ArticleNo.:1005–6629 (2016) 3–0064–04 China Library ClassificationNo.: G633.8 Document ID: B.
At present, the chemistry textbooks for junior high school and senior high school are all equipped with experiments in which active metals react with dilute hydrochloric acid or dilute sulfuric acid to replace hydrogen. For example, when discussing the chemical properties of magnesium, the textbook [1] for senior high school of Jiangsu Education Press sets the reaction between magnesium and dilute sulfuric acid: put a short piece of magnesium strip with oxide film wiped off into a test tube, and then add a certain amount of dilute sulfuric acid into the test tube to observe the phenomenon.
When students were divided into groups, because of the shortage of dilute sulfuric acid in the laboratory, the experimenter temporarily chose dilute hydrochloric acid instead. Some students reported that there was gray precipitation in the test tube! After class, the author asked the students to repeat the whole experiment process: with a small test tube, take 4 ml of 0. 1 mol L- 1 dilute hydrochloric acid, add a magnesium tape about 4cm long, remove the oxide film, and let it stand for a while. Precipitation really arrived as scheduled!
Why is there such an abnormal phenomenon in a seemingly simple middle school experiment? In this regard, after class, teachers and students conducted more in-depth experimental research together.
In the experiment 1 4 small test tube, take 4 ml of 0. 1 mol L- 1 dilute hydrochloric acid, and then add a certain length of magnesium tape to remove the oxide film, observe the phenomenon and compare the precipitation amount. See table 1 for the results.
Conclusion: The experiment 1 shows that there is no abnormal phenomenon when a certain amount of dilute hydrochloric acid reacts with a small amount of magnesium, but when the amount of magnesium tape is increased, bubbles will be generated when it is dissolved, accompanied by gray (or white) precipitation. It can be seen that the amount of magnesium is the reason to determine whether the system is abnormal.
So, what is the gray-white substance? In order to find out the reason, the students had a heated discussion, and finally formed three representative views, and then designed relevant experiments to verify them.
2. 1 viewpoint 1: crystallization of magnesium chloride
Some students think that due to the large amount of magnesium metal, the amount of magnesium chloride produced after the reaction is too large, which may be supersaturated and precipitated in the solution. According to the Chemical Dictionary [2], magnesium chloride is a colorless cubic crystal, and the amount of magnesium chloride dissolved in 100g water is 54.8g(20℃) and 73.0g (100g). It shows that magnesium chloride is easily soluble in water at room temperature, and the concentration of saturated solution can exceed 5 mol L-1even at 20℃.
In experiment 2, the D test tube of experiment 1 was left for a long time, and the supernatant solution was decanted to obtain gray precipitate, and then 5mL distilled water was added, and the amount of precipitate did not decrease. If the magnesium chloride solid (analytically pure) with the same mass is taken in a small test tube, the same amount of distilled water is added, and it is completely dissolved by gently shaking to obtain a clear solution.
It can be seen that magnesium chloride is easily soluble in water, while the solubility of off-white products is relatively poor, and the view of 1 is not credible.
2.2 Viewpoint 2: Magnesium powder remains after the reaction.
If you have done the experiment of the reaction between aluminum foil and hydrochloric acid, the following phenomena may not be unfamiliar: the reaction is intense, bubbles leap up, the solution is turbid, and a large number of light gray precipitates. Many scholars [3, 4] confirmed that light gray white material can react with acid or strong alkali and release a small amount of bubbles. Therefore, it is considered that the gray suspension should contain fine granular aluminum, which is caused by the excessive reaction and falling off the surface of aluminum foil. Then, is the gray precipitate after a sufficient amount of magnesium reacts with dilute hydrochloric acid also fine particles of magnesium powder?
In experiment 3, an appropriate amount of gray precipitate after the above experiment was taken out from a small test tube, and 3 ml of 0.1mol L-1dilute hydrochloric acid was added, and the precipitate immediately dissolved and disappeared, but no bubbles were generated.
