Hot topic type 3 Sodium carbonate and sodium bicarbonate

Example 3. The following methods cannot be used to identify Na2CO3 and NaHCO3 solids are ( )

A． Heat the two solids separately and pass the released gas into the clarified lime water

B． Take two solids dissolved in water, add Ba(OH)2 solution dropwise, and observe whether a precipitate is generated

C． Take 1 g each of the two solids in the tube, drop a few drops of water to shake, and touch the bottom of the tube with your hand

D． The solids of equal mass were taken separately and fully reacted with a sufficient amount of dilute hydrochloric acid, and then the resulting solution was heated, steamed dry, and burned and weighed

Analysis: Item A, NaHCO3 decomposes in heat to generate CO2, AND CO2 can make the clarified lime water turbid; Na2CO3 does not decompose when heat is encountered, so it can be identified. In term B, Na2CO3 and Ba(OH)2 solution reacted: Ba2++CO3(2-)===BaCO3↓, NaHCO3 and Ba(OH)2 solution reacted: Ba2++2OH-+2HCO3(-)===BaCO3↓+2H2O+CO3(2-), the solution became turbid, so it could not be identified. In item C, Na2CO3 is soluble in water exothermic, and NaHCO3 is soluble in water heat absorption, so it can be identified. Item D, the molar mass of Na2CO3 and NaHCO3 is different, the amount of the substance is different at the same mass, and the quality of the final sodium chloride is also different, so it can be identified.

Answer: B

【Fractional Cheats】 Use the combination of numbers and shapes to understand the reaction of Na2CO3, NaHCO3 and hydrochloric acid

(1) Add hydrochloric acid to the Na2CO3 solution drop by drop, the first step: CO3 (2-) is converted to HCO3 (-), no gas is generated; the second step: HCO3 (-) reacts with H+ to produce CO2. The relationship between the volume of HCl consumed and the volume of CO2 produced is shown in Figure 1.

(2) Add hydrochloric acid to the NaHCO3 solution drop by drop, and the relationship between the volume of consumption HCl and the volume of CO2 produced is shown in Figure 2.

(3) Add hydrochloric acid to the mixed solution of NaOH and Na2CO3 drop by drop, and the relationship between the volume of HCl consumed and the volume of CO2 produced is shown in Figure 3 (the ratio of the amount of NaOH and Na2CO3 to the amount x:y=1:1, the image of other proportions is slight).

(4) Hydrochloric acid is added drop by drop to the mixed solution of Na2CO3 and NaHCO3, and the relationship between the volume of HCl consumed and the volume of CO2 produced is shown in Fig. 4 (the ratio of the amount of Na2CO3 and NaHCO3 to the amount of nahCO3 is m:n=1:1, and the image is slightly in other proportions).

【One example and three lessons】

Mix 0.4 g NaOH and 1.06 g Na2CO3 into a solution and add 0.1 mol· L-1 dilute hydrochloric acid. The following image correctly represents the relationship between the volume of hydrochloric acid added to the amount of the substance that generates CO2 is ( )

Answer: C