoluble substance. Pay attention to how easy it is to combine them. At this time, the ion equation cannot be written at will, especially the ion equation cannot be fabricated at will. For example, the ionic equation of the reaction between the solution of NaH2PO4 and the clear solution of Ca(OH)2 in the ratio of 8:5 is: 8H2PO4-+5ca2+10oh-= = Ca3 (po4) 2 ↓+cahpo4 ↓+4hpo42-+10h2o, because of CA3.
5. Effect of redox on ionic reaction.
If one ion in the solution can react with other ions, we should pay attention to the difficulty of the reaction between them. Generally, the same oxidant is easy to react with reducing agent with strong reducibility first; In the same way, the same reductant is easy to react with oxidant with strong oxidation first. For example, the ionic equation of 3mol Cl2 in 4mol FeBr2 solution is 4fe2++2br-+3cl2 = = 4fe3++Br2+6cl-. In this reaction, Cl2 firstly oxidizes Fe2+, and excess Cl2 oxidizes a part of Br-.
Fourth, an issue that is easily overlooked.
Besides paying attention to some laws and methods of ionic equation's writing mentioned above, we should also emphasize the influence of hidden factors on ionic equation's correct writing. Because many reactions contain implicit conditions and implicit ion reactions, it is easy to make mistakes if you don't pay attention to mining implicit factors when writing ionic equation. For example, the ionic equation of Fe(OH)2 dissolved in a sufficient amount of dilute HNO3 is 3fe (OH) 2+10h+NO3-= 3fe 3+NO ↑+8H2O; But it is not Fe (OH) 2+2h+= Fe2++2h2o, because this reaction means an ionic reaction, in which excess dilute HNO3 continues to oxidize Fe2+ to Fe3+. For another example, sufficient so2 does not react when introduced into BaCl2 solution, but reacts when introduced into Ba(NO3)2 solution, and its ionic equation is: 3so2+Ba2+2no3-+2h2o = baso4 ←+2no+4h+2so4-,because the reaction implies that SO2 is acidic when dissolved in water, and NO3- in Ba(NO3)2 solution.