If the sample is (or exists) magnesium powder, adding dilute hydrochloric acid will produce H2. The phenomenon of experiment 3 shows that the gray precipitate obtained by the reaction of sufficient magnesium with dilute hydrochloric acid is not fine magnesium powder.
2.3 Viewpoint 3: Magnesium hydroxide is generated.
Students who hold this view understand that magnesium can react with water at room temperature because its reducibility is slightly lower than that of sodium. In the later stage of the reaction of sufficient magnesium with dilute hydrochloric acid, it can be considered that the residual magnesium continues to react with water in magnesium chloride solution. Due to the existence of chloride ion in the solution, magnesium hydroxide, an insoluble product on the metal surface, has the function of drilling holes, which makes it fall off, destroying the protection of the internal metal, so that magnesium can continuously react with magnesium chloride solution at room temperature, and a large number of bubbles and magnesium hydroxide precipitates are continuously produced.
In experiment 4, 5mL distilled water was taken from a small test tube, and a short section of magnesium tape was added to remove the oxide film. A small amount of bubbles first formed on the surface of magnesium strip and then disappeared, and distilled water remained clear. A few drops of saturated magnesium chloride solution were added continuously, and fine bubbles were immediately generated on the surface of magnesium strip and dissolved continuously. After about 3 minutes, white turbidity appeared in the test tube, and after 20 minutes, the magnesium band was completely dissolved, leaving a large number of gray-white precipitates in the small test tube.
In experiment 5, the gray precipitate in experiment 4 was fully distilled and washed with water until the turbidity was not obvious after adding silver nitrate solution to the washing solution for the last time. Then add a proper amount of 0. 1 mol L- 1 dilute nitric acid, the precipitate will dissolve immediately, and continue to drop a few drops of 0. 1 mol L- 1 silver nitrate solution, resulting in a lot of white turbidity. If the experiment is repeated with the precipitate obtained in the experiment 1, the phenomenon is the same.
In experiment 6, a small amount of gray precipitate was distilled, washed with water and dried at room temperature. Then sample, heat the solid as shown in figure 1, and check the gas volume with wet purple litmus paper, and the paper turns red; If another glass rod soaked in silver nitrate solution is hung at the mouth of the test tube, white turbidity will appear.
Experiment 4 shows that magnesium can react in magnesium chloride solution to form gray precipitate. Combining experiment 5 and experiment 6, the precipitation samples obtained by magnesium and dilute hydrochloric acid or magnesium chloride solution are all dissolved in strong acid, and the presence of chlorine can be detected whether the solution dissolved by nitric acid or the solid sample is directly heated. Therefore, the view that the precipitate is (or just) magnesium hydroxide is also untenable.
On the basis of argumentation and analysis, the original ideas were rejected one by one. Just when everyone was puzzled, a student put forward a new idea: anhydrous magnesium chloride, the raw material for producing electrolytic magnesium, needs to be heated in dry HCl gas flow to get MgCl2 6H2O, and if heated in air, it will produce Mg(OH)Cl or MgO；; Then, is it possible that the gray precipitate generated by the reaction of sufficient magnesium with dilute hydrochloric acid is also a basic salt?
2.4 Viewpoint 4: Formation of Basic Magnesium Chloride
In experiment 7, about 50ml of 0.01mol l-1dilute hydrochloric acid was put into a small beaker, and then two 5cm magnesium strips were added to remove the oxide film. Observe the experimental phenomenon, and monitor and read the change of pH value of the solution with an acid meter. The experimental results are shown in Table 2.
Experiment 7 shows that during the reaction of sufficient magnesium with dilute hydrochloric acid, the pH value of the solution gradually increases, but there is no turbidity in acidic medium. Only when the reaction system of magnesium with dilute hydrochloric acid (actually magnesium and magnesium chloride solution) is weakly alkaline, and the alkalinity is gradually enhanced, the gray precipitate gradually appears and increases. According to the qualitative test results of Experiment 5 and Experiment 6, we have reason to believe that the gray precipitate is basic magnesium chloride.
According to literature [5], there are many structures of basic magnesium chloride, and the general chemical formula can be expressed as mgx (oh) yclz mh2o (where 2x-y-z=0, 0≤m≤6), which was first discovered when studying the chemical reaction between MgO and MgCl2 aqueous solution, and mainly exists in Mg2 (oh) 3cl 4h2o and Mg3 (. The analysis of modern scientific instruments shows that the white precipitate obtained at a certain pH environment is a mixture of various basic magnesium chloride solids, and the measured chemical composition of basic magnesium chloride solid products obtained at different pH values is also different (see Table 3)[6].
To sum up, due to the reaction of sufficient magnesium with dilute hydrochloric acid, the solution will go through the process from acidity to alkalinity, and the alkalinity will gradually increase, thus promoting the formation of gray precipitate. Because the chemical composition of basic magnesium chloride produced at different pH values will also change, we think that the turbidity obtained in the experiment should be a mixture of basic magnesium chloride with different chemical compositions.
3. 1 Understanding of textbook experiments
Using dilute hydrochloric acid instead of dilute sulfuric acid to demonstrate the reaction between magnesium and acid, the reaction speed between magnesium and acid (or water) will be greatly accelerated due to the chloride ion action of Cl- on the metal surface [7]. Once the amount of magnesium is too much, there will be a lot of gray-white precipitation during classroom teaching. Based on the above theoretical and experimental research, we believe that the deep-seated reason of the abnormal phenomenon is the formation of basic magnesium chloride mixtures with different chemical compositions at different pH values. It is suggested that it is reasonable to choose dilute sulfuric acid as the teaching material in this experimental teaching, and it should not be changed at will. Even if dilute hydrochloric acid is used, it should be noted that the amount of magnesium tape is less than more.
3.2 Handling of abnormal phenomena
Based on experiments is one of the important characteristics of chemistry. Chemical experiment is a means to provide students with perceptual knowledge and an effective way to cultivate students' scientific attitude and inquiry spirit. Experimental teaching under the background of teaching reform is an equal, open and dynamic dialogue and exchange. Due to various factors affecting chemical experiments, some unexpected and incomprehensible anomalies often appear in the specific operation process, and the teaching process will also mean more uncertainty [8]. Among many abnormal phenomena, some can be reasonably explained by the knowledge that students have mastered, while others cannot be explained under the current knowledge background of students. But whether it can be explained temporarily does not prevent these abnormal phenomena from becoming our teaching resources. Teachers should dare to break the preset framework, adjust teaching strategies, keep pace with the development of students' thinking, actively carry out classroom innovation, and carry out experimental inquiry activities in time to guide students' thinking to a deeper level.
References:
Editor Wang Zuhao. Chemistry 1 (compulsory) [M], the standard experimental textbook for senior high school. Nanjing: Jiangsu Education Press, 2007: 56.
Zhou Gongdu. Chemical Dictionary (Second Edition) [M]. Beijing: Chemical Industry Press, 20 1 1: 438.
[3] Fan Yanhua and Chen Lijuan. Study on "Abnormal Phenomenon" of Aluminum Foil Reacting with Hydrochloric Acid with Different Concentrations [J]. Chemistry Teaching, 201,(3): 42 ~ 43.
[4][7] Xiang Taiping. Study on Grey Matter in the Reaction of Aluminum with Hydrochloric Acid [J]. Chemistry Teaching, 2004, (3):1~12.
Chen, Lu Shuangshuang, Xia Meisheng, et al. Progress in preparation and application of basic magnesium chloride whiskers [J]. Material Guide (Review), 2009, (4): 52 ~ 55.
Li Chunzhong, Gu Qingshan, Cheng Qilin, et al. Synthesis and morphological analysis of acicular basic magnesium chloride [J]. Journal of East China University of Science and Technology (Natural Science Edition), 2005, (6): 3 14 ~ 3 18.
[8] Ren. Analysis of difficult problems in senior high school chemistry teaching [M]. Hangzhou: Zhejiang Education Press, 20 15: 133.